The superionic phase transition in hydrogen-bonded systems of the M₃H(XO₄)₂ class: the effect of nearest-O(2) distortions on the proton dynamics

We study the behaviour of protonic conductivity at the transition between the superionic state with a disordered system of hydrogen bonds and low-temperature states at which the different types of ordering in the H-bond network appear (as an example, the M₃H(XO₄)₂ class of crystals is considered). We apply a lattice-gas-type model incorporating the two-stage Grotthuss mechanism for proton transport and the interaction between the protons and distortions of nearest oxygen ions causing a strong quasi-polaron effect. The temperature dependences of the conductivity and its reorientational inter-bond and intra-bond contributions are analysed and compared with experimental measurements as well as with the results obtained on the basis of a simplified one-minimum approximation for the H-bond potential. Detailed analysis of the activation energies in the low- and high-temperature phases allows us to infer a more complex character of the superionic transition, suggesting that a possible intermediate transition from the state with more strongly localized protons to the weaker proton-localized state occurred on heating between the ordered and superionic phases.