Microscopic study of superionic transition

The mechanism of superionic transition is discussed both phenomenologically and microscopically in the framework of the ''chemical bond peculiarities'' approach which was originally proposed by Phillips. The proximity of p-halogen and d-metal electronic levels is proved to be a critical prerequisite to the formation of a local additional minimum of crystalline potential. The fold coordination number change involved in the transition is regarded as a redistribution of ionic occupancies, rendering this additional minimum absolute. The low-energy excitations are ascribed to the ionic motion between the local and absolute potential minima, with their consequent appearance both at temperatures below and above the phase transition.