Coexistence of the 1qand 2qincommensurate phases and memory effect in barium sodium niobate

Using high-resolution x-ray diffraction on single crystals we specify the phase diagram of barium sodium niobate between 20 and 300 ^∘{}C. Its main characteristics are the coexistence of two incommensurate phases in the vicinity of the normal-incommensurate transition, a 2q tetragonal phase and a 1q orthorhombic phase, and a progressive 2q/1q transformation in the temperature range where a large thermal hysteresis of the misfit parameter is observed. A complex memory effect is evidenced when the sample is annealed inside the incommensurate phases or the quasicommensurate phase: a pinning of the modulation on a very large plateau of temperature and a delay or an anticipation of the 2q/1q transformation and of the lock-in transition is evidenced. We discuss our results on the basis of the 2q/1q phase coexistence and the existence of mobile effects in the structure of barium sodium niobate (sodium vacancies) interacting with the modulations.