Copper lanthanide selenite oxohalides with francisite structure: Synthesis and structural characteristics

Article abstract of DOI:10.1134/S0036023608090027

The reaction of lanthanide oxohalides with CuO and SeO₂ gave the products Cu₃Ln(SeO₃)₂O₂X (Ln = lanthanide, X = Cl, Br). The oxochlorides are formed with all lanthanides, while oxobromides are formed

Full text of DOI:10.1134/S0036023608090027

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 9, pp. 1353–1358. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © P.S. Berdonosov, V.A. Dolgikh, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 9, pp. 1451–1456.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPUNDS
Copper Lanthanide Selenite Oxohalides with Francisite
Structure: Synthesis and Structural Characteristics
P. S. Berdonosov and V. A. Dolgikh
Moscow State University, Moscow, 119992 Russia
Received July 12, 2007
Abstract—The reaction of lanthanide oxohalides with CuO and SeO2 gave the products Cu3Ln(SeO3)2O2X
(Ln = lanthanide, X = Cl, Br). The oxochlorides are formed with all lanthanides, while oxobromides are formed
only for Ln = La–Gd. The structures of the Cu3Ln(SeO3)2O2Cl phases for Ln = Nd (I) and Y (II) were deter-
mined by the Rietveld method. It was found that Cu3Ln(SeO3)2O2X has the structure of francisite (orthorhombic
system, space group Pmmn), which does not change substantially upon the variation of the Ln3+ radius. No sim-
ilar compounds with tellurium(IV) were found.
DOI: 10.1134/S0036023608090027
Inorganic compounds containing simultaneously a
complex anion and an elemental anion have attracted in
recent years keen attention of researchers due to
expected unusual structures of such phases and the
presence of important applied properties. Thus a large
The variation of the types of halogens in bismuth-
containing phases of this family results in a regular
increase of unit cell parameters and elongation of the
corresponding bonds [4].
The replacement of selenium by the larger tellurium
number of structures with low dimensionality and with atom induces a substantial transformation of the proto-
type structure. In Cu3Bi(TeO3)2O2Cl (V) [5], the oxygen
expected for the selenite halide class [1]. In addition, atoms are displaced from the highly symmetrical crys-
open frameworks containing channels or cavities is
tallographic positions, which gives rise to an additional
position, and the distance from one of the two crystal-
lographically independent copper atoms (Cu(2)) to the
chlorine atom is markedly shortened, resulting in trans-
formation of Cu(2) coordination into square-pyramidal
[Cu(2)O4Cl]. Finally, the unit cell parameter Ò in tellu-
ride oxochloride is doubled and the space group is
changed compared to francisite (Pcmn for V and Pmmn
for francisite III).
At the same type, the structure of erbium-containing
phase IV [6] has no key differences from the parent
francisite structure III, which prompts the idea that a
broad series of analogous phases with other lanthanides
the presence of an acentric SeO3 group in the crystal
increases the probability of formation of a noncen-
trosymmetric structure [2], which is a necessary condi-
tion for the appearance of nonlinear-optical and piezo-
and pyroelectric properties.
Meanwhile, selenite halides are represented in most
cases by only parent compounds of the potential fami-
lies. These include the family of phases with a fran-
cisite-type structure. The prototype of this structural
type, francisite Cu3Bi(SeO3)2O2Cl (III) has been
described rather long ago [3]. Subsequently, both its
synthetic analogue and isostructural compounds with
other halogens, Cu3Bi(SeO3)2O2X (X = Cl, Br, I), have could exist.
been obtained [4]. The possibility of replacement of
selenium by tellurium (Cu3Bi(TeO3)2O2Cl [5]) and bis-
muth by lanthanide (Cu3Er(SeO3)2O2Cl (IV) [6]) with
retention of the key features of the francisite structure
was demonstrated.
However, apart from IV, we found in the literature
only a mention of the preparation of analogous com-
pounds with La and Nd [6] but no structural data were
reported. On the other hand, it is known that lanthanide
compounds with the same stoichiometry may have dif-
ferent structures with different lanthanide cations or
exist only for a particular set of lanthanides [7–9].
This structure is based on an open framework
formed by highly distorted [BiO8] cubes, [CuO4]
squares, and [SeO3] pyramids; the voids of this frame-
In this paper, we report the results of a search for
Cu–Ln telluride oxochlorite, the synthesis conditions,
work accommodate chloride anions separated from the and X-ray diffraction characteristics for the
Cu3Ln(SeO3)2O2X phases (X = Cl, Br; Ln = La, Nd, Sm,
Eu, Gd, Dy, Ho, Er, Yb).
bismuth atoms by distances of more than 4 Å and from
copper atoms by 3.07 Å.
1353

