Structure of Cs1 - X Rb x H2PO4 solid solutions

Article abstract of DOI:10.1134/S0020168510070149

Cs_{1 - x} Rb _x H₂PO₄ solid solutions have been synthesized for the first time in a broad composition range, x = 0.03-0.9. At room temperature, the Cs_{1 - x} Rb _x H₂PO₄ solid solutions are isostructural with the low-temperature phase of CsH₂PO₄ over the entire composition range studied. In the CsH₂PO₄-based solid-solution series, the unit-cell parameters and volume decrease with increasing Rb content. At high temperatures, the Cs_{1 - x} Rb _x H₂PO₄ solid solutions exist in the range x = 0-0.4, are isostructural with cubic CsH₂PO₄, and have a smaller unit-cell parameter.

Full text of DOI:10.1134/S0020168510070149

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 7, pp. 765–769. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.V. Martsinkevich, V.G. Ponomareva, T.N. Drebushchak, G.V. Lavrova, S.S. Shatskaya, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46,
No. 7, pp. 855–859.
Structure of Cs1 – RbxH2PO4 Solid Solutions
x
V. V. Martsinkevicha, V. G. Ponomarevaa, T. N. Drebushchaka,b,
G. V. Lavrovab, and S. S. Shatskayaa
a Institute of SolidꢀState Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences, ul. Kutateladze
18, Novosibirsk, 630128 Russia
b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia
eꢀmail: ponomareva@solid.nsc.ru
Received June 16, 2009; in final form, December 24, 2009
Abstract
range, = 0.03–0.9. At room temperature, the Cs1 – xRbxH2PO4 solid solutions are isostructural with the lowꢀ
temperature phase of CsH2PO4 over the entire composition range studied. In the CsH2PO4ꢀbased solidꢀsoluꢀ
tion series, the unitꢀcell parameters and volume decrease with increasing Rb content. At high temperatures,
—Cs1 – xRbxH2PO4 solid solutions have been synthesized for the first time in a broad composition
x
the Cs1 – xRbxH2PO4 solid solutions exist in the range
x
= 0–0.4, are isostructural with cubic CsH2PO4, and
have a smaller unitꢀcell parameter.
DOI: 10.1134/S0020168510070149
INTRODUCTION
influence the phase transition of CsH2PO4 and its
chemical stability in the superionic state. The monoꢀ
clinic phase undergoes a phase transition to the cubic,
superionic phase, whose temperature decreases with
+
The highꢀtemperature phase of CsH2PO4 has the
highest electrical conductivity (
acid salts of alkali metals with the general formula
MnHm(XO4)y (M = Li, Na, K, Rb, Cs, NH4; = S, Se,
6 ×
10–2 S/cm) among
increasing
concentration, by up to 10
°
С
during
during cooling, whereas the
> 0.4) undergoes no superionic
transition up to its melting point, just like the unsubꢀ
stituted salt NH4H2PO4
X
NH4
heating and by up to 37
tetragonal phase (
P, A s ) . I n a d d i t i o n , CsH2PO4 is one of the most promisꢀ
ing mediumꢀtemperature electrolytes. However, it
undergoes a transition at 230°С from a highꢀtemperaꢀ
ture, cubic phase to a lowꢀtemperature, monoclinic
phase, accompanied by a drop in electrical conductivity
by four orders of magnitude.
°
С
х
.
RbH2PO4, yet another salt of this family, is known to
have a highꢀtemperature form with a superionic transiꢀ
tion temperature of 280°С [8]. CsH2PO4ꢀbased double
salts containing other alkaliꢀmetal cations have not been
systematically studied. In this paper, we report the synꢀ
thesis of Cs1 – хRbxH2PO4 double salts in a wide compoꢀ
sition range and a detailed study of their structure.
One common way of modifying the transport propꢀ
erties of ionic salts is by doping. In particular, the
effect of both cation (Li+, K+, Rb+) and anion
(
−) substitutions in CsHSO4 on the phase compoꢀ
H2PO4
sition of the resulting materials has been studied in a wide
concentration range [1–5]. In a number of composiꢀ
tions, stabilization of an amorphous state was observed
(e.g., in Cs0.67K0.33HSO4) or individual phases with other
crystal structures were formed: Cs3(H2PO4)(HSO4)2,
Cs2(H2PO4)(HSO4), CsNa2(HSO4)3, and Cs2Na(HSO4)3
[6].
According to Xꢀray diffraction (XRD) data,
CsH2PO4 has a monoclinic (
temperature, with unitꢀcell parameters
6.3831 = 4.8803 = 107.73
[9]. Its lowꢀtemperature phase has a twoꢀdimensional
P
21/m) structure at room
= 7.9122
= 250.19 Å3
a
V
Å, b =
, Z =
Å,
c
Å,
β
°,
2
hydrogenꢀbond network, in which the РО4 tetrahedra are
linked by both asymmetric (one energy minimum) and
symmetric (two minima) hydrogen bonds. The phosꢀ
phate tetrahedra are linked into chains by short hydrogen
bonds, 2.47 Å in length, and the chains are linked into
+
In contrast to what occurs in CsHSO4, partial
NH4
substitution for Cs+ in CsH2PO4 causes neither amorꢀ
phization of the salt nor formation of other phases [7].
There are two Cs1 – x(NH4)xH2PO4 solidꢀsolution
phases, one based on the lowꢀtemperature, monoꢀ
twoꢀdimensional layers by longer hydrogen bonds, 2.54
in length. The highꢀtemperature phase crystallizes in
cubic symmetry (Pm ), with = 4.9613
122.1 Å3
and = 1 [10]. At the center of each unit cell,
), which is caused by the structural distincꢀ there is an ideal PO4 tetrahedron disordered in six equally
tion between the endꢀmembers. In the intermediate probable positions. Accordingly, hydrogen bonds may
composition range, 0.1 < < 0.4, no solid solutions have six directions. The cesium atoms are located at the
Å
clinic phase of CsH2PO4
the lowꢀtemperature, tetragonal phase of NH4H2PO4
(0.4 <
(0
≤
х < 0.1), and the other on
3m
а
Å, V =
,
Z
х ≤ 1
х
exist. In addition, cation substitutions were shown to corners of the unit cell.
765

