Spontaneous twisting of an incommensurable improper ferroelastic Ba2NaNb5O15

The spontaneous twisting of a y-orientation single-crystal sample of barium-sodium niobate Ba₂NaNb₅O₁₅ at a phase transition and into an incommensurable ferroelastic phase is investigated. Anomalies are found in the temperature dependence of the torsional deformation at temperatures 453, 533, and 573 K and anomalous hysteresis of the temperature dependences of the torsional deformation, measured on heating and cooling of the sample. The results obtained are discussed on the basis of an analysis of the temperature dependence of incommensurable 1q and 2q structures and ferroelastic twins. Fiz. Tverd. Tela (St. Petersburg) 41, 1848–1850 (October 1999)