Studies on some ternary oxyborates of the Na2O-Me2O3-B2O3 (Me=rare earth or aluminum) systems: Synthesis, structure and crystal growth

Article abstract of DOI:10.1016/j.jssc.2006.01.027

Sodium rare-earth oxyborates Na₂RE₂O(BO₃)₂ (RE=Y, Nd, Er) were prepared for the first time in the present study. They were found to be isostructural with phases of the same composition containing Sm, Eu or Gd and reported by Corbel et al. [J. Solid State Chem.144 (1999) 35-44]. It was shown that the yttrium and erbium compounds could be synthesized at 900-1000 °C by a solid-state reaction between oxides in an equimolecular ratio. With both oxyborates melting led to decomposition into a mixture of Y(Er)BO₃, Y₂(Er₂)O₃ and Na₂B₄O₇. Just the opposite was observed during thermal treatment of the oxide mixture containing Nd₂O₃, from which a practically pure phase of Na₂Nd₂O(BO₃)₂ was only obtained after melting. The attempts to synthesize the oxyborate Na₂La₂O(BO₃)₂ showed it to be unstable, this leading to the formation of a mixture containing, in addition to Na₂La₂O(BO₃)₂, also other already known stable phases of the system Na₂O-La₂O₃-B₂O₃ along with an unknown ternary oxide phase. This phase was found to represent a new oxyborate of sodium and lanthanum with the formula Na₃La₉O₃(BO₃)₈, whose single crystals were obtained by flux growth. It was established that synthesis of a polycrystalline material with the same composition was also possible using solid-state interaction between Na₂CO₃, La₂O₃ and H₃BO₃ at 1000-1100 °C. X-ray diffraction experiments on single crystals were used to solve the structure of Na₃La₉O₃(BO₃)₈. The unit cell was found to be hexagonal, space group P62m (No. 189) with Z = 1. The compound can be regarded as the forefather of a second group of oxyborates representing a new family of isostructural compounds, Na₃RE₉O₃(BO₃)₈. Such phases were obtained with RE=Nd, Sm and Eu whereas with RE=Y and Gd, the synthesis experiments failed. The concentration and temperature regions of crystallization of the double-oxyborate Na₂Al₂O(BO₃)₂ in the ternary system Na₂O-Al₂O₃-B₂O₃ were determined. This compound was shown to melt incongruently at 970±3 °C, which made high-temperature solution growth most appropriate for obtaining its single crystals with NaBO₂ as the best solvent. On the basis of the data obtained, a composition of the initial solution was proposed, the validity of the choice being demonstrated by the growth of Na₂Al₂O(BO₃)₂ single crystals on a seed using the top-seeded solution growth (TSSG) technique and slow cooling of the solution.

Full text of DOI:10.1016/j.jssc.2006.01.027

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 2834–2849
www.elsevier.com/locate/jssc
Studies on some ternary oxyborates of the Na O–Me O –B O
2 3
2
2 3
(
Me ¼ rare earth or aluminum) systems: Synthesis, structure and
crystal growth
a,
Ã
b
a
b
b
a
P. Peshev , S. Pechev , V. Nikolov , P. Gravereau , J.-P. Chaminade , D. Binev ,
a
D. Ivanova
a
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Building 11, 1113 Sofia, Bulgaria
b
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux (ICMCB),CNRS (UPR 9048), Universit e´ Bordeaux 1, 87 avenue du Docteur A. Schweitzer, 33608
Pessac, France
Received 25 August 2005; received in revised form 24 November 2005; accepted 16 January 2006
Available online 13 February 2006
Abstract
Sodium rare-earth oxyborates Na
found to be isostructural with phases of the same composition containing Sm, Eu or Gd and reported by Corbel et al. [J. Solid State
2
RE
2
O(BO
3
)
2
(RE ¼ Y, Nd, Er) were prepared for the first time in the present study. They were
Chem.144 (1999) 35–44]. It was shown that the yttrium and erbium compounds could be synthesized at 900–1000 1C by a solid-state
reaction between oxides in an equimolecular ratio. With both oxyborates melting led to decomposition into a mixture of Y(Er)BO
(Er )O and Na . Just the opposite was observed during thermal treatment of the oxide mixture containing Nd , from which a
practically pure phase of Na Nd O(BO ) was only obtained after melting. The attempts to synthesize the oxyborate Na La O(BO )
3 2
3
,
Y
2
2
3
2
4
B O
7
2 3
O
2
2
3 2
2
2
showed it to be unstable, this leading to the formation of a mixture containing, in addition to Na
stable phases of the system Na O–La –B along with an unknown ternary oxide phase. This phase was found to represent a new
oxyborate of sodium and lanthanum with the formula Na La (BO , whose single crystals were obtained by flux growth. It was
established that synthesis of a polycrystalline material with the same composition was also possible using solid-state interaction between
2 2 3 2
La O(BO ) , also other already known
2
2
O
3
2 3
O
3
9
O
3
3 8
)
Na
Na
2
CO
La
3
, La
2
O
3
and H
3 3
BO at 1000–1100 1C. X-ray diffraction experiments on single crystals were used to solve the structure of
3
9
O
3
(BO )
3 8
. The unit cell was found to be hexagonal, space group P62m (No. 189) with Z ¼ 1. The compound can be regarded as
the forefather of a second group of oxyborates representing a new family of isostructural compounds, Na RE O (BO ) . Such phases
3 8
3
9
3
were obtained with RE ¼ Nd, Sm and Eu whereas with RE ¼ Y and Gd, the synthesis experiments failed.
The concentration and temperature regions of crystallization of the double-oxyborate Na Al O(BO
Na O–Al O –B O were determined. This compound was shown to melt incongruently at 97073 1C, which made high-temperature
2
2
3 2
) in the ternary system
2
2
3
2
3
2
solution growth most appropriate for obtaining its single crystals with NaBO as the best solvent. On the basis of the data obtained, a
composition of the initial solution was proposed, the validity of the choice being demonstrated by the growth of Na
crystals on a seed using the top-seeded solution growth (TSSG) technique and slow cooling of the solution.
r 2006 Elsevier Inc. All rights reserved.
2 2 3 2
Al O(BO ) single
Keywords: Oxyborates; Crystal structure; Solid-state synthesis; Flux growth; Rare earths; Aluminum
1
. Introduction
Fang [5] published the results of the first investigations on
borate crystal structures. Since then a great diversity of new
structures, which may perhaps exceed even that of the
silicate family, have been found. At present over 800 borate
crystal structures have been determined [6,7].
The foundations of borate crystal chemistry were
established some 70 years ago when Zachariasen and Ziegler
[
1], Zachariasen [2,3], Goldschmidt and Hauptmann [4] and
The diversity and versatility of borate structures due to
the flexibility of boron to adopt either trigonal or
tetrahedral oxygen coordination are probably the cause
Ã
Corresponding author. Fax: +359 2 8705024.
E-mail address: ppeshev@svr.igic.bas.bg (P. Peshev).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.01.027