766
MARTSINKEVICH et al.
Table 1. Unitꢀcell parameters of lowꢀtemperature Cs1 – xRbxH2PO4
х
a
,
Å
b,
Å
c
,
Å
β, deg
0
7.912(2)
7.9056(8)
7.892(2)
7.845(1)
7.834(2)
7.786(2)
7.62(3)
6.384(4)
6.374(2)
6.371(5)
6.332(3)
6.320(4)
6.267(3)
6.23(1)
4.886(6)
4.875(2)
4.876(7)
4.858(5)
4.851(7)
4.818(5)
4.75(2)
107.84(4)
107.82(2)
108.01(5)
108.09(3)
108.03(5)
108.19(4)
108.1(3)
0.03
0.1
0.3
0.4
0.6
0.9
RbH2PO4 has a tetragonal (
temperature and a monoclinic (
to that of lowꢀtemperature CsH2PO4, above 79
I
P
42
21/
d
a
) structure at room
) structure, similar
[11].
RESULTS AND DISCUSSION
According to XRD data, the structures of the individꢀ
ual salts CsH2PO4 and RbH2PO4 were similar to those
°С
reported earlier [9, 11].
Figure 1 shows the XRD patterns of Cs1 – хRbxH2PO4
double salts of various compositions. The structure of
lowꢀtemperature CsH2PO4 is seen to persist in the double
EXPERIMENTAL
Cs1 – хRbxH2PO4 ( = 0–0.6) single crystals were
grown from aqueous solutions containing appropriate
ratios of phosphoric acid, cesium carbonate, and
rubidium carbonate by roomꢀtemperature evaporaꢀ
tion. The crystals were washed with acetone and calꢀ
x
salts up to х = 0.9. The relative XRD intensities vary little,
and the reflections gradually shift to larger angles, indiꢀ
cating the formation of CsH2PO4ꢀbased solid solutions
with reduced unitꢀcell parameters. Table 1 lists the unitꢀ
cell parameters of Cs1 – хRbxH2PO4 with х = 0–0.9 (calꢀ
culations with the IK program). The lowꢀtemperature
cined at 150
water. Polycrystalline Cs1 – хRbxH2PO4 (
samples were prepared by reacting appropriate mixꢀ
tures at 200 . The cation composition of the samples
was determined by a combination of atomic absorpꢀ
tion and emission flame photometry ( = 852.1 nm)
°С
for 2 h in order to remove the residual
x
= 0.8–0.9)
phase of the synthesized CsH2PO4 has a monoclinic
(P21/m) structure with lattice parameters a = 7.912(2) Å,
°С
b
= 6.384(4) Å,
c
= 4.886(6) Å, and V =
= 107.84(4)o (
β
234.96 Å3), in agreement with those reported by Matsuꢀ
λ
naga et al. [9]. The lowꢀtemperature phase of the syntheꢀ
−
[12].
was determined by differential colorimeꢀ
H2PO4
sized RbH2PO4 has a tetragonal (
tice parameters = 7.611(1) Å and
agreement with those reported by Rusakov and
I42
d
) structure with latꢀ
try as the yellow vanadomolybdate complex [13].
From the chemical analysis and XRD data, the accuꢀ
a
c
= 7.301(3) Å, in
racy in the compositions obtained was estimated at Kheiker [11].
3–5%.
As seen in Figs. 2–4, the lattice periods and unitꢀcell
volume decrease with increasing Rb content,
whereas increases. The change in parameter is
slightly smaller than those in and . These changes
lead to a reduction in the difference in length
between the hydrogen bonds along the and axes.
As seen in Figs. 2 and 3, the unitꢀcell parameters and
011 interplanar spacing are linear functions of Rb+
content, in accordance with Vegard’s law.
Powder XRD patterns were collected on DRONꢀ
3
β
с
(Сu
К
radiation,
λ
= 1.5418 Å) and Bruker
α
a
b
D8ꢀGADDS (Сu
К
radiation, = 1.5418 Å, graphite
λ
α
monochromator, 0.5ꢀmm receiving slit, area detector)
diffractometers. Highꢀtemperature XRD measureꢀ
ments were made on a Stoe MP powder diffractometer
а
с
(
Сu
radiation,
λ
= 1.5406 Å, bent germanium
K
α
1
monochromator, linear positionꢀsensitive detector)
equipped with a highꢀtemperature attachment (transꢀ
mission geometry, 0.5ꢀmmꢀdiameter capillary).
Thus, as distinct from Cs1 – x(NH4)xH2PO4
solid solutions Cs1 – xRbxH2PO4 exist over the entire comꢀ
position range. Note that the solid solutions were
,
Unitꢀcell parameters were determined from 11 indeꢀ obtained by different procedures: via both coprecipitaꢀ
pendent reflections using the IK program [14]. tion from aqueous solutions and heating of mechanical
INORGANIC MATERIALS Vol. 46
No. 7
2010

