Electrical and thermodynamic properties of Cs0.97 Rb 0.03H2PO4

Article abstract of DOI:10.1134/S0020168509070164

The transport properties of Cs_0.97Rb_0.03H ₂PO₄ have been studied using polycrystalline samples and single crystals. The mixed salt is isostructural with cesium dihydrogen phosphate and has slightly smaller unitc

Full text of DOI:10.1134/S0020168509070164

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 7, pp. 795–801. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © G.V. Lavrova, V.V. Martsinkevich, V.G. Ponomareva, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 7, pp. 857–863.
Electrical and Thermodynamic Properties of Cs0.97Rb0.03H2PO4
G. V. Lavrova, V. V. Martsinkevich, and V. G. Ponomareva
Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
e-mail: ponomareva@solid.nsc.ru
Received May 4, 2008
Abstract—The transport properties of Cs0.97Rb0.03H2PO4 have been studied using polycrystalline samples and
single crystals. The mixed salt is isostructural with cesium dihydrogen phosphate and has slightly smaller unit-
cell parameters. The cation substitution increases the low-temperature ionic conductivity of the material by
about two orders of magnitude but has an insignificant effect on the conductivity of the high-temperature phase.
The low-temperature conductivity of single-crystal samples exhibits significant anisotropy, with σa < σb c. The
conductivity of the polycrystalline material is close to σb c. The substitution reduces the temperature of the
superionic phase transition by 20°C and enhances the thermal stability of the high-temperature phase at low
humidity (1 mol % H2O).
DOI: 10.1134/S0020168509070164
INTRODUCTION
formula Cs1 – x(NH4)xH2PO4 (ı ≤ 0.1), reduces the phase
transition temperature by 10°C during heating and by
37°C during cooling [9, 10]. In addition, it stabilizes the
high conductivity of the salt, but the mechanism behind
this effect is not yet understood. In particular, it is
unclear whether this is due to the difference in cation
configuration or to the greater number of protons in the
ammonium ion and the higher concentration of hydro-
gen bonds in the system, even though these are known
not to be involved in proton transport in other acid salts.
In this context, it is of interest to assess the effect of par-
tial Rb+ substitution for Cs+ on the properties of cesium
dihydrogen phosphate, because these cations differ
less.
The high-temperature phase of CsH2PO4 (t ≥ 230°C)
has almost the highest electrical conductivity among
superprotonic salts. In addition, CsH2PO4 is potentially
attractive as an electrolyte for medium-temperature
fuel cells [1, 2]. The stability of this superionic material
is, however, influenced by the relative humidity; prepa-
ration, storage, and experimental conditions; and trace
impurities and water on its surface.
Humidity has a particularly strong effect on the
properties of this material: in a dry atmosphere, the
superionic phase transition is accompanied by dehydra-
tion, and the conductivity of the high-temperature
phase of CsH2PO4 drops sharply by orders of magni-
tude. At the same time, at an increased water vapor con-
tent in the ambient atmosphere (~30 mol %) the salt is
thermodynamically stable [3, 4]. This high humidity,
however, has an adverse effect on the mechanical prop-
erties of the electrolyte, and the necessity to humidify
the ambient atmosphere adds complexity to the fuel cell
design.
The purpose of this work was to study the transport
properties of Cs0.97Rb0.03H2PO4 and the thermodynamic
stability of its high-temperature phase.
EXPERIMENTAL
CsH2PO4 and Cs0.97Rb0.03H2PO4 single crystals were
grown from aqueous solutions containing appropriate
ratios of phosphoric acid, cesium carbonate, and rubid-
ium carbonate by isothermal evaporation. The crystals
Along with fine-particle oxide additions [5–8], one
possible way of extending the temperature stability
range of the superionic material is by cation or anion
substitution. Superionic acid salts obey the so-called
chemical pressure principle: an increase in the radius of were washed with acetone and heated at 150°C for 2 h
a cation or anion is equivalent to an increase in hydro- to remove the residual water. The cation composition of
static pressure, and vice versa. This suggests that sub-
stitution of a smaller sized cation for cesium may
reduce the phase transition temperature.
the crystals was determined by a combination of atomic
absorption and emission flame photometry (λ = 852.1 nm)
[11]. ç2PO–4 was determined by differential colorime-
Indeed, partial ammonium substitution for cesium try as the yellow vanadomolybdate complex [12]. From
in cesium dihydrogen phosphate, as represented by the the X-ray diffraction (XRD), colorimetry, and chemical
795