1354
BERDONOSOV, DOLGIKH
Table 1. Unit cell parameters and volumes for
Cu3Ln(SeO3)2O2X (X = Cl, Br), space group Pmmn
The above-described precursors were used to pre-
pare stoichiometric mixtures according to the equation
3CuO + LnOX + 2AO2
Cu3Ln(AO3)2O2X
X = Cl
a, Å
b, Å
c, Å
V, Å3
(A = Se, Te; X = Cl, Br).
The components were weighed on a Sartorius Gemplus
balance (accuracy 0.0002 g). The overall sample
weight did not exceed 0.7 g. The mixtures were thor-
oughly ground in an agate mortar and transferred into
quartz ampoules, which were sealed off under vacuum
(~10–2 mm Hg) and placed into annealing furnaces with
controlled heating. The annealing mode was varied
depending on the starting components. The batch contain-
ing SeO2 was heated to 300°ë over a period of 6–12 h and
kept at this temperature for 12–24 h. Then the temper-
ature was raised to 550°ë during 6 h, and the batch was
kept at this temperature for 7 days. Samples containing
TeO2 were heated directly to 550°ë during 3 h and kept
at this temperature for 7 days.
La
6.398(1)
6.362(1)
6.333(3)
9.725(1)
9.606(1)
9.497(4)
7.154(1)
7.074(1)
7.016(3)
445.05(8)
432.36(8)
422.00(2)
Nd
Eu
Gd
Dy
Ho
Er
6.3220(6) 9.501(1)
6.313(1) 9.465(2)
6.2999(6) 9.440(1)
6.2921(7) 9.4257(9) 6.9632(9) 412.97(6)
7.0202(8) 421.66(6)
6.987(2) 417.5(1)
6.9723(8) 414.65(5)
Yb
Y
6.2803(8) 9.381(1)
6.2938(9) 9.438(1)
6.927(2)
6.970(1)
408.1(1)
414.33(8)
The annealing gave green-colored powdered prod-
ucts. A visual examination of the samples under an
optical microscope showed the presence of white-col-
ored grains in all tellurium-containing samples. All
selenium chloride samples looked uniformly colored
and the bromide products fell into two groups as
regards the appearance: uniformly colored samples
(Ln = La, Nd, Sm, Gd) and powders containing grains
with different tints (Ln = Dy, Er, Yb).
X = Br
La
6.405(2)
6.382(2)
6.348(1)
6.337(1)
9.888(4)
9.698(3)
9.581(2)
7.142(3)
7.091(2)
7.079(2)
452.4(2)
438.9(2)
430.6(1)
Nd
Sm
Gd
9.5515(8) 7.0540(9) 426.96(7)
Powder X-ray diffraction of tellurium derivatives
confirmed that they were not single phases. No desired
compounds with a francisite-type structure were found
among them, lanthanide tellurites Ln2Te4O11 being the
major components. Other reaction products were
apparently copper oxochloride derivatives, which were
unstable against hydrolysis and, hence, were not
detected by X-ray diffraction.
EXPERIMENTAL
Synthesis and characteristics of samples. Lan-
thanide oxohalides LnOX (Ln = La, Nd, Sm, Eu, Gd,
Dy, Ho, Er, Yb, and Y; X = Cl, Br), copper oxide (spe-
cial purity grade 9-2), and selenium or tellurium diox-
ide served as the starting compounds.
Powder X-ray diffraction of selenium chloride
derivatives and bromide derivatives with Ln = La, Nd,
Sm, and Gd showed the absence of the starting copper
or lanthanide selenite phases. The X-ray diffraction pat-
terns of the samples of the two indicated groups were
similar for all tested lanthanides and could be fully
indexed under the assumption of the type IV structure.
The unit cell parameters are given in Table 1.
The X-ray diffraction patterns of the samples with
the formal composition Cu3Ln(SeO3)2O2Br for Ln = Dy,
Er, Yb differed from the above-described ones. The
reflections present could not be assigned to any phases
known in these systems; indexing of these X-ray dif-
fraction patterns was impossible. This fact, together
with the visual examination of samples noted above,
indicate most likely that the products of annealing of
mixtures involving late lanthanides are multiphase
materials.
The oxohalides LnOX were prepared by pyrolysis
of lanthanide trihalide hydrates in an air flow at 550–
400°ë depending on the particular halogen and lan-
thanide. Selenium dioxide was obtained from selenous
acid (98%), which was dehydrated under dynamic vac-
uum at moderate temperatures. The product was then
sublimed in a flow of a mixture of dry air and NO2 pre-
pared by thermal decomposition of Pb(NO3)2 (analyti-
cal grade). All weighing and preparatory operations
with SeO2 were carried out in a dry box purged with
argon. Tellurium dioxide was prepared by a reported
procedure [10]: tellurium was dissolved in aqua regia at
80°ë and crystalline TeO2 was precipitated by aqueous
ammonia.
The single-phase nature of the products was con-
firmed by powder X-ray diffraction. The measurements
were carried out in an FR-552 Guinier type camera
(Enraf-Nonius) or on a Stadi-p Stoe diffractometer
(CuKα1 radiation). The conclusion about the phase
In order to elucidate the structural features of these
phases, we refined the structures of two selenite
composition of the product was based on PDF-2 data- oxochlorides by the Rietveld method. This was carried
base [11].
out for Nd and Y derivatives, because the radii of these
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

