COEXISTENCE OF THE 1Q AND 2Q INCOMMENSURATE 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 degrees 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.