Theory of Unusual Lattice Constant Changes in Glassy Phase of Rb1-x(NH4)xH2PO4

Abstract
Within the cluster approximation, a theory is given to explain the unusual temperature dependence of the lattice constants observed in the glassy phase of Rb 1- x (NH 4 ) x H 2 PO 4 . It is shown that the lattice constant a (or c ) always expands (or contracts) below the glass transition point T g irrespective of the value of x in the intermediate region.