NEW MECHANISM OF IONIC-CONDUCTIVITY IN HYDROGEN-BONDED CRYSTALS M3H(XO4)(2) [M=RB, CS, X=S, SE]

A model for the mechanism of ionic conductivity in the high temperatur e paraelastic phase of M3H(XO4)(2) [M=Rb, Cs; X=S, Se] type crystals i s proposed. The key features of the conduction mechanism are the follo wing; (1) two kinds of defect states, H2XO4(+e) and XO4(-e) are formed thermally by breaking of a hydrogen-bond; (2) the H2XO4(+e) defect st ate and the XO4(-e) defect state move coherently from an XO4 tetrahedr on to a distant XO4 as the result of successive proton tunneling among hydrogen-bonds. The density of states and the mobility are calculated for the coherent motions of these defect states by the recursion meth od and the Kubo formula, respectively. The density of states shows the characteristic feature of the Bethe lattice, i.e., the twin peak stru cture due to self-similarity, while the conductivity is obtained as an order of a magnitude of 10(-3)Omega(-1)cm(-1) at the ferroelastic tra nsition temperature, consistent with experiments.