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2835
of the great variety of properties in borates. Thus, in a
series of these compounds, the structural characteristics of
the boron–oxygen groups determine their enhanced UV
transparency which, combined with a high polarizability
and resistance against radiation-induced damage, implies
that many borates are attractive candidates for laser hosts
and nonlinear optical (NLO) materials with harmonic
generation applications [8–12]. Borates are also interesting
as promising lithium-ion battery electrode materials
oxyborates, similarly to the sodium gadolinium oxyborate
[30], should be the matrix while the neodymium- and
erbium-containing oxyborates should be activators.
For the sake of comparison, experiments on the
synthesis of the compound Na Gd O(BO ) prepared for
the first time by Corbel et al. [30] were performed under the
conditions of the investigations on Y-, La-, Nd- and Er-
containing samples.
2
2
3 2
[
13–18] and ion conductors [19–21]. Some of them have
2.1. Experimental
specific magnetic [22,23] and superconducting [24] proper-
ties. Recently, borates with a tunnel structure were
proposed as basic substances in the development of non-
siliceous frameworks for separations and storage as well as
in heterogeneous catalysis [25].
The syntheses of the samples were carried out in
platinum crucibles using Na CO , H BO and RE O
3
2
3
3
3
2
(RE ¼ Y, La, Nd, Gd, Er) in a stoichiometric ratio. The
rare-earth oxides were subjected to preliminary heating at
1000 1C for 4 h in order to decompose hydroxides and
carbonates that may be present on the particle surfaces.
Homogenized mixtures of the reagents were subjected to
threefold thermal treatment at 900 1C for 24 h and a single
heating at the same temperature for 48 h, with intermediate
grinding between the separate heatings. Further on, three
24 h treatments with intermediate grinding were performed
at 900, 1000 and 1050 1C, respectively. After each of the
seven stages of solid-state synthesis, the product was
subjected to X-ray phase analysis. With each series the
sample obtained after the last stage was tested in order to
establish the initial and total melting temperatures. Each
test was followed by X-ray phase analysis of the
corresponding material.
The heat treatments proceeded in a resistance furnace,
the desired temperature being maintained with an accuracy
of 75 1C. The X-ray powder diffraction patterns needed
for phase identification of the samples and for structural
analysis were obtained using a Philips X’Pert MPD
diffractometer with a Bragg-Brentano geometry (CuKa
radiation). Structure refinement was performed by the
Rietveld method, the diffraction patterns being analyzed
by a whole pattern fitting procedure using the FULLPROF
programme [34].
The present paper reviews some results obtained recently
by the authors in the framework of a program of
investigations on Na O–Me O –B O (Me ¼ RE or Al)
2
2
3
2
3
oxide systems. They concern the following ternary oxybo-
rate phases which appear in these systems: (i) sodium rare-
earth oxyborates, Na RE O(BO ) ; (ii) sodium rare-earth
2
2
3 2
oxyborates, Na RE O (BO ) and (iii) sodium aluminum
3
9
3
3 8
oxyborate, Na Al O(BO ) [26–29].
3 2
2
2
2
. Studies on oxyborates of the Na RE O(BO ) family
2 2 3 2
In 1999, Corbel et al. [30] evidenced the existence of a
family of sodium rare-earth oxyborates, Na RE O(BO ) ,
2
2
3 2
with RE ¼ Sm, Eu and Gd. The authors found all the three
compounds obtained by solid-state interaction in equimo-
lecular oxide mixtures to be isostructural. Single-crystal
X-ray diffraction of a Na Gd O(BO ) sample showed it to
2
2
3 2
have a monoclinic structure (S.G. P2 /c, Z ¼ 4), the three-
1
dimensional network consisting of infinite sheets of REO
polyhedra in the (b, c) plane, which are separated by
sodium ions.
8
The three oxyborates mentioned above contain rare-
earth metals (Sm, Eu and Gd) which occupy neighbouring
places in the series of lanthanoids in the Periodic Table and
3
+
have very close values of the RE ionic radii. In our study
we tested the possibility of synthesizing Na RE O(BO )
2
2
3 2
2.2. Results
oxyborates with rare earths having larger and smaller ionic
radii than those of samarium, europium and gadolinium.
As competitive reactions, yielding Na BO , rare-earth
Following the weight loss of the samples, one can see in
all cases that Na CO and H BO present in the initial
3
3
2
3
3
3
borates, REBO , and quaternary phases with the composi-
3
reagent mixtures have already completely decomposed to
oxides after the first 24 h heating at 900 1C. Simultaneously,
differences in the appearance of the samples containing
different rare earths became visible with advancing heat
treatment. Thus, after a 120 h heating at 900 1C, the highest
degree of sintering is observed with the Er-containing
samples. The sintering degree changes in the sequence
tions Na RE(BO ) , Na RE (BO ) and Na RE(BO )
3 7
3
3 2
3
2
3 3
18
[31–33] could also take place, it was of interest to find the
conditions of formation of Na RE O(BO ) samples with a
maximum phase purity.
2
2
3 2
The preparation of oxyborates with yttrium, lanthanum,
neodymium or erbium, respectively, as a rare-earth metal
was studied. The four metals were chosen for two reasons:
Er ¼ Gd4Y ¼ Nd4La,
the La-containing sample showing only a negligible
sintering.
Thermal treatment at 950 1C results in a measurable
increase in grain size with samples containing Y, Gd and Er
(
and Er, having radii RE
i) on the basis of their ionic radius size, two of them, Y
3
+
much smaller than the other
3
+
Sm
3+
two, La and Nd, larger than RE , REEu or REGd; (ii) on
3+
the basis of their possible application, e.g. as optical crystal
hosts. Then the sodium yttrium and sodium lanthanum

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2836
only. After heating at 1000 1C all samples exhibit an
increase in sintering degree. The appearance of some of the
samples significantly changes after treatment at 1050 1C.
The surface of those containing Y and Er is covered with
small needle-like crystals, whereas the particle size of the
La-containing specimen displays a visible increase.
not change in amount which, as estimated on the basis of
measurements, is of the order of 4%. The data for 1050 1C
indicate the presence of a second admixture of rare-earth
oxide, which evidences the beginning of a decomposition
process. Complete decomposition occurs after melting of
the samples. The products obtained contain no phase of the
type Na Gd O(BO ) . They consist of REBO , RE O and
The initial and total melting temperatures of the samples
subjected to all the seven stages of thermal treatment at
2
2
3 2
3
2
3
Na B O .
2
4
7
9
00–1050 1C are given in Table 1. The presence of an
Quite a different behaviour is observed with the
neodymium-containing sample. After all stages of solid-
state interaction at 900–1050 1C the products always
contain two main phases, Na Nd O(BO ) and Na Nd
2
interval of temperatures within which the samples melt
shows that even if the solid-state interaction leads to the
formation of pure Na RE O(BO ) oxyborates, they melt
2
2
3 2
2
2
3 2
3
incongruently. As is evident from the data, the narrowest
20 1C) range of melting corresponds to the sample
containing Nd. Taking into account the accuracy of
determination, it can be assumed that this sample melts
practically without decomposition.
(BO ) , and an impurity of NdBO . However, after
3 3 3
(
melting, X-ray phase analysis indicates the presence of
Na Nd O(BO ) and traces of Na Nd (BO ) only, while
2
2
3 2
3
2
3 3
NdBO3 lines are absent. This result is in complete
agreement with the data in Table 1, according to which
the Nd-containing sample melts in fact congruently.
On the basis of the data in Table 2 it may be concluded
that of the phases isostructural with Na Gd O(BO ) being
Table 2 presents the results obtained by determination of
the phase composition of the products after different stages
of heating. It is obvious that in all experiments the products
of solid-state interaction contain phases which are iso-
structural with the oxyborate Na Gd O(BO ) reported by
2
2
3 2
the subject of this study, only Na La O(BO ) cannot
2
2
3 2
be obtained as a relatively pure product either by solid-
state synthesis or by melting. Thermal treatment at 900 1C
leads to a mixture containing Na La (BO ) along with
2
2
3 2
Corbel et al. [30]. When synthesized at 900 1C, this
compound contains no impurities. At higher temperatures,
3
2
3 3
traces of GdBO appear in addition to the main phase. The
3
Na La O(BO ) , while already at 950 1C traces of an
2
2
3 2
melting of Na Gd O(BO ) is accompanied by its decom-
2
unknown phase appear. This phase becomes one of the
main components in the sample after melting.
2
3 2
position to a mixture of Gd O and Na B O . Samples
7
2
3
2
4
containing yttrium and erbium have shown the same
behaviour. In the temperature range between 900 and
The successful synthesis of practically phase pure sodium
rare-earth oxyborates of Y and Er, which are isostructural
with the corresponding compounds of Sm, Eu and Gd [30],
permitted a structural analysis of the newly prepared
substances with the use of X-ray powder diffraction data
and the Rietveld method. Unfortunately, the quality of our
sample of the Nd-containing compound did not allow us to
make a good structural refinement, so only refined unit cell
parameters for Na Nd O(BO ) are presented in this work.
1
containing traces of YBO or ErBO impurities, respec-
000 1C pure Na Y O(BO ) and Na Er O(BO ) phases
2
2
3 2
2
2
3 2
3
3
tively, can be obtained. Within the above interval of
preparation temperatures these impurities practically do
Table 1
2
2
3 2
Initial
Na O:RE
metals
and
total
melting
points
of
samples
with
a
The conditions of both data collection and structural
refinement as well as some basic crystallographic data on
Na Y O(BO ) and Na Er O(BO ) are summarized in
2
2
O
3
2 3
:B O
¼ 1:1:1 molar ratio containing different rare-earth
2
2
3 2
2
2
3 2
Rare-earth metal
Y
La
Nd
Gd
Er
Table 3. By way of example, Fig. 1 shows comparison
between experimental and calculated patterns for the
yttrium compound. The presence of traces of Y(Er)BO3
as impurity phases in the samples (ticks indexed as 2) has
Initial melting point (1C)
Total melting point (1C)
1260
41350
1090
1150
1170
1190
1230
1290
1170
1260
Table 2
Phase composition of the samples with a Na
2 2 3 2 3
O:RE O :B O molar ratio after different heat treatments
Rare-earth metal
Temperature and duration of the heat treatment
After melting
900 1C 48 h
900 1C 120 h
950 1C 24 h
1000 1C 24 h
1050 1C 24 h
Y
1A,3B
1A,2A
1A,2A,3B
1A
1A,3B
1A,2A
1A,2A,3B
1A
1A,3B
1A,3B
1A,3B,4B
1A,2A,3B,6B
1A,2A,3B
1A,3B
3A,4A,5A
1A,2A,6A
1A,2B
La
Nd
Gd
Er
1A,2A,6B
1A,2A,3B
1A,3B
1A,2A,3B,6B
1A,2A,3B
1A,3B
1A,4A,5B
3A,4A,5A
1A,3B
1A,3B
1A,3B
1A,3B
1A,3B,4B
1
, Na
2
Gd
2
O(BO
3
)
A, main phase; B, traces.
2
-type phase [30]; 2, Na
3
RE
2
(BO
3
)
3
phase; 3, REBO
3
phase; 4, RE O
2 3
phase; 5, Na
2
B
4
7
O ; 6, unknown phase; RE ¼ Y, La, Nd, Gd, Er

