ORIENTATIONAL ORDERING IN MIXED CYANIDE CRYSTALS - (NACN)1-X(KCN)X

The technique of constant-pressure molecular dynamics is used to investigate the influence of random strain fields on the structure of solid (NaCN)1-x(KCN)x mixtures at two concentrations in the orientational-glass-forming range, namely x = 0. 50 and 0. 85. The simulations reveal that Na+ ions act as nucleation centers that lock neighboring CN- ions into one of six equivalent <100> orientations, with the C end preferentially pointing towards Na+. In the x = 0.5 mixture, CN- ions favor <100> orientations at all temperatures studied. At the lower Na+ concentration, x = 0.85, Na+ lock most neighboring anions into <100> directions at a relatively high temperature. Upon further cooling, however, overall freezing into <111> orientations dominates. The competition between these two freezing-in processes leads to an anomalous temperature dependence of the orientational-order parameters for CN- at low Na+ concentration.