On the mechanism of superionic conduction in the zero-dimensional hydrogen-bonded crystals M3H(XO4)(2) with M = K, Rb, Cs and X = S, Se

In this paper we first present recent experimental results related to superionic conduction in the zero-dimensional hydrogen-bonded crystals of alkali-hydrosulfates or hydroselenates, M3H(XO4)(2) [M = K, Rb, Cs and X = S, Se]. Then a novel approach to the mechanism of proton conduction in the paraelastic phase is described. The key features of the conduction mechanism in the high temperature paraelastic phase which we call the superionic phase are: (1) Two kinds of ionic states, H2XO4(+e) and XO4(-e) are formed by breaking a hydrogen-bond thermally, (2) H2XO4(+e) and XO4(-e) ionic states move coherently from an XO4 tetrahedron to a distant XO4 as the result of successive proton tunneling among the hydrogen bonds. We calculate the density of states for the coherent motions of these ionic states by the recursion formula. From this result we show that a characteristic feature of the band-like states obtained from itinerancy of H2XO4(+e) and XO4(-e) ionic states is that of the Bethe lattice; that is the appearance of the twin peak structure due to self-similarity. The calculated conductivity is very high such as the order of 10(-2) S/cm at and above T-c, consistent with experiment. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.