796
LAVROVA et al.
I
1
2
23
24
25
26
2θ, deg
1
2
20
30
40
50
60
70
2θ, deg
Fig. 1. XRD patterns of (1) Cs Rb H PO and (2) CsH PO at 25°C.
0.97 0.03
2
4
2
4
analysis data, the accuracy in the compositions 20–400°C at a heating rate of 10°C/min (argon, flow
obtained was estimated at 2%.
rate of 30 ml/min).
XRD patterns were collected on a Bruker powder
diffractometer (ëuKα1 radiation, continuous scan rate
of 2°/min) at room temperature. The crystal structures
of the salts at 235°C were determined at the Difrakt-
sionnoe Kino Station at the Siberian Synchrotron Radi-
ation Centre. The diffracted radiation (λ = 1.525 Å) was
detected by an OD-3 linear array detector [13] in the
angular range 2θ = 23°–54°. The samples were heated at a
rate of 10°C/min in an Anton Paar XRK900 chamber.
IR absorption spectra of powder samples preheated
at ~140–150°C for 10 h were measured in the range
580–4000 cm–1 on a Digilab Excalibur 3100 (ZnSe)
spectrometer.
RESULTS AND DISCUSSION
According to our XRD data, the low-temperature
phase of the synthesized CsH2PO4 has a monoclinic
structure (sp. gr. P21/m) with unit-cell parameters
a = 7.9120 Å, b = 6.3830 Å, c = 4.8802 Å, and β =
107.73°, V = 234.75 Å3, in agreement with previous data
[14] (Fig. 1). In the XRD pattern of Cs0.97Rb0.03H2PO4,
the reflections are shifted to larger angles, attesting to
the formation of a CsH2PO4-based solid solution with a
reduced unit cell. Its lattice parameters determined by a
fit among the 11 strongest reflections (IK software) are
a = 7.9052 Å, b = 6.3742 Å, c = 4.8757 Å, β =
107.8280°, and V = 233.89 Å3 (∆V within 0.4%).
Electrical conductivity was measured by a two-
probe ac method at frequencies from 12 Hz to 200 kHz
using an Instek LCR-821 impedance meter. For aniso-
tropic conductivity measurements, the crystals were
lapped in various crystallographic directions. The crys-
tals measured ~0.2 × 0.3 × 0.4 cm, and the crystal size
depended on the crystallographic direction (the (100)
face had the largest area). The conductivity of polycrys-
talline material was measured using samples 6 mm in
diameter and 1–2 mm in thickness with a relative den-
sity of ~95%. Electrical contacts were made with fine-
particle palladium paste or pressed fine-particle silver.
The measurements were performed isothermally or
during cooling at a rate of 0.1–0.2°C/min in air at ~15%
relative humidity.
XRD results show that, above 225°C, CsH2PO4
transforms into a cubic phase (sp. gr. Pm3m) with a =
4.961 Å (Fig. 2), in accordance with previous results
[15]. In the XRD pattern of the mixed salt, reflections
Thermal analysis (TG + DTA) was carried out with from the cubic phase emerge below 220°C. The
a Netzsch STA 449C system in the temperature range observed shift of the reflections indicates that the high-
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ELECTRICAL AND THERMODYNAMIC PROPERTIES OF Cs0.97Rb0.03H2PO4
797
2000
2
1000
1
800
35.5
36.0
2θ, deg
600
400
200
0
25
30
35
40
45
50
55
2θ, deg
Fig. 2. XRD patterns of the high-temperature phase of (1) Cs Rb H PO and (2) CsH PO .
0.97 0.03
2
4
2
4
–1
–2
–3
–4
–5
–6
–7
–8
1
2
3
4
1.8
2.0
2.2
2.4
2.6
2.8
3
–1
10 /T, K
Fig. 3. Arrhenius plots of conductivity for (1) polycrystalline and (2, 3) single-crystal Cs Rb H PO in comparison with
0.97 0.03
2
4
(4) CsH PO ; (2) σ , (3) σ
. Polycrystalline Cs Rb H PO was cooled at 0.1–0.2°C/min; the other measurements were
2
4
a
b
c
0.97 0.03 2 4
made at a cooling rate of 1–2°C/min in air.
INORGANIC MATERIALS Vol. 45 No. 7 2009