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2837
Table 3
Crystallographic data and conditions of powder data collection for
Table 4
Atomic coordinates and temperature factors in Na
2
RE
2
O(BO
3
)
2
(RE ¼
a
Y , Er )
b
Na
2
RE
2
O(BO
3
)
2
(RE ¼ Y, Er)
2
˚
Y
Er
Atom
Site
x
y
z
B
iso (A )
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
Symmetry
Space group
Z
Monoclinic
/c
Monoclinic
RE(1)
4e
0.1835(3)
0.1825(3)
0.9842(3)
0.9836(3)
0.526(1)
0.526(2)
0.609(1)
0.608(2)
0.972(2)
0.974(3)
0.320(2)
0.320(2)
0.170(1)
0.171(2)
0.149(1)
0.146(3)
0.399(2)
0.397(3)
0.853(2)
0.854(3)
0.368(2)
0.367(3)
0.318(1)
0.0815(5)
0.0822(4)
0.4591(5)
0.4609(6)
0.0877(3)
0.0874(4)
0.1821(3)
0.1821(3)
0.264(1)
0.264(2)
0.046(1)
0.042(2)
0.099(2)
0.100(2)
0.137(2)
0.134(3)
0.365(2)
0.365(3)
0.087(2)
0.087(3)
0.057(1)
0.057(2)
0.275(2)
0.276(3)
0.335(2)
0.335(3)
0.91(7)
0.71(1)
0.81(7)
0.71(1)
1.6(3)
1.0(4)
1.5(3)
1.6(6)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
0.9(2)
0.8(3)
1.2(5)
2.1(9)
1.2(5)
2.1(9)
P2
1
1
P2 /c
4
293(2)
4
293(2)
RE(2)
Na(1)
Na(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
B(1)
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
Temperature (K)
Cell parameters
ꢀ0.059(2)
ꢀ0.054(3)
0.256(2)
0.256(4)
0.124(3)
0.138(4)
ꢀ0.248(3)
ꢀ0.248(4)
0.258(3)
0.260(4)
0.458(2)
0.455(3)
0.159(3)
0.156(4)
0.255(2)
0.252(4)
0.149(2)
0.146(4)
0.257(5)
0.257(1)
ꢀ0.344(3)
ꢀ0.343(6)
˚
a (A)
10.5980(3)
6.2305(2)
10.2244(3)
117.76(2)
597.40(6)
3.97
10.5757(2)
6.2101(1)
10.1976(2)
117.73(1)
592.80(4)
5.76
˚
b (A)
˚
c (A)
b (deg.)
Volume (A)
˚
3
Calculated density (g/cm )
Date collection range
Step
81o2yo1201
81o2yo1201
0.021
30
0.021
30
Counting time (s/deg.)
˚
Wavelengths, CuKa1,2 (A)
1.54056, 1.54439
881
65
1.54056, 1.54439
874
65
Number of reflections
Number of refined parameters
Reliability factors
R
R
R
p
9.51
12.4
2.78
1.98
10.5
13.6
4.03
3.29
wp
B
2
w
ꢀ0.077(2)
ꢀ0.077(4)
0.147(3)
0
.318(2)
0.208(3)
.206(4)
1
1
1
4000
B(2)
0
0.144(12)
2000
0000
Na Y O(BO )
3 2
2
2
8
6
4
2
000
000
000
000
in the Y and Er compounds are summarized in Table 5.
According to this description the O1 atom occupies a
particular position since it is not bonded to any of the
above polyhedra. Moreover, it is associated with the
shortest RE–O distances in these compounds. So it is
worth examining the structure from the point of view of
oxocentered polyhedra. Fig. 2 shows that O1 participates
0
1
2
-
-
2000
4000
in ORE tetrahedra. These tetrahedra share a common
4
10
20
30
40
50
60
70
2θ (deg)
edge so as to form O RE finite units which are corner
2
6
linked and result in a two-dimensional layer of oxygen-
centered rare-earth tetrahedra in the bc-plane. The average
˚
O1–Y and O1–Er distances are 2.276(8) and 2.27(1) A,
Fig. 1. Experimental (+) and calculated (solid line) X-ray diffraction
patterns of Na O(BO and the difference between them 1, main phase,
Na O(BO ; 2, impurity phase, YBO
2
Y
2
3 2
)
2
Y
2
3
)
2
3
.
which is very close to the predicted values 2.276 and
2
ꢀ
˚
.245 A, respectively, for four coordinated O ions with
2
3
+
3+
been taken into account. Owing to the large number of
variables during the refinement, the temperature para-
meters of the crystallographically distinct boron and
oxygen atoms were considered as being identical. The
B–O distances were also constrained to the average value of
Y
or Er
[35]. Bond-valence calculations were
performed. They revealed no significant deviations from
the theoretical values; the results are available on demand
from the authors.
During the preparation of this paper, two studies
reported relevant results. Thus, Heymann et al. [36]
published data on solid-state synthesis at 900–950 1C of
two new oxyborates, Na RE O(BO ) (RE ¼ Dy, Ho)
3
ꢀ
˚
.38 A, determined on a single crystal for the BO ions in
3
1
Na Gd O(BO ) [30]. The refined atomic coordinates and
2
2
3 2
temperature factors for the two oxyborates are given in
Table 4.
The structure of the Na RE O(BO ) oxyborates was
2
2
3 2
isotypic to the phases which are the subject of the present
work and of the study of Corbel et al. [30]. The second
investigation [37] confirmed our results reported previously
in Ref. [26] concerning the structure parameters of
Na Y O(BO ) . The authors established that doping of
2
2
3 2
already described in terms of a three-dimensional network
of cation-centered REO , NaO and BO polyhedra [30].
The interatomic distances within some of these polyhedra
8
6
3
2
2
3 2

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2838
Table 5
Selected distances (A) and angles (deg.) in Na
˚
2
RE
2
O(BO
3
)
2
(RE ¼ Y, Er)
2 2 3 2
Na Y O(BO )
Y(1)–O(1)
Y(1)–O(1)
Y(1)–O(4)
Y(1)–O(7)
Y(1)–O(2)
Y(1)–O(3)
Y(1)–O(5)
Y(1)–O(6)
2.24(1)
2.31(2)
2.37(1)
2.40(1)
2.42(2)
2.43(2)
2.49(2)
2.60(2)
Y(2)–O(1)
Y(2)–O(1)
Y(2)–O(3)
Y(2)–O(4)
Y(2)–O(3)
Y(2)–O(6)
Y(2)–O(6)
Y(2)–O(4)
2.24(2)
2.31(2)
2.35(1)
2.36(2)
2.38(2)
2.39(2)
2.42(2)
2.49(2)
B(1)–O(7)
B(1)–O(5)
B(1)–O(3)
1.37(3)
1.37(2)
1.39(2)
O(7)–B(1)–O(5)
O(5)–B(1)–O(3)
O(3)–B(1)–O(7)
126(2)
116.7(2)
117(2)
/B–OS ¼ 1.38
B(2)–O(2)
B(2)–O(6)
B(2)–O(4)
1.38(4)
1.38(4)
1.39(2)
O(2)–B(2)–O(4)
O(4)–B(2)–O(6)
O(6)–B(2)–O(2)
125(2)
115(2)
120(3)
/
Y(1)–OS ¼ 2.41
O(BO
/Y(2)–OS ¼ 2.37
Na
2
Er
2
3 2
)
Er(1)–O(1)
Er(1)–O(1)
Er(1)–O(4)
Er(1)–O(7)
Er(1)–O(3)
Er(1)–O(2)
Er(1)–O(5)
Er(1)–O(6)
2.28(4)
2.30(2)
2.35(2)
2.40(2)
2.42(3)
2.43(2)
2.47(3)
2.61(3)
Er(2)–O(1)
2.16(3)
2.33(3)
2.33(3)
2.35(2)
2.37(4)
2.38(3)
2.39(3)
2.49(3)
B(1)–O(7)
B(1)–O(5)
B(1)–O(3)
1.37(6)
1.37(4)
1.39(3)
O(7)–B(1)–O(5)
O(5)–B(1)–O(3)
O(3)–B(1)–O(7)
128(4)
115(3)
117(4)
Er(2)–O(1)
Er(2)–O(4)
Er(2)–O(3)
Er(2)–O(3)
Er(2)–O(6)
Er(2)–O(6)
Er(2)–O(4)
/B–OS ¼ 1.38
B(2)–O(2)
B(2)–O(6)
B(2)–O(4)
1.38(7)
1.40(5)
1.37(9)
O(2)–B(2)–O(4)
O(4)–B(2)–O(6)
O(6)–B(2)–O(2)
126(5)
112(6)
121(7)
/Er(1)–OS ¼ 2.41
/Er(1)–OS ¼ 2.35
between oxides at different temperatures failed. After
melting the product consisted of Na La O(BO ) -type
borate, Na La (BO ) and an unknown ternary oxide
2
2
3 2
3
2
3 3
phase. Determination of the chemical composition showed
this phase to correspond to a ternary borate with the
formula Na La O (BO ) . This oxyborate was prepared
3
9
3
3 8
without admixtures of other phases by solid-state interac-
tion at 1000 and 1100 1C with several intermediate
grindings of a pelletized initial mixture of Na CO ,
2
3
La O and H BO in a stoichiometric molar ratio.
3
2
3
3
3.1. Crystal growth of Na La O (BO ) and determination
of its structure
3 9 3 3 8
Single crystals of Na La O (BO ) were grown by
3
9
3
3 8
spontaneous crystallization on platinum rods immersed in
melts of the constituent binary oxides during their cooling
with a rate of 1 1C/h. The experiments were carried out in a
resistance furnace with SiC heating elements. The melts
were prepared in Pt crucibles, starting compositions with a
constant B O content (33.3 mol%) being used. In the
Fig. 2. View showing the formation in Na
dimensional layer of oxygen-centered RE tetrahedra in the bc-plane.
2 2 3 2
RE O(BO ) of a two-
3+
this compound with Eu ions led to a strong red emission
2
3
5
of the D – F transition at 611 nm.
7
equilibrium diagram they were situated on the line
corresponding to a Na O content between 33.3 and
0
2
The cell parameters of Y-, Nd- and Er-containing
oxyborates determined in this study are listed in Table 6.
For the sake of comparison the parameters of the
compounds with Y, Sm, Eu, Gd, Dy and Ho given in
Refs. [30,36,37] are also presented. It is evident that the
change in lattice parameters is in agreement with evolution
2
40.0 mol% and a La O content varying in the concentra-
2
3
tion range 33.3–26.7 mol%. The crystallization tempera-
ture changed from 1150 to 1100 1C with increasing Na O
2
content. It is evident that Na La O (BO ) crystallizes in a
3
9
3
3 8
region of the Na O–La O –B O ternary diagram richer in
2
2
3
2
3
3
+
of the ionic radii Ri of the rare earths [38].
Na O than the phase itself, the neighbouring regions being
2
the Na La (BO ) region (with a higher B O content) and
3
2
3 3
2
3
3
. Studies on oxyborates of the Na RE O (BO ) family
3
the Na La O(BO ) region (with a higher Na O content).
2 2 3 2 2
9
3
3 8
The single-crystal X-ray diffraction experiments were
carried out on a Nonius Kappa CCD diffractometer using
MoKa radiation. All crystal data and experimental
As was pointed out above, attempts to prepare
Na La O(BO ) as a pure phase by solid-state interaction
2
2
3 2

