Cluster Theory of Glass Transition in Rb_1-x(NH₄)_xH₂PO₄

Abstract

A theory of glass transition in hydrogen-bonded mixed crystals of ferroelectrics and antiferroelectrics, Rb 1- x (NH 4 ) x H 2 PO 4 , is developed within the cluster approximation on the basis of Ising pseudo-spin model for the proton system. By means of the replica method, it is shown that competition between two kinds of proton ordering under the ice rule constraint leads the system to the new glass state, and glass transition temperature T g depends on x in a way characteristic to this system. The phase diagram derived theoretically is in good agreement with the experimental results in all temperature-composition plane.