Orientational ordering in mixed cyanide crystals: (NaCN)1−x(KCN)x

Abstract

The technique of constant-pressure molecular dynamics is used to investigate the influence of random strain fields on the structure of solid (NaCN${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(KCN${)}_{\mathit{x}}$ mixtures at two concentrations in the orientational-glass-forming range, namely x=0.50 and 0.85. The simulations reveal that ${\mathrm{Na}}^{+}$ ions act as nucleation centers that lock neighboring ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ ions into one of six equivalent 〈100〉 orientations, with the C end preferentially pointing towards ${\mathrm{Na}}^{+}$. In the x=0.5 mixture, ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ ions favor 〈100〉 orientations at all temperatures studied. At the lower ${\mathrm{Na}}^{+}$ concentration, x=0.85, ${\mathrm{Na}}^{+}$ 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 ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ at low ${\mathrm{Na}}^{+}$ concentration.