Short-Range Interaction Explanation of Ferroelectric, Antiferroelectric and Proton Glass Behavior in Pure RDP, ADP, and Mixed RDP-ADP Crystals Respectively

Abstract

A unified model is presented for Rb_1-x (NH₄) _x H₂PO₄ crystals over the whole range x=0 (RDP) to x=1 (ADP). Two short-range interactions are postulated. One is the Slater energy ε ₀ which is kept at the value for RDP for all x. The other is an interaction ε _a between two hydrogens in O-H…O bonds across an NH₄ ⁺ ion from each other. It is responsible for the off-center positions observed for ammonium ions in the antiferroelectric phase of ADP. Its strength is assumed proportional to x. By minimizing the free energy, one finds a range of x for which no transition occurs, but instead proton glass behavior sets in. Fox x near 0 and 1 respectively, first-order ferroelectric and antiferroelectric transitions are predicted. Both phase boundaries are close to those observed experimentally.