STRUCTURE OF Cs1 – xRb xH2PO4 SOLID SOLUTIONS
767
8
7
6
5
4
3
2
1
20
40
, deg
60
2
θ
Fig. 1. XRD patterns of (
) 0.9.
1
)
CsH PO , (
2–7)
Cs
Rb H PO , and (
8)
RbH PO ; x
4
= (2) 0.03, (
3) 0.1, (4) 0.3, (5) 0.4, (6) 0.6,
2
4
1 –
х
х
2
4
2
(7
mixtures to 200
range extends up to
cell parameters were determined from XRD data with a
substantially higher uncertainty than those at lower valꢀ
ues, probably because the = 0.9 sample was more
hygroscopic, which broadened the reflections from it and
reduced their intensity.
°С. As seen in Fig. 1, the solidꢀsolution chemical bonding, and the electronic structure of the
х
= 0.9. At х = 0.9, however, the unitꢀ
atoms. If substituents differ considerably in cation
radius, the structure of solid solutions between acid
salts typically differs markedly from those of the endꢀ
members [1, 5, 6]. The likely reason for the large width
of the solidꢀsolution range is that rubidium dihydroꢀ
x
x
gen phosphate undergoes a transition at 79°С to a
monoclinic phase similar in structure to lowꢀtemperaꢀ
ture CsH2PO4 and also that the difference in ionic
radius between Cs and Rb is not very large (Cs+, 1.74 Å;
Isomorphic substitutions are known to be more likely
when the atoms involved have the same coordination
number and, in the case of covalent compounds, when
they are identical in bonding configuration. At a given
temperature and pressure, the degree of isomorphism is
+
determined by the proximity in interatomic distances, Rb+, 1.61 Å;
, 1.42 Å).
NH4
INORGANIC MATERIALS Vol. 46
No. 7 2010

768
MARTSINKEVICH et al.
d, Å
3.78
3.77
3.76
3.75
3.74
3.73
3.72
3.71
3.70
3.69
3.68
3.67
0
0.1
0.2
0.3
0.4
0.5
0.6
x
Fig. 2. 011 interplanar spacing as a function of Rb content
for Cs
Rb H PO
.
1 –
х
х
2
4
3
Δ
a,
Δ
b
,
Δ
c
, %
0
2
1
–0.005
–0.010
–0.015
–0.020
3
1
2
20
30
40
50
0
0.2
0.4
0.6
2θ, deg
x
Fig. 3. Composition
in the unitꢀcell parameters
Cs Rb H PO
dependences of relative changes
Fig. 5. XRD
patterns of
highꢀtemperature
(1)
a, (
2)
b, and (3) c of
Cs
Rb H PO with
х
= (1) 0.01, (2) 0.2, and
1 –
х
х
2
4
.
4
(3) 0.4 (t = 240°С).
1 –
х
х
2
V,
Å3
As shown earlier, at 230°С Cs0.97Rb0.03H2PO4 is isosꢀ
235
tructural with the highꢀtemperature, superionic phase of
CsH2PO4 but has slightly smaller unitꢀcell parameters
[15]. The highꢀtemperature XRD data for the superionic
230
phase of Cs1 – хRbxH2PO4 with
240 also indicate the formation of solid solutions based
on highꢀtemperature CsH2PO4, with a gradual reduction
in (Fig. 5). Their unitꢀcell parameters are listed in
Table 2. For 0.4, the transition to the highꢀtemperaꢀ
ture, superionic phase, isomorphic with CsH2PO4
occurs in the temperature range 220–235 . Partial
rubidium substitution for cesium and the formation of
solid solutions, of course, influence the transport and
thermodynamic properties of the double salts and the
hydrogen bond energetics in them, which will be considꢀ
ered in a subsequent communication.
х = 0.1, 0.2, and 0.4 at
225
220
215
°
С
а
х
≤
,
°С
210
0
0.2
0.4
0.6
0.8
x
Fig. 4. Composition dependence of the unitꢀcell volume
for Cs Rb H PO
.
4
1 –
х
х
2
INORGANIC MATERIALS Vol. 46
No. 7
2010

