A novel approach to the mechanism of ionic conductivity below and at the ferroelastic phase transition in the zero-dimensional hydrogen-bonded crystals M3H(XO4)(2) with (M = Rb or Cs; X = S or Se)

The temperature dependence of the proton conductivity in M3H(XO4)(2) (M = R b or Cs. X = S or Se) is calculated for the low-conducting ferroelastic pha se near the phase transition temperature, based on the phase transition the ory and the novel mechanism of ionic conductivity proposed by Ito and Kamim ura. The conductivity below the phase transition temperature can be explain ed by the following theoretical results: firstly, below T < T-c, in the pre sence of an alternating electric field the ferroelastic phase changes to a new stripe phase consisting of the stripe domains in which the distances be tween XO4 groups are the same and of the intervening regions in which XO4 g roups form XO4-H-XO4 dimers by hydrogen bonds; secondly, the stripe domains contribute to the ionic conductivity for T <less than or equal to> T-c; th irdly, the propagation of protons along the zigzag paths in the stripe doma ins lead to the (T-c - T)(-1/2) power law in the temperature dependence of the ionic conductivity.