A novel approach to the mechanism of ionic conductivity below and at the ferroelastic phase transition in the zero-dimensional hydrogen-bonded crystals M₃H(X0₄)₂ with (M = Rb or Cs; X = S or Se)

Abstract

The temperature dependence of the proton conductivity in M₃H(X0₄)₂ (M = Rb or Cs; X = S or Se) is calculated for the low-conducting ferroelastic phase near the phase transition temperature, based on the phase transition theory and the novel mechanism of ionic conductivity proposed by Ito and Kamimura. The conductivity below the phase transition temperature can be explained by the following theoretical results: firstly, below T < T _c, in the presence of an alternating electric field the ferroelastic phase changes to a new stripe phase consisting of the stripe domains in which the distances between XO₄ groups are the same and of the intervening regions in which XO₄ groups form XO₄-H-XO₄ dimers by hydrogen bonds; secondly, the stripe domains contribute to the ionic conductivity for T ≤ T _c; thirdly, the propagation of protons along the zigzag paths in the stripe domains lead to the (T_c-T)^−½ power law in the temperature dependence of the ionic conductivity.