STRUCTURE OF Cs1 – xRb xH2PO4 SOLID SOLUTIONS
769
Table 2. Unitꢀcell parameters of highꢀtemperature Cs1 – xRbxH2PO4
at 240°C
4. Haile, S.M., Lentz, G., Kreuer, K.ꢀD., and Maier, J.,
Superprotonic Conductivity in Cs3(HSO4)2(H2PO4),
Solid State Ionics, 1995, vol. 77, pp. 128–134.
3
х
a,
Å
V, Å
5. Chisholm, C.R.I. and Haile, S.M., Superprotonic Behavꢀ
ior of Cs2(HSO4)(H2PO4)—A New Solid Acid in the
CsHSO4–CsH2PO4 System, Solid State Ionics, 2000,
vols. 136–137, pp. 229–241.
0
4.961
122.10
121.74
121.43
120.96
0.1
0.2
0.4
4.9561(1)
4.952(2)
4.946(8)
6. Chisholm, C.R.I., Cowan, L.A., Haile, S.M., and
Klooster, W.T., Synthesis, Structure, and Properties of
Compounds in the NaHSO4–CsHSO4 System: 1. Crysꢀ
tal Structures of CsNa2(HSO4)3 and Cs2Na(HSO4)3
,
Chem. Mater., 2001, vol. 13, pp. 2574–2583.
CONCLUSIONS
7. Baranov, A.I., Grebenev, V.V., Khodan, A.N., et al., Optiꢀ
mization of Superionic Acid Salts for Fuel Cell Applicaꢀ
tions, Solid State Ionics, 2005, vol. 176, pp. 2871–2874.
We synthesized Cs1 – хRbxH2PO4 double salts in a
broad composition range and investigated their structural
properties. At 25
are isostructural with lowꢀtemperature cesium dihydroꢀ
gen phosphate and have smaller , and cell parameꢀ
°С, the double salts with x = 0.03–0.9
8. Rapoport, E., Clark, J.B., and Richter, P.W., HighꢀPresꢀ
sure Phase Relations of RbH2PO4, CsH2PO4, and
KD2PO4, J. Solid State Chem., 1978, vol. 24, nos. 3–4,
pp. 423–433.
a,
b
c
ters. At high temperatures, the Cs1 – хRbxH2PO4 solid
solutions are isostructural with cubic cesium dihydrogen
phosphate and exist in the range х = 0–0.4.
9. Matsunaga, H., Itoh, K., and Nakamura, E., XꢀRay
Structure Study of Ferroelectric Cesium Hydrogen Phosꢀ
phate at Room Temperature, J. Phys. Soc. Jpn., 1980,
vol. 48, pp. 2011–2014.
ACKNOWLEDGMENTS
10. Preisinger, A., Mreiter, K., and Bronowska, W., The Phase
Transition of CsH2РО4 (CDP) at 505 K, Mater. Sci.
Forum, 1994, vols. 166–169, pp. 511–516.
We are grateful to S.V. Tsibulya for his assistance with
the structure calculations.
This research was supported in part by the Russian
Foundation for Basic Research, grant no. 07ꢀ03ꢀ12151.
11. Rusakov, A.A. and Kheiker, D.M., RoomꢀTemperature
Structure Refinement of RbH2PO4, Kristallografiya,
1978, vol. 23, no. 2, pp. 409–411.
12. Stolyarova, I.A. and Filatova, M.F., Atomnoꢀabsorbtsionꢀ
naya spektrometriya (Atomic Absorption Spectroscopy),
Leningrad: Nedra, 1981, p. 24.
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,
INORGANIC MATERIALS Vol. 46
No. 7 2010