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2839
Table 6
Cell parameters of sodium rare earth oxyborates, Na
2
RE
2
3
O(BO )
2
3
V (A )
3+
i
˚
a (A)
˚
b (A)
˚
c (A)
˚
˚
(A) [38]
Oxyborate
b (1C)
R
Source
Na
Na
Na
Na
Na
Na
Na
Na
Na
2
2
2
2
2
2
2
2
2
Y
Y
2
O(BO
O(BO
3
)
)
2
10.5980(3)
10.5993(1)
10.8204(7)
10.754(2)
10.721(7)
10.695(6)
10.639(1)
10.612(5)
10.5757(2)
6.2305(2)
6.2311(1)
6.4297(6)
6.369(2)
6.343(6)
6.320(4)
6.262(1)
6.237(2)
6.2101(1)
10.2244(2)
10.2247(1)
10.4617(9)
10.381(2)
10.347(7)
10.328(6)
10.253(1)
10.225(3)
10.1976(2)
117.76(2)
117.756(1)
117.96(3)
117.85(2)
117.84(5)
117.80(4)
117.76(1)
117.7(1)
597.40(6)
597.59(3)
642.87(9)
628.6(2)
622.1(6)
617.5(9)
604.95(3)
599.20(3)
592.80(4)
1.019
1.019
1.019
1.079
1.066
1.053
1.027
1.015
1.004
This work
[37]
2
3
2
Nd
Sm
2
O(BO
O(BO
3
)
)
2
2
This work
[30]
2
3
Eu
2
O(BO
O(BO
O(BO
O(BO
O(BO
3
)
2
[30]
[30]
Gd
2
3
)
2
Dy
Ho
2
3
)
2
[36]
[36]
2
3
)
2
Er
2
3
)
2
117.73(1)
This work
Table 7
Table 8
Crystal data and structure refinement for Na
3
La
9
3
O (BO
3
)
8
Atomic coordinates and equivalent isotropic displacement parameters
2
A ) for Na
˚
(
3
La
9
O
3
(BO
3
)
8
U(eq) is defined as one third of the trace of the
Crystal data
ij
orthogonalized U tensor
Empirical formula
Formula weight
Crystal system
Na
1837.64 g
Hexagonal
3 9 3 3 8
La O (BO )
Atom
Site
x
y
z
U(eq)
La1
La2
Na
B1
3g
6i
0.28323(4)
0.34806(2)
0.3366(2)
0
0.28323(4)
0
1/2
0.23333(2)
0.0051(1)
0.0056(1)
0.0145(7)
0.0075(14)
0.0063(15)
0.0072(10)
0.0102(5)
0.0092(5)
0.0098(4)
0.0075(7)
Space group
Temperature (K)
Unit cell dimensions (from 1182
P-6 2m (No. 189)
293(2)
a ¼ b ¼ 8.9033(3)
c ¼ 8.7131(3)
598.14(4)
3f
0.3366(2)
0
1/3
0
˚
2e
2c
4h
6i
0.2084(8)
0
reflections with y ¼ 1.0–30.01) (A)
3
Volume (A )
˚
B2
B3
2/3
2/3
1/3
0.3234(6)
0.2120(3)
0
Z
1
5.06(2)
D
m
(by hydrostatic pressure meas. in
)
O1
O2
O3
O4
0.1558(3)
0.5047(4)
0.5129(3)
0.2572(4)
0.1558(3)
0.1824(4)
0.3334(2)
0
ꢀ3
C
6
H
5
Br) (Mg m
ꢀ
6j
3
12l
3g
0.3171(2)
1/2
D
x
(Mg m
)
5.102
802
F(000)
Crystal shape
Crystal colour
Crystal size (mm )
Wedge-shaped plate
Transparent colourless
3
0.220 ꢁ 0.060 ꢁ 0.040
Data collection
Diffractometer
Collect
agreement with the hexagonal 6/mmm Laue class
(Rint ¼ 0.040).
Nonius Kappa CCD
f/o scan
As no particular reflection conditions were observed in
the data set, five possible space groups had to be
considered: P6/mmm, P-6m2, P-62m, P6mm and P622.
After several trials the structure was solved by Patterson
function deconvolution and the heavy atom method in the
P-62m non-centric space group. At first, two sites for the
lanthanum and one site for the sodium atoms were located
˚
Radiation
MoKa (l ¼ 0.71073 A)
ꢀ1
Absorption coefficient (mm
y range (deg.)
Index ranges
)
15.9
3.5–30.0
ꢀ12php12
ꢀ
12pkp12
12plp12
ꢀ
Reflections collected [I42s(I)]
Independent reflections [I42s(I)]
(Friedel not merged)
Refinement
Absorption correction
6709
684 (Rint ¼ 0.040)
(SHELXS-86 program [39]). Then the boron and oxygen
positions in the structure were obtained by refinement and
successive Fourier difference functions (SHELXL-93 pro-
gram [40]). The final results on the atomic coordinates,
equivalent isotropic displacements and anisotropic displa-
cements parameters are indicated in Tables 8 and 9.
Selected interatomic distances and angles are listed in
Table 10.
Empirical, Scalepack
T
min ¼ 0.150; Tmax ¼ 0.540
2
Refinement method
Data/parameters refined
Weighting scheme
Full-matrix least-squares on F
684/49
o ¼ 1/
2
2
2
[
o
s (F )+(0.0165P) +0.07P]
2
2
where P ¼ (F
0.054(1)
1.24
o c
+2F )/3
Extinction coefficient
Goodness-of-fit on F
2
3.2. Structure description of Na La O (BO )
3 9 3 3 8
Final R indices [I42s(I)]
Absolute structure parameter
˚
Largest diff. peak and hole (eA
R
1
¼ 0.015, wR
ꢀ0.03(3)
1.25 (on La1 site) and ꢀ1.78
2
¼ 0.030
ꢀ3
)
In a general manner the crystallographic structure of
Na La O (BO ) can be described as a layer-type structure,
since there is alternate stacking along the c-axis of:
3
9
3
3 8
conditions in Na La O (BO ) structure determination are
3
summarized in Table 7. The test of equivalent data merging
in the different possible Laue classes indicates good
NaB2(O2)3 (z ¼ 0); B1(O1) (z ¼ 0.21); La2 (z ¼ 0.23);
9
3
3 8
3
B3(O3) (z ¼ 0.32); La1O4 on the mirror plane at z ¼ 1/2
3
(Fig. 3).

