Ferroelasticity and phase transformation in Rb2Hg(CN)4 spinel

Abstract

The low-temperature form of Rb2Hg(CN)4 [space group R\bar 3c, Z = 12, a 1 = 9.076, a 3 = 46.050 Å (273 K)] possesses a slightly distorted spinel structure with parameters similar to those of cubic cyanospinels. It transforms into a cubic spinel modification at ca 398 K (a = 13.140 Å at 399 K). Below the transition temperature the crystals exhibit ferroelastic properties, which allow the switching of the distortion directions into any one of four equivalent states. The critical stress is less than 1 x 10-3 kp mm-2. The resulting deformations do not exceed 2 x 10-2. Larger twinned crystals return to their primary state after the release of stress. They behave like extremely soft elastic springs. The crystals belong to the ferroelastic Aizu species m3m F\bar 3m. The transition shows a constant hysteresis of ca 12 K in all crystals investigated. The pressure derivatives of the upper and lower transition temperatures are observed to be almost equal and constant over a wide temperature range (dT/dP ≈ O. 245 K bar-l). The observed enthalpy of transformation is 1.03 J g-1 in fair agreement with the prediction of the Clapeyron-Clausius relation. Crystals of K2Hg(CN)4 exhibit similar properties; their transition temperature, hysteresis, enthalpy of transformation, and dT/dP are considerably less.