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 t he superionic state with a disordered system of hydrogen bonds and low-temp erature states at which the different types of ordering in the H-bond netwo rk appear las an example, the M3H(XO4)(2) class of crystals is considered). We apply a lattice-gas-type model incorporating the two-stage Grotthuss me chanism for proton transport and the interaction between the protons and di stortions 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 experiment al measurements as well as with the results obtained on the basis of a simp lified one-minimum approximation for the H-bond potential. Detailed analysi s of the activation energies in the low- and high-temperature phases allows us to infer a more complex character of the superionic transition, suggest ing that a possible intermediate transition from the state with more strong ly localized protons to the weaker proton-localized state occurred on heati ng between the ordered and superionic phases.