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2840
The description can be done on the basis of the near
‘compact stacking’’ of the Na, La cations and the usual A,
will be hexagonal compact (h.c.) (Fig. 4). Within this
description, the BO triangles are arranged in rows running
‘
3
B, C compact planes notation. So, if Na ions (z ¼ 0) form
an A-type plane, La2-ions (z ¼ 0.23) are a B-type plane and
La1-ions (z ¼ 1/2) are again in an A-type plane, then the
stacking of Na and La cations on the sequence ABAB y
along the c-axis and they could be projected in the C positions
of the compact packing. We can distinguish between two
types of BO rows: type 1, with ‘‘eclipsed’’ triangles (two BO
3
3
per cell in (0, 0) projection) and type 2, with a succession of
two ‘‘eclipsed’’ and one ‘‘rotated’’ triangle (six BO per cell in
3
(1/3, 2/3) and (2/3, 1/3) projections. A similar kind of
structure can be found for Hg (BO ) . One can compare
[Na La ](BO ) with Hg (BO ) , which also has an
3 9 3 8 12 3 8
Table 9
Anisotropic displacement parameters (A ) for Na
2
3
La
9
O
3
(BO
3
)
anisotropic displacement factor exponent takes the form:
8
.The
3
3 2
2
2
2
ꢀ
2p ðh an U11 þ . . . þ 2hkanbnU12 þ . . . Þ
ABAB yh.c. network of Hg atoms and rows of ‘‘rotated’’
BO triangles in C positions, parallel to the c-axis [41].
3
U
11
U
22
U
33
U
23
U
13
U
12
The O4 atoms, which are independent of the BO3
triangles and justify the oxyborate label of the compound
are on the mirror plane (z ¼ 1/2) in tetrahedral sites of a
B–A–B lanthanum ‘‘compact sandwich’’.
La1 0.0052(2) 0.0052(2) 0.0055(2)
La2 0.0058(1) 0.0057(1) 0.0052(1)
Na 0.011(1) 0.011(1) 0.021(2)
B1 0.009(2) 0.009(2) 0.005(3)
B2 0.008(2) 0.008(2) 0.002(3)
B3 0.009(2) 0.009(2) 0.004(2)
0
0
0
0
0
0
0
0.0032(2)
0.0006(1) 0.0028(1)
0
0
0
0
0.006(1)
0.004(1)
0.004(1)
0.004(1)
Another description of the framework can be done using
the above O4La4 tetrahedra. Three tetrahedra share a
common La corner and constitute a La O ring parallel to
O1 0.008(1) 0.008(1) 0.013(1) ꢀ0.001(1) ꢀ0.001(1) 0.003(1)
9
3
O2 0.007(1) 0.010(1) 0.008(1)
O3 0.009(1) 0.009(1) 0.013(1)
O4 0.009(1) 0.009(2) 0.004(2)
0
0
0.002(1)
0.001(1) 0.005(1)
0.005(1)
the (ab) plane (Fig. 5). The rings are separated by
Na–B2(O2) planes and BO rows of type 2. The BO
3
0.000(1)
0
3
3
0
rows of type 1 run inside the rings.
Table 10
Selected bond lengths (A) and angles (deg) in Na
˚
3
9 3 3 8
La O (BO )
I
La1
2 ꢁ
O4
O3
O1
2.414(2)
2.450(2)
2.754(2)
La2
2 ꢁ
2 ꢁ
O4
O2
O3
O3
O1
O1
2.460(2)
2.540(2)
2.568(2)
2.673(2)
2.695(2)
1.388(3)
II, III, IV
III
V
4
2
ꢁ
ꢁ
VI, VII
VIII
IX
2
2
ꢁ
ꢁ
X
I
I, IX
Na
2 ꢁ
O1
2.450(3)
2.486(3)
2.632(3)
97.85(14)
90.93(6)
123.74(6)
177.2(2)
56.27(14)
2.414(2)
2.460(2)
129.5(2)
98.06(2)
141.6(2)
B1
O1–B1–O1
3 ꢁ
3 ꢁ
3 ꢁ
I
2
2
ꢁ
ꢁ
O2 ꢁ
119.95(2)
XII, XIII
O2
X
VI, XII
O1–Na–O1
O1–Na–O2
O1–Na–O2
O2–Na–O2
O2–Na–O2
O4
B2
O2–B2–O2
O2
O3
1.395(3)
120
VI
XII
XI
VI, XII
B3
O3–B3–O3
1.371(3)
119.84(4)
XII
VI
IX
III
2 ꢁ
La1
La2
2
ꢁ
IX
III
La1–O4–La1
La1–O4–La2
La2–O4–La2
Symmetry codes: (I) ꢀy,xꢀy,z ; (II) y,x,ꢀz+1 ; (III) x,y,ꢀz+1 ; (IV) y,x,z ; (V) xꢀy,ꢀy,ꢀz ; (VI) ꢀy+1,xꢀy,z (VII) ꢀx+1,ꢀx+y,z (VIII) xꢀy,ꢀy,z ;
(IX) ꢀx+y,ꢀx,z ; (X) x,y,ꢀz ; (XI) y,x,ꢀz (XII) ꢀx+y+1,ꢀx+1,z ; (XIII) ꢀx+1,ꢀx+y+1,ꢀz.
c
Na
La1
La2
La2
O
La1
La2
Na
b
BO3
a
Na
3 9 3 3 8
Fig. 3. The layer-type structure of Na La O (BO ) .

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2841
O3
O2
type 1
type 2
O1
O1
O1
O1
O4
O4
O2
O2
O2
O4
O3
O1
O3
O3
O2
O3
O2
O3
O3
O1
O3
c
La1O
8
La2O9
NaO6
Fig. 6. Coordination polyhedra of the La1, La2 and Na cations.
3.2.2. Boron atom environment
All borate groups consist of a boron triangularly
coordinated to oxygen atoms. Three types of crystal-
3
ꢀ
lographically-independent isolated (BO3) groups parallel
to the (ab) plane are present in the unit cell similarly to the
case of other borate and oxyborate compounds [43]. In
Na La O (BO ) only small deviations from the ideal
b
a
3
9
3
3 8
borate group having a B–O distance and O–B–O angle
˚
values equal to 1.38 A and 1201, respectively, are observed
(Table 10). For the sake of comparison, it should be noted
that the same order of B–O distances has been reported for
other borates containing isolated BO3 triangles, e.g.
3
Fig. 4. View of the two types of BO rows.
˚ ˚
.360–1.399 A for LiGd O (BO ) [43] and 1.34–1.42 A
6 5 3 3
1
for La (BO ) O [42].
2
6
3 8 27
B2(O2)3
3.2.3. Sodium atom environment
The unit cell contains one type of Na atoms only.
Sodium is surrounded by six oxygen atoms (two O1 and
four O2), forming a highly distorted octahedron (Table 7,
Fig. 6).
B3(O3)3
B1(O1)3
c
3.2.4. Oxygen atom environment
Oxygen atoms can be classified into three groups
b
depending on their bonding with other atoms in the unit
cell. Thus, O1 and O2 atoms are connected to Na, B and
La atoms, O3 to B and La, while O4 are bonded to four La
atoms only. The appearance of a Ln–O bonding of the
latter type was reported previously for LiLn O (BO )
3 3
a
6
5
(
Ln ¼ Pr–Tm) [43], Li Ln O (BO ) (Ln ¼ Yb, Lu) [44]
2
5
4
3 3
and Na Ln O(BO ) (Ln ¼ Sm, Eu, Gd) [30]. Similarly to
2
2
3 2
these substances, Na La O (BO ) can be labelled as an
3
9
3
3 8
oxyborate of sodium and lanthanum. It is worth noting
that the La–O4 bond distances belong to the shortest ones
˚
Fig. 5. View of Na
3
La
9
O
3
(BO
3
)
8
along the c-axis. Three O4La tetrahedra
3
rings. Types 1 and 2 BO rows run
4
share common corners to form La
inside and between the rings.
9
O
3
(2.414–2.460 A). The O4 oxygen atoms are located in the
four coordinated La O tetrahedra mentioned above, which
4
form rings (Fig. 5), the bond distances and angle values
being presented in Table 10.
3
.2.1. Lanthanum atom environment
Lanthanum atoms occupy two different crystallographic
sites: La1 in 3g and La2 in 6i, eight and nine coordinated,
3.3. Na RE O (BO ) (RE ¼ Nd, Sm, Eu) oxyborates
3
9
3
3 8
isostructural with Na La O (BO )
3 8
3
9
3
respectively (Fig. 6). La–O bond distances range from
˚
Solid-phase interactions between Na CO , H BO and
2 3 3 3
2
.414(2) to 2.754(2) A for La1, and from 2.460(2) to
Nd O (Sm O or Eu O , respectively) in a stoichiometric
2 3 2 3 2 3
˚
.695(2) A for La2. These values are close to those reported
2
molar ratio under the regime used for the preparation of
polycrystalline Na La O (BO ) were successfully applied
to obtaining analogues of this compound containing
for the La–O distances in lanthanum borate, La26
BO ) O [42].
3
9
3
3 8
(
3
8
27

