A GEOMETRICAL MODEL FOR MIXED CYANIDE CRYSTALS

A model of diluted random field sustained by quenched volume deformati ons is shown to reproduce puzzling physical features found in X(CN)(x) Y1-x mixed cyanide crystals. X is an alkali metal(K, Na or Rb) and Y i s a spherical halogen ion (Br, Cl or I). Critical thresholds x(c) at w hich associated first order ferroelastic transitions disappear are cal culated exactly. The diluted random field is shown to compete with com pressibility in making the transition first order. Transitions are the n found to remain first order down to x(c) except in the case of bromi ne dilution where they become continuous. All the results are in agree ment with available experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.