Optical studies of thermal cycling and hysteresis effects in elastic order-disorder phase transformations. I. Pure alkali-metal cyanide crystals

The order-disorder transformations in NaCN, KCN, and RbCN are studied under thermal cycling around $T_c$ by monitoring the change in optical transmission produced by the light-scattering properties of their multidomain ordered structures. The phase transitions in both NaCN (large-hysteresis cubic ⇆ orthorhombic) and RbCN (hysteresis-free cubic ⇆ monoclinic) remain basically unchanged under multiple thermal recycling. In contrast to these persistently ferroelastic (NaCN) and antiferroelastic (RbCN) ordering systems, KCN acts as a ‘‘bistable’’ borderline case at its critical temperature $T_c$≊168 K. After passing a large-hysteresis cubic ⇆ orthorhombic transition in its first thermal cycle, a ‘‘memory’’ of it (preserved up to room temperature) causes, in all subsequent thermal cycles, hysteresis-free cubic ⇆ monoclinic transitions at nearly the same $T_c$. When cooling beyond $T_c$, a gradual monoclinic → orthorhombic transition occurs around 155 K, which, under subsequent heating, returns with large (∼15 K) hysteresis back to the cubic structure. The well-correlated optical- and dielectric-constant changes of these transitions in KCN under various thermal-cycling conditions are presented and discussed.