Temperature and Concentration Dependence of Lattice Constants in a Rondom Mixture of Ferroelectric and Antiferroelectric; Rb_1−x(NH₄)_xH₂PO₄

Abstract

The measurements of thermal expansion in a random mixture of ferroelectric RbH₂PO₄(RDP) and antiferroelectric NH₄H₂PO₄(ADP), (RDP)_1−x(ADP)_x, were made by X-ray diffraction. Anomalous expansion of lattice constants was observed in glassy (RDP)_1−x(ADP)_x(0.22<x<0.75) at low temperatures, while the crystal symmetry was still tetragonal. Temperature dependence of the anomalous expansion can be related to the idea of cluster condensation. The NH₄-concentration dependence of the anomalous expansion implies that the glass is the mixture of two kinds of local strains at low temperatures.