COPPER LANTHANIDE SELENITE OXOHALIDES WITH FRANCISITE STRUCTURE
1355
Ln3+ ions differ considerably [12]; hence, this is the pair
of choice for comparison.
Table 2. Crystal data and X-ray diffraction experiment de-
tail for Cu3Ln(SeO3)2O2Cl (Ln = Nd(I), Y(II))
Polycrystalline Cu3Ln(SeO3)2O2Cl samples (Ln =
Nd (I), Y (II)) were studied on a Stoe Stadi-p diffracto-
meter equipped with a germanium monochromator
(CuKα1 radiation) in the 2θ range of 10°–115° with a
step of 0.01°. The resulting X-ray diffraction patterns
were processed by the GSAS program [13]. As the ini-
tial model, the structural data for IV were used [6]. The
X-ray diffraction experiment details are summarized in
Table 2, the atomic coordinates and thermal parameters
are given in Table 3, and selected interatomic distances
are in Table 4. The final Rietveld refinement profiles are
shown in Fig. 1. The X-ray diffraction pattern of sample
II showed four weak peaks from an impurity in the 2θ
range of 24°–28°, which could not be assigned to any
known phases. This accounts for somewhat higher R
factors in the solution of structure II as compared to I.
I
II
System
Orthorhombic
Orthorhombic
Space group
Pmmn
Pmmn
Unit cell parameters
a, Å
6.37775(10)
9.62685(16)
7.09341(11)
2
6.30963(5)
9.45637(7)
6.98355(5)
2
b, Å
c, Å
Z
2θ range, deg
10–114
39
10–115
41
The number of refined
parameters
RESULTS AND DISCUSSION
The desired selenite oxohalides formed in most of our
experiments. The structures of phases I and II (Fig. 2)
are typical of the francisite family, which seems to
cover the selenite oxochlorite derivatives of all lan-
Rp
0.0273
0.0390
0.0499
0.0726
wRp
Table 3. Atomic coordinates and isotropic thermal parameters for I and II
Atom
Position
x
y
z
Uiso, Å2
I
Nd(1)
Cu(1)
Cu(2)
Se(1)
Cl(1)
O(1)
O(2)
O(3)
2a
4c
2a
4e
2b
4e
8g
4e
0.25
0.25
0.0
0.26709(21)
0.0
0.0102(6)
0.0479(10)
0.0301(13)
0.0202(7)
0.0427(23)
0.0328(17)
0.0328(17)
0.0328(17)
0.0
0.25
0.25
0.7890(6)
0.59867(27)
0.1596(8)
0.9864(14)
0.7499(11)
0.5760(15)
0.25
0.55626(20)
0.75
0.25
0.25
0.1097(10)
0.5833(6)
0.1250(9)
0.0450(9)
0.25
II
Y(1)
Cu(1)
Cu(2)
Se(1)
Cl(1)
O(1)
O(2)
O(3)
2a
4c
2a
4e
2b
4e
8g
4e
0.25
0.25
0.26583(18)
0.0
0.0036(4)
0.0120(4)
0.0108(6)
0.00434(31)
0.0257(12)
0.0195(20)
0.0199(16)
0.0175(21)
0.0
0.0
0.25
0.25
0.79490(29)
0.59026(15)
0.1461(5)
0.9972(9)
0.7398(7)
0.5726(9)
0.25
0.55992(11)
0.75
0.25
0.25
0.1128(6)
0.5871(5)
0.1214(6)
0.0397(7)
0.25
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