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P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2842
Table 11
Cell parameters of sodium rare-earth oxyborates, Na
of a compound with the composition Na Al B O
2 2 2 7
3
9 3 3 8
RE O (BO )
(
Na O:Al O :B O ¼ 1:1:1). The short communication
2
2
3
2
3
3
V (A )
3+(C.N.)
i
[38]
contains a poor X-ray powder diffraction pattern of the
compound which has not been indexed. According to the
authors, Na Al B O melts congruently at 1010710 1C. In
˚
a (A)
˚
c (A)
˚
Oxyborate
R
a
VIII
8.903(3) 8.713(3) 598.14(4) 1.160 ; 1.216
IX
IX
IX
IX
Na
Na
Na
Na
3
3
3
3
La
9
O
3
(BO
3
)
8
2
2
2
7
VIII
1.109 ; 1.163
Nd
Sm
9
O
3
3
(BO
(BO
3
)
8
8.767(1) 8.476(1) 564.2(1)
8.706(1) 8.359(1) 548.7(1)
8.682(1) 8.308(1) 542.3(1)
another paper [51] some of the same authors have
confirmed the results in Ref. [50] concerning Na Al B O
7
VIII
1.079 ; 1.132
9
O
3
)
8
VIII
1.066 ; 1.120
2
2
2
Eu
9
O
3
(BO
3
)
8
and, studying the section of Na O ꢂ 2B O –Al O , have
2
2
3
2
3
a
Single-crystal XRD data.
established the existence of a second ternary borate,
NaAlB O (Na O:Al O :B O ¼ 1:1:2), with a melting
2
5
2
2
3
2
3
point of 1025710 1C.
The structure of Na Al B O was determined for the first
neodymium, samarium and europium. However, the
attempts to synthesize Y- and Gd-containing phases in
the same manner, failed.
2 2 2 7
time in 2000 by Corbel and Leblanc [52]. They found the
compound, defined by them as oxyborate, Na Al O(BO ) ,
2
2
3 2
The X-ray powder diffraction experiments needed for
the identification of the new phases and their structural
analysis were carried out using the instruments and
procedures described in Section 2.1. The cell parameter
values obtained by whole pattern refinement of powder
XRD data are presented in Table 11, where, for the sake of
comparison, data from the single-crystalline structure
determination of Na La O (BO ) are also given.
to be isostructural with Na Ga O(BO ) and to crystallize
2 2 3 2
in the trigonal system (S.G. P31c; Z ¼ 2). More recently,
the structure of Na Al O(BO ) was refined by He et al.
2
2
3 2
[53,54], while Corbel et al. [21] established a sodium ionic
conduction in this oxyborate.
According to the data available in the literature, the
system Na O–Al O –B O has not yet been investigated
2
2
3
2
3
with respect to the possibilities and conditions of growth of
ternary borate single crystals. Prognostication of such
possibilities and conditions was one of the principal tasks
of our studies. The importance of any information of that
kind increases on the background of some results reported
during the preparation of this paper. They show that
substitution of up to 60% of Na for K in K Al O(BO )
3 2
3
9
3
3 8
3.4. Some perspectives of the application of
Na RE O (BO ) oxyborates
3
9
3
3 8
As was shown above, the new family of sodium rare-
earth oxyborates, Na RE O (BO ) , have a structure
3
9
3
3 8
2
2
belonging to the non-centrosymmetric group P-62m, which
permits assuming that they have NLO properties. In the
case of Na La O (BO ) , we observed a second-harmonic
(KABO) may result in a NLO material that seems to be
more promising than KABO [55]. On the other hand,
3
+
doping with 3d metal ions which partially replace the Al
ions in Na Al O(BO ) could result in the formation of
3
9
3
3 8
generation (SGH) effect, which was measured by the
powder technique [45]. This was confirmed by the results of
Zhang et al. [46] published during the preparation of the
present work. These authors found Na La O (BO ) to
2
2
3 2
single crystals appropriate for application as solid-state
laser hosts.
3
9
3
3 8
exhibit an optical SHG effect exceeding about 3–5 times
that of KH PO (KDP), which revealed a significant
4.1. Determination of phase crystallization regions in the
Na O–Al O –B O system
2
4
2
2
3
2
3
potential of the material for applications of laser frequency
conversion in the blue-green wavelength range.
The remaining isostructural Na RE O (BO ) oxybo-
The temperature and concentration regions of crystal-
lization of the different phases in the system Na O–Al O –
3
9
3
3 8
2
2
3
rates might prove not less promising as, say, luminescent
materials or self-frequency-doubling (SFD) laser hosts.
B O were determined on the basis of the results obtained
2 3
during slow cooling and spontaneous crystallization of
numerous homogeneous high-temperature solutions with
various compositions. In contrast with the investigations on
the products of solid-state interactions between the compo-
nents or the use of data from the thermal analysis (DTA and
TGA) of suitable samples, the method in question has the
advantage of facilitating a faster and more reliable reaching
of the thermodynamic equilibrium. In addition, along with
data on the crystallization regions, information is also
obtained on a series of properties of the solutions (viscosity,
creeping, evaporation, homogeneity degree, etc.), which are
very important for the processes of single-crystal growth
from these solutions.
4. Regions of phase crystallization in the ternary system
Na O–Al O –B O and conditions of single-crystal growth
2
2
3
2
3
of the oxyborate Na Al O(BO ) from high-temperature
3 2
2
2
solutions of this system
The interest in double borates of alkaline metals and
aluminum increased during the past years after it was
discovered that the oxyborate K Al O(BO ) (KABO)
2
2
3 2
crystals are a very promising NLO material for frequency
conversion of high-power solid-state ultraviolet (UV) lasers
[
47–49]. The data available in the literature concerning
double-sodium aluminum borates are rather scarce. In-
vestigating melts of the pseudobinary system NaBO2–
Al O , Samedov et al. [50] first established the formation
The experiments were carried out in vertical resistance
furnaces ensuring maintenance of temperature with an
accuracy of 71 1C (Eurotherm controller). The solution
2
3

ARTICLE IN PRESS
P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2843
containers were cylindrical platinum crucibles with a height
of 40 mm and a diameter of 40 mm. The solutions used had
a mass of 50–70 g, which corresponded to a layer of about
absent and the crystallization was not accompanied by
noticeable losses due to evaporation. The contents of the
three solution components varied between 20 and 55 mol%
for Na O, 0 and 35 mol% for Al O and 25 and 55 mol% for
2
from Na CO , Al O and H BO in preset molar ratios. In
0–25 mm thickness in the crucible. They were prepared
2
2 3
B O . Determination of the crystallization regions of the
2 3
2
3
2
3
3
3
order to avoid losses due to sprinkling, a mechanical
mixture of Na CO and H BO was at first melted and
phases in the solution Na O–Al O –B O within the above
2 2 3 2 3
concentration limits required investigation of about 50–60
different solutions distributed inhomogeneously, according to
the importance of the corresponding portion of the system.
The thermal behaviour of some characteristic samples
was studied by obtaining their DTA and TGA curves using
a Stanton Redcroft STA 760 instrument. Microprobe
analysis with a JEOL SUPERPROBE 733 apparatus was
used in experiments determining the melting characteristics
of Na Al O(BO ) .
2
3
3
3
decomposed in the crucibles, and only after the formation
of a clear solution, the necessary amount of Al O was
2
3
added in small portions. After that the solution was
homogenized with continuous stirring using a Pt stirrer at a
temperature exceeding by 50–100 1C the expected crystal-
lization temperature, Tcryst. The homogenization continued
until a thoroughly transparent solution was obtained,
which usually was achieved in 2–6 h. After that the solution
temperature was gradually decreased within 1 h down to
values exceeding Tcryst by 5–10 1C only.
2
2
3 2
The experimentally determined regions of phase crystal-
lization in the investigated part of the ternary system
Na O–Al O –B O are presented in Fig. 7. Regions of one
The high-temperature solutions of the system Na O–
Al O –B O have a high viscosity and a very pronounced
2
2
2
3
2
3
binary, two ternary and one quaternary oxide are visible as
follows:
2
3
2
3
trend to glass formation. For that reason, to achieve the
most precise determination of Tcryst we ensured: (i) an
appropriate temperature distribution in the solution with
clearly expressed (about 10 1C) axial and radial tempera-
ture differences, resulting in a position of the surface center
in the coldest zone; (ii) crystallization in a dynamic regime
on a platinum disc (diameter 10–15 mm) rotating on the
solution surface with a speed of 20–30 rpm and (iii)
introduction, with each temperature change, of a small
amount of fine powder from pre-synthesized Na Al O
(i) Al
in the solutions;
(ii) NaBO , at Al O concentrations ranging from 0 to
2 3
O crystallizing at high concentrations of this oxide
3
2
2
21 mol% and temperatures from 966 1C (melting point
of the compound) to about 800 1C;
(iii) NaAlO
content above 27.5 mol%;
(iv) Na Al O(BO at an Al O concentration between 21
2 3
, at low B O concentrations and an Al O
2
2 3 2 3
2
2
(
BO ) to the solution surface. This was made at every
2
2
2
3
)
2
3
2–3 h with a step of 1–2 1C. The purpose was to introduce
of a seed to initiate the crystallization process.
and 33 mol% and a temperature of 800–970 1C.
Special experiments showed that, as a result of neglecting
the above requirements, the larger part of the solutions did
not crystallize or the crystallization proceeded with a
significant supersaturation at temperatures lower than the
real Tcryst by 50–100 1C. The necessity of observing the
above requirements in order to determine Tcryst with
sufficient exactness is confirmed by the fact that when the
temperature at which crystallization begins is maintained
constant for 1–2 days, the mass of the crystals formed on
the rotating disc seldom exceeds 1 g.
The nature of the phases crystallizing in each of the four
regions is confirmed by the X-ray diffraction patterns of
ground crystals grown in these regions, which completely
correspond with the data available in the literature concern-
ing the compounds under consideration. No crystallization
In each composition of the high-temperature solution
the crystals obtained at Tcryst during slow cooling were
cleaned by water treatment and part of them were
characterized by X-ray phase determination. The composi-
tion of the solution was then altered in the desired direction
with the appropriate step by addition of Na CO , Al O , or
2
3
2
3
H BO or of two of these components simultaneously and
3 3
the procedure of Tcryst determination and phase character-
ization of the crystals was repeated. After 5–6 successive
changes of the composition, a new solution was prepared
from the initial reagents in order to minimize the errors
associated with possible losses due to evaporation, decrease
of the total mass that turned to crystals, etc.
Fig. 7. Crystallization regions of the phases: NaBO
B), NaAlO (C), and Al (D) in the system Na
Na Al O(BO stoichiometric composition.
2
(A), Na
2
Al
2
O(BO
3 2
)
The study was restricted to solution compositions where
the viscosity was not unusually high, the creeping trend was
(
2
2
O
3
2
O–Al
2
O
3
–B
2 3
O ; m,
2
2
3 2
)

