Regions of phase crystallization in the ternary system Na 2O-Al2O3-B2O3

Article abstract of DOI:10.1016/j.jallcom.2004.08.046

The concentration and temperature regions of formation of crystals from different phases in the ternary system Na₂O-Al₂O ₃-B₂O₃ were determined using the results on spontaneous crystallization in vari

Full text of DOI:10.1016/j.jallcom.2004.08.046

Journal of Alloys and Compounds 391 (2005) 256–261
Regions of phase crystallization in the ternary
system Na O–Al O –B O
2
2 3
2 3
∗
D. Binev, V. Nikolov, P. Peshev
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Building 11, 1113 Sofia, Bulgaria
Received 6 August 2004; accepted 25 August 2004
Available online 12 October 2004
Abstract
The concentration and temperature regions of formation of crystals from different phases in the ternary system Na
determined using the results on spontaneous crystallization in various high-temperature solutions of this system. The study was limited to
solutions in which the contents of the three oxides varied between 20 and 55 mol% for Na O, 0 and 35 mol% for Al and 25 and 55 mol%
for B , respectively. Evaporation losses, creeping and extremely high viscosities were not observed in these solutions. In the above
limits, the crystallization regions of Al
Na Al
its single crystals. Studies on the habit of Na
conclusions concerning the composition of the flux growth solutions.
2004 Elsevier B.V. All rights reserved.
2 2 3 2 3
O–Al O –B O were
2
2 3
O
2
O
3
2
O
3
, NaBO
2
, NaAlO
2
and the double oxyborate Na
2
Al
2
B
2
O
7
(Na
2
Al
2
(BO
3
)
2
O) were determined. As
◦
2
2
B
2
O
7
was found to melt incongruently at 970 ± 3 C, this made the high-temperature solution growth most appropriate for obtaining
2
2 2 7
Al B O crystals grown in different zones of its crystallization region led to some preliminary
©
Keywords: Optical materials; Oxide materials; Phase diagrams; Crystallization; Crystal growth
1
. Introduction
(CLBO) [4], Sr2Be2B2O7 (SBBO) [5], Li2B4O7 (LTB) [6]
and K2Al2B2O7 (KAB) [7]. The ternary borate YAl3(BO3)4
(YAB) is the principal member of another family of bo-
rates, LnM3(BO3)4 (Ln = Y, rare earth; M = Al, Ga, Sc, Cr,
Fe) with huntite structure. Doped with several trivalent ions,
YAB can find application as a self-frequency doubling (SFD)
laser crystal, the most commonly used being Nd:YAl3(BO3)4
(NYAB). A large number of huntite type borates are subjected
to intense studies [8,9]. A third family of borate materials
with lasing properties is that of oxoborates, LnM’4O(BO3)3
(Ln = rare earth; M’ = alkaline earth metal). The most studied
representatives of this family are GdCa4O(BO3)3 (GdCOB),
YCa4O(BO3)3 (YCOB) and their solid solutions [9–11].
Detailed information about the synthesis, structure, crys-
tal growth and properties of borate NLO and laser mate-
rials is given in several review papers published recently
[8,9,11–16].
Oxide materials of the borate group have been the subject
of increasing interest during the past years. A large num-
ber of papers dealing with the synthesis, crystal growth and
properties of new borates as well as borate crystal chemistry
have appeared in the literature. This interest is due, above
all, to the excellent non-linear optical (NLO) and/or lasing
properties of the crystals of some borates. The unique struc-
ture characteristics of the boron–oxygen groups in a series
of these compounds determine their enhanced UV trans-
parency, good non-linearity and relatively high resistance
against laser-induced damage. On the other hand, the extraor-
dinary versatility of borate structures facilitates the design of
new compounds with appropriate optical properties.
Borates with NLO properties form actually a large fam-
ily whose most prominent members are ␤-BaB2O4 (BBO)
[
1], LiB3O (LBO) [2], CsB3O (CBO) [3], CsLiB O10
Regardless of the large number of known borate materials
with promising NLO or laser properties, the studies on them
and the intense search for new compounds continue. This is
due to (i) the extraordinary high viscosities of most borate
5
5
6
∗
Corresponding author. Tel.: +359 297 92573; fax: +359 287 05024.
E-mail address: ppeshev@svr.igic.bas.bg (P. Peshev).
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.08.046

