0020-1685/04/4007- © 2004 MAIK “Nauka /Interperiodica” 0721 Inorganic Materials, Vol. 40, No. 7, 2004, pp. 721–728. Translated from Neorganicheskie Materialy, Vol. 40, No. 7, 2004, pp. 826–834. Original Russian Text Copyright © 2004 by Ponomareva, Shutova, Matvienko. INTRODUCTION Alkali-metal and ammonium hydrogen sulfates and selenates with the general formulas MHXO 4 , M 5 H 3 (XO 4 ) 4 , and M 3 H(XO 4 ) 2 (M = Cs, Rb, K, Na, Li, NH 4 ; X = S, Se) constitute one of the largest families of solid proton electrolytes [1–3]. These compounds, as a rule, undergo a phase transition from a superionic phase, characterized by a dynamically disordered hydrogen-bond network, to a low-temperature phase. This transition is accompanied by a sharp drop in elec- trical conductivity, by several orders of magnitude. The conductivity of the high-temperature phase is 10 –2 to 10 –3 S/cm, with an activation energy E a = 0.2–0.3 eV, and that of the low-temperature, ordered phases is 10 –6 to 10 –10 S/cm, with E a = 0.6–1.6 eV. Owing to the chemical similarity between alkali- metal hydrogen sulfates and hydrogen phosphates and the structural similarity between their low-temperature phases, there are a large number of mixed compounds with different S : P ratios and a rather broad range of hydrogen contents. This suggests that, in spite of the marked difference in electrical properties between the constituent sulfates and phosphates, the mixed salts may have enhanced proton conductivity in the low-tem- perature phase. The properties of hydrogen sulfate phosphates, including Cs 3 (HSO 4 ) 2 (H 2 PO 4 ), are essentially unex- plored. CsHSO 4 - and CsH 2 PO 4 -based solid solutions were investigated in [4–6]. The low-temperature con- ductivity of the intermediate phases existing in this system was found to be higher than that of the end members. Interesting data were obtained on the structure, phase transitions, and transport properties of the mixed salts Cs 3 (HSO 4 ) 2 H 2 PO 4 (α phase) and Cs 3 (HSO 4 ) 2 (H x (P, S)O 4 ) (β phase). Those data point to the existence of a superprotonic high-temperature phase and a rather high low-temperature conductivity [4, 5]. According to Haile et al. [4], Cs 3 (HSO 4 ) 2 (H 2 PO 4 ) under- goes a disorder–order phase transition on cooling, accompanied by a reduction in conductivity by about two orders of magnitude, from 6 × 10 –3 to 2 × 10 -5 S/cm, and an increase in activation energy from 0.45 to 0.9 eV. As in the case of CsHSO 4 , the increase in conductivity is associated with rotation of the sulfate and phosphate tetrahedra. The low-temperature phase of Cs 3 (HSO 4 ) 2 H 2 PO 4 has a monoclinic structure (sp. gr. P2 1 /n) with lattice parameters a = 19.546 Å, b = 7.8798 Å, c = 9.1854 Å, and β = 100.534°. The struc- ture is made up of zigzag rows of AO 4 sulfate and phos- phate groups, which alternate with Cs rows. The anion rows are connected by hydrogen bonds into chains, which, in turn, form a dense three-dimensional frame- work [4]. The high-temperature phase has a bcc struc- ture with a = 6.961 Å. As reported by Haile et al. [4], this phase decomposes at 190°C. The structure of the β phase, which is close in lattice parameters to the α phase, has a different arrangement of the phosphorus atoms and a random distribution of the P and S atoms over the tetrahedra, which gives rise to compositional instability of the β phase (a tendency toward phospho- rus deficiency). The sharp change in conductivity across the phase transition restricts the application field of the com- pounds in question. The introduction of fine-particle silica as a dopant may stabilize the high-conductivity, disordered state of these salts and enhance their thermal stability [7–10]. The study of the effect of second-phase inclusions on the behavior of mixed salts is of interest Conductivity of Proton Electrolytes Based on Cesium Hydrogen Sulfate Phosphate V. G. Ponomareva, E. S. Shutova, and A. A. Matvienko 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 November 4, 2003 Abstract—The conductivity and thermal behavior of cesium hydrogen sulfate phosphate are studied. New composite proton electrolytes (1 – x)Cs 3 (HSO 4 ) 2 (H 2 PO 4 )–xSiO 2 with a high conductivity in the range 60– 200°C are prepared, and their transport properties are studied in a broad composition range (x = 0.3–0.95). Their conductivity exceeds that of CsHSO 4 –SiO 2 composites and depends strongly on composition: it reaches a max- imum at x = 0.7 (22 vol % SiO 2 ) and drops at higher silica contents on account of percolation disruption. It is shown by differential scanning calorimetry and x-ray diffraction analysis that the introduction of fine-particle silica stabilizes the high-conductivity, disordered state of the mixed salt on the surface of the silica particles.