The temperature dependence of the proton conductivity was calculated, for the low-conductivity ferroelastic phase near to the phase transition temperature, on the basis of phase transition theory and the Ito-Kamimura mechanism of ionic conductivity. The conductivity below the phase transition temperature could be explained by various behaviors. Firstly, at temperatures below Tc, in the presence of an alternating electric field, the ferroelastic phase changed into a new stripe phase which consisted of stripe domains where the distances between XO4 groups were the same, and intervening regions in which the XO4 groups formed XO4-H-XO4 dimers via H bonds. Secondly, the stripe domains contributed to the ionic conductivity below Tc. Thirdly, the propagation of protons along zig-zag paths in the stripe domains led to a (Tc-T)-½ dependence of the ionic conductivity.

A Novel Approach to the Mechanism of Ionic Conductivity below and at the Ferroelastic Phase Transition in Zero-Dimensional Hydrogen-Bonded Crystals M3H(XO4)2 with M = Rb or Cs; X = S or Se. H.Kamimura, S.Watanabe: Philosophical Magazine B, 2001, 81[9], 1011-9