D. Binev et al. / Journal of Alloys and Compounds 391 (2005) 256–261
257
melts and high-temperature solutions, which is a serious hin-
drance to the growth of high-quality single crystals; (ii) the
lack of transparency with most of the known borates in the
UV region far below 200 nm, which is needed for a series of
new applications.
equilibrium. Inaddition, alongwithdataonthecrystallization
regions, information is also obtained on a series of properties
of the solutions (viscosity, creeping, evaporation, homogene-
ity degree, etc.), which are very important for the processes
of single crystal growth from these solutions.
Within the framework of a programme of the authors
aimed at search for new borates with NLO characteristics and
growth conditions of their single crystals, the present study
is focused on the concentration and temperature regions of
crystallization of phases of the system Na2O–Al2O3–B2O3.
Varying the component concentrations within a wide range,
attention is mainly paid to finding the regions where ternary
borates crystallize.
The data available in the literature concerning double
sodium aluminum borates are rather scarce. Investigating
melts of the pseudobinary system NaBO2–Al2O3, Same-
dov et al. [17] have for the first time established forma-
tion of a compound with the composition Na2Al2B2O7
The experiments were carried out in vertical resistance
furnaces ensuring maintenance of temperature with an ac-
◦
curacy of ± 1 C (Eurotherm controller). The solution con-
tainers 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 about 20–25 mm
layer thickness in the crucible.
Analytical reagent grade purity Na2CO3, Al2O3 and
H3BO3 in preset molar ratios were used for the preparation
of the solutions. In order to avoid losses due to sprinkling,
a mechanical mixture of Na2CO3 and H3BO3 was at first
melted and decomposed in the crucibles, and only after the
formation of a clear solution, the necessary amount of Al2O3
was added in small portions. After that the solution was ho-
mogenized with continuous stirring using a Pt stirrer at a
(Na2O:Al2O3:B2O3 = 1:1:1). The short communication con-
tains a poor X-ray powder diffraction pattern of the com-
pound which has not been indexed. According to the au-
◦
temperature exceeding by 50–100 C the expected crystal-
◦
thors, Na2Al2B2O7 melts congruently at 1010 ± 10 C. In
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 tem-
perature was gradually decreased within 1 h down to values
another paper [18] part of the same authors have con-
firmed the results in Ref. [17] concerning Na2Al2B2O7
and, studying the section of Na2O·2B2O3–Al2O3, have
established the existence of a second ternary phase,
NaAlB2O5 (Na2O:Al2O3:B2O3 = 1:1:2), with a melting
◦
exceeding Tcryst by 5–10 C only.
The high-temperature solutions of the system
Na2O–Al2O3–B2O3 have a high viscosity and a very
pronounced trend to glass formation. For that reason,
to achieve the most precise determination of Tcryst we
ensured (i) an appropriate temperature distribution in the
◦
point of 1025 ± 10 C.
The structure of Na2Al2B2O7 was determined for the first
time not long ago by Corbel and Leblanc [19]. They found the
compound, defined by them as oxyborate, Na2Al2(BO3)2O,
to be isostructural with Na2Ga2(BO3)2O and to crystallize
in the trigonal system (S.G. P-31c; Z = 2). More recently the
structure of Na2Al2B2O7 was refined by He et al. [20]. These
◦
solution with clearly expressed (about 10 C) axial and
radial temperature 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;
(iii) introduction, with each temperature change, of a small
amount of fine powder from pre-synthesized Na2Al2B2O7
to the solution surface. This was made at every 2–3 h with a
authors reported the compound to be melting congruently at
◦
9
86 C.
According to the data available in the literature, the
system Na2O–Al2O3–B2O3 has not yet been investigated
with respect to the possibilities and conditions of growth of
ternary borate single crystals. Prognostication of such pos-
sibilities and conditions is an additional task of the present
work.
◦
step of 1–2 C. The purpose was introduction of a seed to
initiate the crystallization process.
Special experiments showed that, as a result of neglection
of 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
2
. Experimental
◦
Tcryst by 50–100 C. The necessity of observing the above
The temperature and concentration regions of crystalliza-
requirements in order to determine Tcryst with a 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.
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 composition of the solution
was then altered in the desired direction with the appropriate
tion of the different phases in the system Na2O–Al2O3–B2O3
were determined on the basis of the results obtained during
slow cooling and spontaneous crystallization of numerous
homogeneous high-temperature solutions with various com-
positions. In contrast to the investigations on the products of
solid state interactions between the components or the use of
data from the thermal analysis (DTA and TGA) of suitable
samples, themethodinquestionhastheadvantageoffacilitat-
ing a faster and more reliable reaching of the thermodynamic