798
LAVROVA et al.
temperature phase also has a reduced unit-cell parame-
ter: a = 4.940 Å (∆V also within ~0.4%).
Cesium dihydrogen phosphate contains a system of
strong hydrogen bonds, which give rise to prominent IR
absorption bands in the spectral range 1700–2800 cm–1
[20]. The IR spectrum of the polycrystalline CsH2PO4
obtained in this study (Fig. 4a, spectrum 1) is very sim-
ilar to that reported by Marchon and Novak [20]. The
cation substitution gives rise to changes in the system
of hydrogen bonds (Fig. 4a). In particular, in the spec-
trum of Cs0.97Rb0.03H2PO4 a broad, strong absorption
band emerges in the range 3200–3400 cm–1, and the
2650-cm–1 band, arising from the OH stretching mode
(δéç), is shifted to higher frequencies (~2700 cm–1). In
addition, the substitution increases the intensity of the
broad bands in the range 1600–2400 cm–1 and shifts
them from 2320 and 1680 to 2300 and 1660 cm–1,
respectively. In the spectral range of the phosphate
group (600–1300 cm–1), we also observe significant
changes in the shape and relative intensity of absorption
bands (Fig. 4b): the bands at 868, 931, and 1068 cm–1
become broader and shift to lower frequencies (to 852, 918,
and 1055.4 cm–1, respectively), the band at 1130 cm–1
disappears almost completely, and the spectrum
becomes more symmetric. This behavior suggests that
partial rubidium substitution for cesium leads to a
reduction in hydrogen bond strength and structural dis-
ordering of the phosphate tetrahedra.
Figure 3 shows the Arrhenius plots of conductivity in
different crystallographic directions for Cs0.97Rb0.03H2PO4
in comparison with CsH2PO4. The proton conductivity
of the mixed salt in the superionic state is seen to be
close to that of CsH2PO4, whereas its low-temperature
proton conductivity is about two and half orders of
magnitude higher. Over the entire temperature range
studied, the activation energy for conduction in the
mixed salt is lower, which seems to be due to a higher
degree of structural disordering.
The conductivity of single-crystal Cs0.97Rb0.03H2PO4
depends on crystallographic orientation (Fig. 3). Note
that a correlation of conductivity anisotropy with the
crystal structure and hydrogen-bond configuration was
reported previously for a variety of salts. For example,
in the low-temperature phase of CsHSO4, where the
hydrogen bonds have the form of zigzag chains in the
[010] direction, the values of σ[010] and σ[001] differ by
two orders of magnitude [16]. The conductivity of
Cs5H3(SO4)4 · H2O and Cs5H3(SeO4)4 · H2O in the hexag-
onal or pseudohexagonal plane is one to two orders of
magnitude higher than that along the normal to this
plane [17], which is consistent with the two-dimen-
sional geometry of the dynamically disordered hydro-
gen bond network [18]. The compound CsH5(PO4)2 also
has the lowest conductivity in the [100] direction, nor-
mal to the layers of hydrogen-bonded phosphate tetra-
hedra [19].
The DTA curve of CsH2PO4 (Fig. 5) shows endot-
hermic peaks at 230, 280, and 345°ë, due to the superi-
onic phase transition, the formation of dimers, trimers,
or polymers as a result of partial dehydration, and melt-
ing, respectively. The dehydration of CsH2PO4 begins
near ~220°C, and the total weight loss on heating to
400°C amounts to ~7.8%, which corresponds to the
removal of two water molecules per formula unit. The
temperatures of the endotherms and the total weight
loss agree with previous results [3]. The DTA curve of
the mixed salt shows a broad endotherm between 138–
175°C, accompanied by a weight loss of 1.2%. This is
most likely due to the presence of hydrous phases on
the surface of the mixed crystals: the samples with par-
tial Rb substitution for Cs were more hygroscopic. The
substitution reduces the enthalpy of the superionic
phase transition. In addition, the dehydration endot-
herm around 280°C shifts to higher temperatures and
disappears almost completely. In the temperature range
230–310°C, the dehydration process slows down, and
the total weight loss decreases, indicating a rise in the
thermal stability of the high-temperature phase.
The low-temperature phase of CsH2PO4 has hydro-
gen bonds of different lengths: disordered hydrogen
bonds 2.48 Å in length and more ordered bonds 2.52 Å
in length [14]. As would be expected on structural
grounds, σb c (in the plane of the disordered hydrogen
bonds) is an order of magnitude higher than σa. The
observed reduction in phase transition temperature
(~20°C) upon partial rubidium substitution for cesium
is less significant than that upon ammonium substitu-
tion [10], which correlates with the ionic radii of Cs+,
Rb+, and NH+4 : 1.81, 1.66, and 1.42 Å, respectively. The
lower activation energy for conduction in ammonia-
substituted cesium dihydrogen phosphate was attrib-
uted to the lower N–O–H hydrogen bond strength, the
activation energy being lower at higher ammonium
contents. At the same time, partial rubidium substitu-
tion produces no additional hydrogen bonds but
reduces the hydrogen bond strength. Conductivity mea-
surements at different cooling rates (0.1–0.2 and
Indeed, long-term holding of Cs0.97Rb0.03H2PO4 at
235–240°C in air (1 mol % H2O) showed that the stabil-
2°C/min) demonstrate that partial Rb+ substitution for ity of the cation-substituted high-temperature phase
Cs+ hinders the kinetics of the transition from the supe-
rionic to the low-temperature, ordered phase.
was markedly higher than that of CsH2PO4 (Fig. 6). The
conductivity of the unsubstituted salt dropped by five
INORGANIC MATERIALS Vol. 45 No. 7 2009

