Detwinning cell for high-transition-temperature atmosphere-sensitive ferroelastic crystals

A universal cell is reported for detwinning ferroelastic crystals, with transition temperatures that range from 300 to 1300 K, in a controlled atmosphere. A carefully oriented compressive stress on the order of 0.1-1 MN m^-2 is applied to the twinned crystal at room temperature. The crystal temperature is raised from 300 K through the phase transition and is slowly lowered again under stress. The cell has been used to detwin crystals of Na₅W₃O₉F₅ with a Curie temperature of 800 K in an atmosphere of flowing high-purity oxygen, and crystals of Rb₂KMoO₃F₃ with a Curie temperature of 328 K in air. Successful detwinning is demonstrable by the complete conversion of high-angle multiple-component line profiles diffracted by the as-grown crystal to single-component reflection profiles.