2
58
D. Binev et al. / Journal of Alloys and Compounds 391 (2005) 256–261
by noticeable losses due to evaporation. The contents of the
three solution components varied between 20 and 55 mol%
for Na2O, 0 and 35 mol% for Al2O3 and 25 and 55 mol%
for B2O3. Determination of the crystallization regions of the
phases in the solution Na2O–Al2O3–B2O3 within the above
concentration limits required investigation of about 50–60
different solutions distributed inhomogeneously, according
to the importance of the corresponding portion of the sys-
tem.
The phase characterization of the crystals growing in each
of the system regions was made by X-ray diffraction (XRD).
◦
Powder XRD data were recorded for 2θ values of 5–80 at
room temperature on a DRON-3 automatic powder diffrac-
tometer using filtered Cu K␣ radiation.
The thermal behaviour of some characteristic samples was
studied by obtaining their differential thermal analysis (DTA)
and thermogravimetric analysis (TGA) curves using a Stan-
ton Redcroft STA 760 instrument.
Microprobe analysis with a JEOL SUPERPROBE 733
apparatus was used in experiments determining the melting
characteristics of Na2Al2B2O7.
Fig. 1. Crystallization regions of the phases: NaBO2 (A), Na2Al2B2O7
(
B), NaAlO2 (C), and Al2O3 (D) in the system Na2O–Al2O3–B2O3 ꢀ,
Na2Al2B2O7 stoichiometric composition.
step by addition of Na2CO3, Al2O3, or H3BO3 or of two of
these components simultaneously and the procedure of Tcryst
determination and phase characterization 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 by that turned to
crystals, etc.
3. Results and discussion
3.1. Concentration and temperature regions of phase
crystallization
The present study was restricted to solution compositions
where the viscosity was not unusually high, the creeping
trend was absent and the crystallization was not accompanied
The experimentally determined regions of phase crys-
tallization in the investigated part of the ternary system
Na2O–Al2O3–B2O3 are presented in Fig. 1. Regions of one
Fig. 2. Microphotographs of NaBO2, NaAlO2, Na2Al2B2O7 and Al2O3 crystals grown in the corresponding regions of the Na2O–Al2O3–B2O3 system.

D. Binev et al. / Journal of Alloys and Compounds 391 (2005) 256–261
259
binary, two ternary and one quaternary oxide are visible, as
follows:
A much more definite answer to the question about the
character of melting of the double borate under considera-
tion was obtained as a result of a series of experiments on
thermal treatment at different temperatures of isolated sin-
gle crystals. Microphotographs of some of the treated crys-
tals are presented in Fig. 3. It is obvious that after heating
(i) Al2O3 crystallizing at high concentrations of this oxide
in the solutions;
(
ii) NaBO2, at Al2O3 concentrations ranging from 0 to
◦
2
1 mol% and temperatures from 966 C (melting point
◦
of a Na2Al2B2O7 crystal for 36 h at 968 C it shows no
◦
of the compound) to about 800 C;
change, remaining transparent (Fig. 3a). No change is also
observed after 10 h if the heating temperature is increased
(iii) NaAlO2, at low B2O3 concentrations and an Al2O3 con-
tent above 27.5 mol%;
◦
◦
by 5 C up to 973 C. However, prolongation of the heat-
treatment at this temperature to 36 h results in the appear-
(iv) Na2Al2B2O7 (or Na2Al2(BO3)2O) at an Al2O3 concen-
tration between 21 and 33 mol% and a temperature of
◦
8
00–970 C.
Thenatureofthephasescrystallizingineachofthefourre-
gions is confirmed by the X-ray diffraction patterns of ground
crystals grown in these regions, which completely correspond
to the data available in the literature concerning the com-
pounds under consideration. Microphotographs of crystals
from the four regions are shown in Fig. 2.
No crystallization region of the double borate NaAlB2O5
reported in the paper of Abdullaev et al. [18] has been es-
tablished. Our experiments have shown crystal formation of
Na2Al2B2O7 only in the region where the above compound
should have appeared.
The Na2Al2B2O7 borate deserves special attention due to
its non-centrosymmetirc crystal structure, which permits the
assumption that this compound could be a new NLO material.
In addition, the Al3 ions present in it could be partly replaced
by 3d metal activator ions, this leading to new laser media.
According to the data from the literature cited in the intro-
duction, Na2Al2B2O7 has a congruent character of melting.
+
◦
Abdullaev et al. [18] have mentioned 1010 C as a melt-
ing point, whereas He et al. [20] are of the opinion, on
the basis of the DTA curve of Na2Al2B2O7, that this tem-
◦
perature is 986 C. However, the crystallization region of
the double borate determined in the present study lies to
the left of the point for the stoichiometric molar composi-
tion of the compound (Na2O:Al2O3:B2O3 = 33.3:33.3:33.3)
although the upper boundary of the region (Na2O:
Al2O3:B2O3 = 34.3:31.5:34.2) is rather close to this com-
position. In other words, the results obtained indicate that
Na2Al2B2O7 should be incongruently melting, its composi-
tion in the peritectic point being close to the stoichiometric.
The choice of a method for growing Na2Al2B2O7 sin-
gle crystals requires explicit elucidation of the character of
melting of this compound. In this connection, additional
experiments were performed. At first the DTA curves of
Na2Al2B2O7 were obtained at three different heating rates.
They showed a strong temperature dependence of the en-
dothermic peak of melting on the heating rate. Thus, at a
◦
◦
heating rate of 5 C/min the effect was observed at 993 C,
◦
◦
◦
while at 10 C/min this happened at 996 C and at 15 C/min,
at 1001 C. In fact, with such not large differences be-
◦
tween the heating rates, the shift of the endothermic effect
should not be substantial if the compound is congruently
melting.
Fig. 3. Microphotographs of Na2Al2B2O7 crystals after a 36 h thermal treat-
ment at: (a) 968 C; (b) and (c) 980 C.
◦
◦