ELECTRICAL AND THERMODYNAMIC PROPERTIES OF Cs0.97Rb0.03H2PO4
799
(‡)
0.015
0.010
0.005
2
1
0
4000
3500
3000
2500
2000
1500
–1
Wavenumber, cm
(b)
1
2
0.06
0.04
0.02
0
1000
800
600
1200
–1
Wavenumber, cm
Fig. 4. IR spectra of (1) CsH PO and (2) Cs Rb H PO .
2
4
0.97 0.03
2
4
orders of magnitude after 90 h, whereas that of the may influence both the thermodynamics of the dehy-
mixed salt dropped by no more than a factor of 1.5 after
170 h. XRD examination showed that the reflections
from the low-temperature phase of cesium dihydrogen
phosphate remained unchanged.
dration process (a reduction in the equilibrium water
vapor pressure in the mixed salt) and its kinetics. Both
possibilities may be associated with the formation of a
more stable, disordered high-temperature phase, with
partial amorphization, which would hinder cooperative
processes. To shed light on this problem, further inves-
tigation of the dehydration process in CsH2PO4 and par-
The question that now arises is what can be respon-
sible for the considerable rise in the thermal stability of
the salt in spite of the low degree of cation substitution?
There are two possible answers: cation substitution tially substituted analogs is needed.
INORGANIC MATERIALS Vol. 45 No. 7 2009

800
LAVROVA et al.
∆m/m, %
100
TG
98
96
94
92
90
88
86
84
1
2
DTA
82
50
100
150
200
250
300
350
400
Temperature, °C
Fig. 5. TG and DTA data for (1) Cs Rb H PO and (2) CsH PO ; heating in Ar at a rate of 10°C/min.
0.97 0.03
2
4
2
4
–1
–2
–3
–4
–5
–6
–7
1
2
0
20
40
60
80
100
Time, h
120
140
160
Fig. 6. Conductivity as a function of time for (1) Cs Rb H PO and (2) CsH PO during isothermal holding in air (235°C,
0.97 0.03
2
4
2
4
0.5 mol % H O).
2
CONCLUSIONS
dihydrogen phosphate based solid solution with
slightly reduced lattice parameters.
The mixed salt Cs0.97Rb0.03H2PO4 was synthesized,
and its structural, electrical, and thermodynamic prop-
erties were studied. The structure of the partially substi-
Rubidium substitution for 3 mol % cesium in
CsH2PO4 increases the low-temperature electrical con-
tuted salt Cs0.97Rb0.03H2PO4 corresponds to a cesium ductivity of the material by about two orders of magni-
INORGANIC MATERIALS Vol. 45 No. 7 2009

ELECTRICAL AND THERMODYNAMIC PROPERTIES OF Cs0.97Rb0.03H2PO4
801
CsH2PO4/xSiPyOz (x = 0.2–0.7) Composites, Neorg.
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tude but has an insignificant effect on the conductivity
of the high-temperature phase.
The low-temperature conductivity of single-crystal
8. Ponomareva, V.G. and Shutova, E.S., High-Temperature
Behavior of CsH2PO4 and CsH2PO4–SiO2 Composites,
J. Solid State Ionics, 2007, vol. 178, pp. 729–734.
samples exhibits significant anisotropy, with σa < σb
.
c
The substitution changes the enthalpy of the superionic
phase transition and the enthalpy of fusion. The thermal
stability of the high-temperature phase at low humidity
increases, suggesting that the mixed salt is a candidate
material for medium-temperature fuel cells.
9. Baranov, A.I., Dolbinina, V.V., Lancerose-Mendez, S.,
and Schmidt, V.H.S., Phase Diagram and Dielectric
Properties of Mixed Cs1 – x(NH4)xH2PO4 Crystals, Ferro-
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mization of Superprotonic Acid Salts for Fuel Cell
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ACKNOWLEDGMENTS
We are grateful to T.N. Drebushchak, S.V. Tsibulya
(Boreskov Institute of Catalysis, Siberian Division,
Russian Academy of Sciences), and M.R. Sharafutdi-
nov for their assistance with the X-ray work; to
A.P. Fedotov (Novosibirsk State University) for mea-
suring the IR spectra; to S.S. Shatskaya for determining
the chemical composition of the compound; and to
A.A. Matvienko for useful discussions.
11. Stolyarova, I.A. and Filatova, M.F., Atomno-absorbt-
sionnaya spektrometriya (Atomic Absorption Spectros-
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This work was supported in part by the Russian
Foundation for Basic Research, grant no. 07-03-12151.
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