Breakdown of translational symmetry at the 300°C ferroelastic transition of barium sodium niobate

Abstract The temperature dependence of superlattice reflections has been investigated, in Ba 2 NaNb 5 O 15 , by X-ray oscillation photographs taken along the c -axis. The reflections vanish above the 300°C ferroelastic transition. This result confirms that the transition's order parameter corresponds to the Z -point of the Brillouin zone of the high-symmetry phase.