Orientational glasses. I. A random compressible model for KCN-KBr

Abstract

The full phase diagram of K(CN)~Bri _~ mixtures is obtained using a mean-field treatment of a dilute random anharmonic compressible ferroelastic model. Integration of elastic degrees of freedom leads to a non-linear percolating problem. No random competing interactions are found. In parallel a simple linear hypothesis for an effective density of cyanides is assumed to account for the asymmetric shape of the cyanide versus the spherical bromide shape. Site percolation of cyanides is thus discriminated from the corresponding orientational percolation. Three characteristic values of cyanide concentration x = 0.2, 0.5 and 0.6 are obtained. For x < 0.6 the ferroelastic transition is lost. In the range 0.5 < x < 0.6, at low temperatures, cyanides form ferroelastic domains with random orientations. When 0.2 < x < 0.5 a collective dynamical glass is obtained. It tums to a single ion state for x < 0.2. It is then shown that the corresponding phase diagram of K~(Naj ~)CN mixtures is of a different physical nature. It involves in particular random competing interactions. A critical review of the orientational random field model is presented. The results are found to provide a unified and consistent framework to various experimental data. 1196 JOURNAL DE PHYSIQUE I M 8