1356
BERDONOSOV, DOLGIKH
4
I × 10 , arb. units
(a)
2.0
1.0
0.0
(b)
1.0
0.5
0.0
0
20
40
60
80
100 2θ, deg
Fig. 1. Final Rietveld refinement profiles for (a) I and (b) II. The experimental profiles are shown by crosses and the calculated ones
are shown by the continuous line. The strokes correspond to the theoretical positions of peaks in the X-ray diffraction patterns.
Figs. 1a and 1b present also the difference curve between the calculated and theoretical profiles.
thanides. As in prototype III, in the structures of I and
Table 4. Interatomic distances (d) in structures I and I
II, two copper atoms have square coordination by oxy-
I
II
gen atoms. These squares share vertices and form lay-
ers perpendicular to the [001] direction; the layers are
cross-linked by LnO8 cubes to form a hollow frame-
work.
Bond
d, Å
Bond
d, Å
Nd(1)–O(1) ×2 2.406(9) Y(1)–O(1) ×2 2.281(6)
Nd(1)–O(2) ×4 2.476(6) Y(1)–O(2) ×4 2.391(4)
Nd(1)–O(3) ×2 2.500(10) Y(1)–O(3) ×2 2.464(7)
Cu(1)–Cl(1) ×2 3.101(2) Cu(1)–Cl(1) ×2 3.020(1)
Cu(1)–O(1) ×2 1.915(5) Cu(1)–O(1) ×2 1.904(3)
Cu(1)–O(2) ×2 1.968(8) Cu(1)–O(2) ×2 2.011(5)
Cu(2)–O(1) ×2 1.945(10) Cu(2)–O(1) ×2 1.918(6)
Cu(2)–O(3) ×2 1.932(9) Cu(2)–O(3) ×2 1.972(6)
Cu(2)–Cl(1) ×2 3.210(1) Cu(2)–Cl(1) ×2 3.182(1)
Se(1)–O(2) ×2 1.711(7) Se(1)–O(2) ×2 1.708(4)
The selenium atoms have a usual trigonal bipyrami-
dal SeO3 coordination completed by the lone electron
pair (LEP) of Se(IV) functioning as an additional
ligand. The SeO3 pyramids share vertices with CuO4
squares of only one layer (Fig. 2). Chloride ions not
incorporated in the coordination polyhedra are
arranged in the framework tunnels together with LEP.
The main specific features introduced in the struc-
ture by the lanthanide nature are related to the change
in bond lengths. A comparison of the Cu(1)–O and
Cu(2)–O bond for erbium [10], neodymium, and yttrium
Se(1)–O(3)
1.753(9) Se(1)–O(3)
1.719(6)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

