The mechanisms of superionic transitions were considered phenomenologically and microscopically within the framework of the so-called chemical bond peculiarities approach. The proximity of p-halogen and d-metal electronic levels was proved to be a critical requirement for the formation of a local additional minimum in the crystalline potential. The coordination number change which was involved in the transition was explained in terms of a redistribution of ionic occupancies which rendered this additional minimum absolute. The low-energy excitations were attributed to ionic motion between the local and absolute potential minima, with their resultant appearance at temperatures both below and above the phase transition.

A.Rakitin, M.Kobayashi: Physical Review B, 1996, 53[6], 3088-94