2
60
D. Binev et al. / Journal of Alloys and Compounds 391 (2005) 256–261
Fig. 4. Habit of Na2Al2B2O7 crystals obtained from high-temperature solutions with compositions 1–8 (Fig. 1).
ance of fine particles of another phase in the crystal. The
number of these particles becomes significant with rising
temperature and prolongation of the heat treatment, as is ev-
crystallization region of the compound (region B in Fig. 1)
form colorless transparent and homogeneous glasses after
quick cooling.
The explicit conclusion from the above experiments is that
the double borate Na2Al2B2O7 is a compound which, when
melted, decomposes peritectically according to the reaction
◦
ident from Fig. 3b where a crystal heated for 36 h at 980 C
is shown. A larger magnification (Fig. 3c) shows that the
particles have a hexagonal shape. The microprobe analysis
and X-ray determinations indicate them to be consisting of
alumina.
Na2Al2B2O7 → 2NaBO2 + Al2O3
◦
When the temperature of heat-treatment of the
The decomposition occurs at T = 970 ± 3 C. However, since
◦
Na2Al2B2O7 crystals is increased to 1050 C, the product
the transition needs formation of Al O nuclei, which is a
2
3
becomes a fully opaque glass due to dispersed Al2O3
particles. In contrast to this glass, the compositions in the
relatively slow process, the effect on the DTA curve is some-
what retarded and appears at a temperature increasing with

D. Binev et al. / Journal of Alloys and Compounds 391 (2005) 256–261
261
the heating rate at which the curve has been registered. The
second endothermic peak expected as a result of the peritec-
tic decomposition, which would correspond to the complete
dissolution of Al2O3 (the so-called liquidus point) would not
be observed due to the very high temperature of location of
solutions with a B2O3 concentration of more than 45 mol%,
solvents with a Na2O:B2O3 ratio (in molar parts) ranging
from 0.7:0.3 to 0.5:0.5 should be used.
However, a final choice of the initial solution for the
growth of Na2Al2B2O7 single crystals would be only pos-
sible after investigating other important properties (supersat-
uration degree, viscosity, density, etc.) of the solutions in the
crystallization region of this compound. These studies are
now in progress.
◦
this point (probably above 1200 C) when also processes of
intense evaporation of Na2O and B2O3 begin.
3
.2. Habit of Na2Al2B2O7 single crystals
A characteristic feature of most of the borate single crys-
tals is their pronounced trend to split in the plane perpendic-
ular to the c-axis and to form hexangonal 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 under consideration, we had to pay attention to
the habit of spontaneously growing crystals of Na2Al2B2O7
in different zones of its crystallization region. Fig. 4 shows
microphotographs of crystals obtained from compositions in
the crystallization region which correspond to points 1–8 of
the diagram in Fig. 1. It is obvious that the composition of
the high-temperature solution strongly affects the habit of the
grown crystals. With rising B2O3 content in the solution, the
crystals become increasingly thinner and when this content
exceeds 45 mol%, the habit becomes mica-like (Fig. 4.1, 4.2,
Acknowledgement
The financial support of the National Council of Scientific
Investigations of Bulgaria under contract No. X-1306/2003
is highly appreciated.
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[
[