Low-temperature glasslike properties in (NaCl)1−x(NaCN)x

Thermal conductivity, internal friction, transverse sound velocity (60 mK to 300 K), and specific-heat data (100 mK to 40 K) for (NaCl${)}_{1\mathrm{-}\mathit{x}}$(NaCN${)}_{\mathit{x}}$ (x=0, 0.025, 0.05, 0.1, 0.76, 1) show a progression from crystalline to glasslike behavior as the ${\mathrm{CN}}^{\mathrm{-}}$ concentration is increased from 0 to 76 %. The evolution of glasslike properties is compared to that in other crystals in which glasslike properties evolve with increasing disorder, e.g., (KBr${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$ and ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{F}}_{2\mathrm{-}\mathit{x}}$. For (KBr${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$, Sethna and Chow have shown that as the concentration of the almost freely rotating ${\mathrm{CN}}^{\mathrm{-}}$ ions is increased the average potential barrier for ${\mathrm{CN}}^{\mathrm{-}}$ reorientation also increases through elastic quadrupolar interactions. For x∼0.5, only a small density of low-energy states is left, which equals that observed in structural glasses. In ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{F}}_{2\mathrm{-}\mathit{x}}$, on the other hand, the crystal field for small doping x is so large that no atomic motion occurs at low temperatures. (NaCl${)}_{1\mathrm{-}\mathit{x}}$(NaCN${)}_{\mathit{x}}$ is shown to represent an intermediate case, in that the crystal field is non-negligible at small x, yet glasslike low-energy excitations indicative of very small potential barrier heights evolve with increasing x. It is argued that random internal strains cause a decrease of the barrier heights in these crystals, which lead to the low-energy excitations. It is proposed that random strains have a similar effect in other disordered crystals as in ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{F}}_{2\mathrm{-}\mathit{x}}$, which for small x show no low-energy mobile states, yet which for large x become glasslike.