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P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2844
region of the double-borate NaAlB O reported in the paper
2
Decomposition occurs at T ¼ 97073 1C. However, since
5
of Abdullaev et al. [51] has been established.
According to the literature data cited above, Na Al O
the transition needs formation of Al O nuclei, which is a
2
3
relatively slow process, the effect on the DTA curve is
somewhat retarded and appears at a temperature increas-
ing with the heating rate at which the curve has been
registered. The second endothermic peak expected as a
result of the peritectic decomposition, which would
correspond to the complete dissolution of Al O (the so-
2
2
(
BO ) has a congruent character of melting. Abdullaev et
3 2
al. [51] have mentioned 10101C as a melting point, whereas
He et al. [53] are of the opinion, on the basis of the DTA
curve of Na Al O(BO ) , that this temperature is 986 1C.
2
2
3 2
However, the crystallization region of the double oxyborate
determined in this study lies to the left of the point for
the stoichiometric molar composition of the compound
2
3
called liquidus point) would not be observed due to the
very high temperature of location of this point (probably
above 1200 1C) when also processes of intense evaporation
of Na O and B O begin.
(
Na O:Al O :B O ¼ 33.3:33.3:33.3) although the upper
2
2
3
2
3
boundary of the region (Na O:Al O :B O ¼ 34.3:31.5:34.2)
2
2
3
2
3
2
2
3
is rather close to this composition. In other words, the
results obtained indicate that Na Al O(BO ) should be
A characteristic feature of most of the borate single
crystals is their pronounced tendency to split in the plane
perpendicular to the c-axis and to form hexagonal
pinacoids with a small thickness. A habit of that kind
creates serious problems especially with respect to the
growth of single crystals with practical application. For
that reason, simultaneously with the determination of the
regions of phase crystallization in the system, we had to
pay attention to the habit of spontaneously growing
crystals of Na Al O(BO ) in different zones of its crystal-
2
2
3 2
incongruently melting, its composition in the peritectic point
being close to the stoichiometric.
The choice of a method for growing Na Al O(BO ) single
2
2
3 2
crystals requires explicit elucidation of the character of
melting of this compound. In this connection, additional
experiments were performed. At first DTA curves of
Na Al O(BO ) were obtained at three different heating
2
2
3 2
rates. They showed a strong temperature dependence of the
endothermic peak of melting on the heating rate. Thus, at a
heating rate of 5 1C/min the effect was observed at 993 1C,
while at 10 1C/min this happened at 996 1C and at 15 1C/min,
at 1001 1C. In fact, with such a small differences between the
heating rates, the shift of the endothermic effect should not be
substantial if the compound is congruently melting.
2
2
3 2
lization region. Observations on the crystals obtained from
compositions in the crystallization region which corre-
spond to points 1–8 of the diagram in Fig. 7 show that the
composition of the high-temperature solution strongly
affects the habit of the grown crystals. With rising B O
2
3
content in the solution, the crystals become increasingly
thinner and when this content exceeds 45 mol%, the habit
becomes mica-like. The drop of Tcryst also makes the
crystals thinner but in this case the trend is much less
pronounced.
A much more definite answer to the question about the
character of melting of the double oxyborate under
consideration was obtained as a result of a series of
experiments on thermal treatment at different temperatures
of isolated single crystals. Microphotographs showed that
after heating a Na Al O(BO ) crystal for 36 h at 968 1C it
4.2. Conditions of growth of Na Al O(BO ) single crystals
3 2
2
2
3 2
2
2
presented no change and remained transparent. No change
was also observed after 10 h when the heating temperature
was increased by 5 1C up to 973 1C. However, prolongation
of the heat treatment at this temperature to 36 h resulted in
the appearance of fine particles of another phase in the
crystal. The number of these particles increased with rising
temperature and prolongation of the heat treatment.
Heating for 36 h at 980 1C led to the formation of particles
with a hexagonal shape clearly visible in the microphoto-
graphs. The microprobe analysis and X-ray determinations
indicated them to be consisting of alumina.
As was shown above, Na Al O(BO ) melts incongru-
2
2
3 2
ently, which makes high-temperature solution growth most
appropriate for obtaining single crystals. In this connection
the choice of a solvent is of special importance. This choice
should be based on analysis of characteristics such as width
of the oxyborate concentration and temperature crystal-
lization regions in the potential solvents, change in
supersaturation degree of the solutions with decreasing
temperature, temperature and concentration dependences
of the solution viscosity and density, evaporation losses,
etc. [56].
When the temperature of heat treatment of the
Na Al O(BO ) crystals is increased to 1050 1C, the
product becomes a fully opaque glass due to dispersed
2
2
3 2
4.2.1. Solvents of Na Al O(BO ) from the system
2
2
3 2
Al O particles. In contrast to this glass, the compositions
2
Na O–B O
3
3
2
2
in the crystallization region of the compound (region B in
Fig. 7) form colourless transparent and homogeneous
glasses after quick cooling.
In order to avoid crystal contamination by foreign
admixtures, it is recommended to grow single crystals from
high-temperature solutions whose solvents consist of
components participating in the composition of the crystal
to be obtained. That is why solvents of the system
Na O–B O were chosen.
The explicit conclusion from the above experiments is
that Na Al O(BO ) is a compound which, when melted,
decomposes peritectically according to the reaction:
2
2
3 2
2
2
3
From the diagram in Fig. 7 it is evident that the
crystallization region of the Na Al O(BO ) oxyborate
Na2Al2OðBO3Þ ! 2NaBO2 þ Al2O3.
2
2
2
3 2

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2845
does not include the point (denoted by m) corresponding to
the stoichiometric composition of the compound. The
region is situated entirely to the left of this point, in a zone
of enhanced Na O and B O contents with respect to the
and 70:30 solvents are rather low. This, along with the high
concentrations of B O when 30:70 and 40:60 solvents are
2
3
used, would lead to crystal growth in solutions with too
high viscosities and all the inconveniences resulting from
this circumstance. With the solutions of the above three
solvents the lowest supersaturation values are observed, i.e.
the same solution amount would give the lowest yield. The
unfavourable prognosis with respect to the use of these
solutions was pointed out above in connection with the
habit of the crystals grown by spontaneous crystallization.
Of the remaining two solvents, the one with an equimo-
lecular ratio between Na O and B O , i.e. the ratio in the
2
2
3
above composition, i.e. Na Al O(BO ) single crystals
2
2
3 2
could be grown from solutions in which the ratio (in
mol%) between the Na O and B O components of the
2
2
3
solvent varies between 70:30 and 30:70. The crystallization
of Na Al O(BO ) with decreasing temperature is accom-
2
2
3 2
panied by a composition change of the solutions along the
a, b, c, d and e lines for 70:30, 60:40, 50:50, 40:60 and 30:70
solvents, respectively, as shown in Fig. 8. The experimen-
tally obtained data on the widths of the concentration and
temperature crystallization regions of Na Al O(BO ) and
2
2
3
Na Al O(BO ) oxyborate, was preferred for the further
2
2
3 2
investigations.
2
2
3 2
the mean supersaturation (expressed as grams of substance
precipitated during cooling of 100 g solution by 1 1C)
of solutions based on the above solvents are shown in
Table 12. It can be seen that both the concentration and
temperature regions where Na Al O(BO ) crystallizes from
solutions of all the five solvents are broad enough, i.e. on
this basis each of them would be suitable for the growth of
the oxyborate single crystals. However, it is obvious that
the saturation temperatures of the solutions in 30:70, 40:60
4.2.2. Solubility of Na Al O(BO ) in the solvent where
2
2
3 2
Na O:B O ¼ 50:50 (NaBO )
2
2
3
2
The solubility of Na Al O(BO ) in NaBO was inves-
2
2
3 2
2
2
2
3
tigated by determining the saturation temperature in
solutions with preset concentration and a mass of 50–70 g
each, cylindrical platinum crucibles with a height of 40 mm
and a diameter of 40 mm being used as containers. The
experiments were carried out in vertical resistance furnaces
ensuring maintenance of temperature with an accuracy of
71 1C (Eurotherm controller).
In order to determine the saturation temperature of a
given solution, in the bulk of this solution one had to
achieve temperature differences that would lead to crystal-
lization of Na Al O(BO ) in the centre of the solution
2
2
3 2
surface. There a platinum disc, 15–20 mm in diameter,
rotating continuously with a speed of 20 rpm was placed. In
the above dynamic regime the crystals appeared and grew
on the disc. Nucleation was initiated by introducing a very
small amount of fine powder of pre-synthesized Na2
Al O(BO ) to the solution surface. The particles of this
2
3 2
powder were dissolved when under the experimental
conditions the solution temperature exceeded the satura-
tion temperature. When the solution temperature was
lower than the saturation temperature, the powder
particles became centres of nucleation and growth of the
oxyborate crystals. Thus, using careful periodic tempera-
ture decrease, it was possible to reach temperature values at
which slow growth of a group of small crystals began. Then
the solution and the crystals on the rotating disc were
allowed to stand at this temperature for 1–2 days until
2 2 3 2 2
Fig. 8. Crystallization region of Na Al O(BO ) in the system Na O–
Al
Na
0:70.
2
O
3
–B
2
O
3
and composition change of its solutions in solvents with a
molar ratio of: (a) 70:30; (b) 60:40; (c)50:50; (d) 40:60; (e)
2
O:B
2
O
3
3
Table 12
Width of the crystallization regions of Na
Al
2 2
O(BO
3
)
2
and supersaturation of its solutions in different solvents of the system Na
2
O–B
2
O
3
Solvent
Na O:B
mol%)
Maximum Na
2
Al
2
O(BO
3
)
2
Maximum Na
2
Al O(BO
2
3
)
2
Na
2
Al O(BO
2
3
)
2
Na
2
Al
2
O(BO
3
)
2
Supersaturation
(g/1C per 100 g
solution)
2
O
2 3
concentration (mol%) vs.
saturation temperature (1C)
concentration (mol%) vs.
saturation temperature (1C)
concentration region
width (mol%)
temperature region
width (mol%)
(
3
4
5
6
7
0:70
0:60
0:50
0:40
0:30
91/980
93/985
94.5/990
96/985
97/985
40/760
48/810
54/850
60/835
61/800
51
45
220
175
140
150
185
0.36
0.45
0.66
0.62
0.44
40.5
36
35

