CuCN: An orientational glass

Rhombohedral crystal phase of CuCN forms irreversibly on heating a mixture of its orthorhombic and monoclinic phases to 593 K. In its space group, R3m (a=b=0.6026 nm, c=0.4823 nm, α=β=90°, γ=120°, Z=3, V0=0.1516 nm3, and Dx=2.94 g/cm3), positions of C and N atoms are disordered, and therefore orientations of the CN group in the (N-Cu-C) chain structure are random. Its heat capacity increases in the manner of a sigmoid shape over the 350–380 K range, analogous to that observed on kinetic unfreezing of a canonical glass, as does that of a mixture of crystal phases obtained by flocculation process. The CuCN phase appears as the simplest orientational glass whose state is trapped in a deep minimum with a corrugated bottom, and each corrugation corresponding to one configurational excitation.