Solid acid proton conductors: the influence of hydrogen subsystems on physical properties

Abstract

Interest in superprotonic crystals of M m H n (XO 4 ) (m+n)/2 (М = K, Rb, Cs, NH 4 ; X = S, Se, P) is associated with the solution to fundamental problems of modern condensed matter physics: establishment of structural conditionality of anomalies of physical properties with the aim of designing new functional materials. Superprotonic crystals belong to a special class. As opposed to other hydrogen-containing compounds, phase transitions in these crystals are accompanied by a hydrogen-bond network rearrangement, resulting in a radical change in the physicochemical properties; in particular, these transitions give rise to proton conductivity of about 10 . These crystals are unique in the class of proton conductors, since the superprotonic conductivity is related to the structural features of these compounds, instead of consequence of impurity doping process. The occurrence of high superprotonic conductivity in the M 3 H(XO 4 ) 2 crystals is associated with the formation of a qualitatively new and dynamically disordered hydrogen-bond system. This fact was found for the first time in the studies of structural phase transitions in the Rb 3 H(SeO 4 ) 2 crystals [1] and was then confirmed for other M 3 H(XO 4 ) 2 crystals, including K 3 H(SO 4 ) 2 [2]. In K 9 H 7 (SO 4 ) 8 •Н 2 О crystals, the only known representative of the M 9 H 7 (XO 4 ) 8 •xН 2 О family, the superprotonic phase transition at 405 K is associated with the diffusion of crystallization water, the hydrogenbond network rearrangement, and the formation of channels for the possible motion of K atoms [3,4]. The hydrogen-bond rearrangement and the hindered back diffusion of water to the crystal stabilize the high-temperature phase and ensure its supercooling to low temperatures.

Investigation of the structure of Cs3(HSO4)2(H2PO4) single crystals

“…In K 3 H(SO 4 ) 2 [8] and K 9 H 7 (SO 4 ) 8 ⋅ Н 2 О [11] crystals, which also belong to the M m H n (XO 4 ) (m + n)/2 ⋅ yН 2 О family, the mean S-O bond lengths in SO 4 tetrahedra are, respectively, 1.481(1) and 1.468(2)-1.476(2) Å. Despite the difference in the systems of hydrogen bonds, the S-O bond lengths in these compounds are also close to the corresponding values for CsHSO 4 crystals.…”

“…A structural analysis of K 9 H 7 (SO 4 ) 8 ⋅ Н 2 О crystals (which are currently a single representative of the M 9 H 7 (XO 4 ) 8 ⋅ yН 2 О subgroup) with the aid of X-ray and synchrotron radiation beams [10,11] showed that the main changes in the atomic structure of crystals at a phase transition are related to the diffusion of crystallization water and subsequent transformation of the system of hydrogen bonds. As a result, a new system of hydrogen bonds is formed which is partially dynamically disordered and contains additional sites for protons.…”