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2846
temperature and at the saturation temperature of each of
the solutions. The device used for measuring the viscosity
was calibrated with pure glycerol at temperatures ranging
from 5 to 40 1C and a series of glycerol–water solutions at
25 1C. Precise data on the viscosity values of these
calibrating liquids can be found in the literature [59]. Error
estimation showed a possibility of viscosity determination
by the above method with an accuracy of about 72%.
The densities of the solutions whose viscosities were
measured, were established optically by determining the
level of a solution layer (i.e. its volume) with a preset mass.
The accuracy of measurement in this case was of the order
of 73%.
The results of viscosity measurements at the saturation
temperatures of solutions with different Al O concentra-
tions are given in Fig. 10. Evidently, the viscosity values
vary within quite large boundaries: from about 500 cP at
2
3
2 2 3 2 2 3 2 3
Fig. 9. NaBO –Al O section of the system Na O–Al O –B O .
8
50 1C for a solution containing 18 mol% Al O to 4000 cP
2
3
equilibrium was achieved. After that the above procedure
was repeated with solutions of higher Na Al O(BO )
3 2
when the solution contains 28 mol% alumina. The
differences are a result of the interaction between two
opposite factors. On the one hand, the increase in amount
of Al O , which is a typical glass-forming oxide, should
2
2
concentrations, which were reached by addition of the
corresponding amount of Al O . Within the crystallization
2
3
2
3
region the saturation temperature was determined for
solutions whose concentration was varied by a step of
contribute to the viscosity rise whereas the temperature
increase should have the opposite effect. Obviously, the
effect of the increasing alumina content is much stronger,
as is confirmed by the data in Figs. 11 and 12. Fig. 11
shows how the viscosity is affected by the temperature
change with solutions containing 18–28 mol% Al O (54
1
mol% Al O , corresponding to 3 mol% Na Al O(BO ) .
2 3 2 2 3 2
The results obtained during the investigation on the
solubility of Na Al O(BO ) in NaBO are illustrated by
the NaBO –Al O section of the Na O–Al O –B O
2
2
3 2
2
2
2
3
2
2
3
2
3
2
3
system (Fig. 9). It is obvious that the crystallization region
of the oxyborate lies to the right of the eutectic
NaBO –Na Al O(BO ) appearing at an Al O concentra-
and 84 mol% Na Al O(BO ) , respectively).The expected
2 2 3 2
viscosity drop with rising temperature is observed. How-
ever, the other figure (Fig. 12) demonstrates a dramatic
increase in viscosity with increasing alumina content in the
solution, the temperature remaning constant.
2
2
2
3 2
2
3
tion of 18.2 mol% (or 18.2 ꢁ 3 ¼ 54.6 mol% Na Al O
2
2
(
eutectic crystallization of NaBO proceeds, while Al O
BO ) ), and a temperature of 985 1C. To the left of the
3
2
The studies on the density of saturated solutions of
Na Al O(BO ) in NaBO and the density changes with
rising Al O (oxyborate, respectively) concentration in-
2
2
2
crystallizes to the right of the peritectic. The calculated
mean supersaturation of the solution is 0.66 g/1C per 100 g
solution. With rising temperature (Na Al O(BO ) con-
2
2
3 2
2
2
3
dicate such changes within relatively narrow limits. Thus,
at 850 1C and with an Al O content in the solution
2
2
3 2
centration, respectively), the supersaturation sharply in-
creases. Thus, at 880–890 1C the supersaturation is 0.45, at
940–950 1C it is 1.15, and at 970–980 1C it reaches 3.46 g/1C
per 100 g solution.
2
3
3
amounting to 18 mol%, the density is 2.28 g/cm , to
3
decrease to 1.94 g/cm for a solution containing 30 mol%
4
.2.3. Viscosity and density of the solutions
The viscosity of a series of solutions of the NaBO2–
Al O system in the crystallization regions of Na Al O
2
3
2
2
(
BO ) was determined by a method described in previous
3 2
papers [57,58]. The principle of the method consists in
measuring the resistance exercised by the solution on a
platinum disc rotating in it, which is determined by the
solution viscosity. The power applied to the motor rotating
the disc was used as quantitative measure of the resistance.
The measurements were performed in a platinum crucible,
4
0 mm in diameter, which contained a 15–20 mm layer of
the corresponding solution. The Pt disc immersed in the
high-temperature solution had a diameter of 20 mm and a
thickness of 1 mm. The rotating power of the disc was
measured at several temperatures above the saturation
2 2 3 2 2
Fig. 10. Viscosity of the saturated solutions of Na Al O(BO ) in NaBO :
(a) liquidus line of the section in Fig. 9; (b) viscosity.

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P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2847
2 2 3 2
Fig. 13. A Na Al O(BO ) single crystal grown by TSSG with slow
cooling.
Fig. 11. Viscosity vs. temperature in solutions with a constant concentra-
tion of Na Al O(BO :(a) 54 mol% (18 mol% Al ); (b) 60 mol%
20 mol% Al ); (c) 66 mol% (22 mol% Al ); (d) 72 mol% (24 mol%
Al ); (e) 78 mol% (26 mol% Al ); (f) 84 mol% (28 mol% Al ).
2
2
3
)
2
2 3
O
and the increase in B O concentration in the solvent above
2
3
(
2
O
3
2 3
O
Na O:B O ¼ 1:1 results not only in a supersaturation drop
2
2
3
2
O
3
2
O
3
2 3
O
but also in the formation of crystals with an inappropriate
habit.
The second conclusion concerns the preferred initial
concentration of Na Al O(BO ) in the solutions used for
2
2
3 2
growing its single crystals. The concentration should not
exceed 78 mol% (26 mol% alumina, respectively). Due to
the rather high values of the solution supersaturations
(above 1 g/ 1C per 100 g solution) and viscosities (above
3
000 cP), the successful growth of Na Al O(BO ) single
2
2
3 2
crystals from highly concentrated solutions seems to be
questionable.
The high viscosity values found with all solutions
indicate that an intense dynamic regime facilitating mass
transfer is needed for growing Na Al O(BO ) single
2
2
3 3
crystals. It could be achieved either by using forced
convection or by creating high temperature gradients in
the solutions. This is the third principal conclusion to be
drawn from the present investigation.
2 2 3 2
Fig. 12. Viscosity vs. Na Al O(BO ) concentration at a constant
temperature of (a) 900 1C; (b) 950 1C and (c) 1000 1C.
The above conclusions were utilized for performing a
preliminary experiment for the growth of a Na Al O(BO )
3 2
2
2
Al O , which is saturated at 980 1C. These values are lower
2
3
single crystal using the top-seeded solution growth (TSSG)
technique with slow cooling. The experiment was carried
out with a 60 g solution of a 78 mol% Na Al O(BO )
3 2
than the theoretical density of Na Al O(BO ) , which is
2
2
2
3 2
3
.532 g/cm [53], this permitting the conclusion that the
2
2
method of crystal growth from a high-temperature solution
on a seed situated on the solution surface will prove
appropriate for the preparation of single crystals of this
oxyborate.
concentration. The oxyborate seed had the shape of a
hexagonal pinacoid (3 ꢁ 1 mm) and the crystal grew in a
direction perpendicular to the c-axis. The process lasted 3
days with a cooling rate of 1 1C/day. Fig. 13 shows the
photograph of the single crystal obtained. This preliminary
result is an indication of the possibility of growing
Na Al O(BO ) single crystals from non-stoichiometric
4
.2.4. Analysis of the results
Some principal conclusions concerning the practice of
2
2
3 2
growing Na Al O(BO ) single crystals from high-
2
2
3 2
high-temperature solutions in spite of the rather high
solution viscosity. Simultaneously, the distinct defects on
the crystal confirm the necessity of intensified mass transfer
and application of high-precision temperature control.
temperature solutions can be drawn on the basis of the
above results.
First of all, this provides evidence that of all possible
solvents of the Na O–B O system, NaBO is the most
2
2
3
2
suitable. This solvent proposes very large concentration and
temperature regions of crystallization of Na Al O
5. Conclusion
2
2
(
BO ) along with a sufficiently high supersaturation degree
3 2
of the solutions, which ensures a high yield of crystals with a
habit suitable for practical application. With solvents of a
higher Na O content the solution supersaturation is lower
The results of this work represent a contribution to the
knowledge of double oxyborates formed in the systems
alkaline oxide–Me O -type oxide–boron oxide.
2
2
3

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P. Peshev et al. / Journal of Solid State Chemistry 179 (2006) 2834–2849
2848
The preparation conditions of three new oxyborates of
the Na RE O(BO ) (RE ¼ Y, Nd, Er) family with a
choice is demonstrated by the growth on a seed using the
TSSG technique and slow cooling of the solution.
Recommendations are made concerning the improvement
of the growth process with a view to obtaining good-
quality crystals.
2
2
3 2
monoclinic structure (S.G. P2 /c) have been established
1
and the cell parameters of the compounds have been
determined. Their behaviour during melting has been
studied and some conclusions about the appropriate
methods for single-crystal growth of these substances have
been drawn with a view to their use as optical materials.
It has been shown that the oxyborate Na La O(BO )
3 2
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2
[
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