COPPER LANTHANIDE SELENITE OXOHALIDES WITH FRANCISITE STRUCTURE
1357
yttrium compounds (∆1 ~ 0.1 Å, ∆2 ~ 0.06 Å), while for
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
the compound with larger Nd3+ cation, ∆1 is ~0.05 Å
and ∆2 is ~0.01 Å. Presumably, this is due to the elonga-
tion of the Ln–O bonds in the case of neodymium as
compared to yttrium and erbium (Table 4) since [CuO4]
squares share edges with lanthanide polyhedra [LnO8]
(Fig. 2).
Cu(2)
Cu(2)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(2)
Cu(2)
The higher degree of distortion of SeO3pyramids in
phase I compared to II (Table 4) may be regarded as
being due to this transformation of Cu–O distances
since the SeO3 polyhedra share vertices with the
[Cu(1)O4] and [Cu(2)O4] squares (Fig. 2).
Cu(1) Cu(1)
Cu(1)
Cu(1)
Cu(1)
Cu(2)
Cu(2)
Cu(1)
Cu(1)
Cu(1)
Cu(1)
In the case of selenite oxobromides, phases with a
francisite-type structure end at gadolinium, which may
be due to the size factor. Figure 3 shows a nearly linear
dependence of the unit cell volume of lanthanide selen-
ite oxohalides with a francisite-type structure on the
Ln3+ ion radius (CN = 8 [12]). Presumably, the large Br–
anion cannot be accommodated in the framework chan-
nels with small Ln3+ cations, which may give rise to the
existence of a “threshold” structure.
Cu
Nd
O
Cu(2)
Cu(2)
b
Cu(1)
Cu(1)
Cu(1)
Cu(1)
Se
Cl
a
Fig. 2. Structure of Cu Nd(SeO ) O Cl (I). A unit cell is
3
3 2 2
shown and Nd and Cu polyhedra are marked.
The absence of the desired phases among our
annealing products of tellurium-containing mixtures
can hardly be attributed to the size factor. This aspect
requires more detailed investigation.
compounds shows that the bond length differences for dif-
ferent copper atoms ∆1 = r(Cu(1)-O(1)) – r(Cu(1)-O(2)) and ∆2 =
r(Cu(2)-O(1)) – r(Cu(2)-O(3)) are relatively great for erbium and
3
V, Å
Cl
Br
450
La
440
Sm
430
Nd
Gd
420
410
Dy
Eu
Er
Y Ho
Yb
1.200
1.150
1.100
1.050
1.000
0.950
, Å
r
3+
Ln
3+
Fig. 3. Unit cell volume of the Cu Ln(SeO ) O X phases with the francisite-type structure vs. Ln ion radius (CN = 8)
3
3 2 2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 9 2008

1358
BERDONOSOV, DOLGIKH
6. R. Berrigan and B. M. Gatehouse, Acta Crystallogr.,
ACKNOWLEDGMENTS
Sect. C: Struct. Sci. Commun. 52, 496 (1996).
This work was supported by the Russian Foundation
for Basic Research, project nos. 06-03-32134 and
05-03-32719, and by the INTAS (YSF 05-109-4474).
7. G. Brandt and R. Dieh, Mater. Res. Bull. 9 (4), 411
(1974).
8. D. G. Shabalin, P. S. Berdonosov, V. A. Dolgikh, et al.,
Izv. Akad. Nauk, Ser. Khim., No. 1, 93 (2003).
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