In order to investigate a microstructure giving rise to a memory effect in the incommensurate phase of barium sodium niobate (BSN), in-situ observations of a tetragonal-to-incommensurate phase transition were made in both non-annealed and annealed samples by means of a transmission electron microscope. Note that only the annealed sample exhibits the memory effect found in the incommensurate phase of BSN. In a case of the non-annealed sample, when the sample is cooled from the high-temperature tetragonal phase, a region having a plate-shaped contrast, a microdomain structure, appears around 260°C, together with a region having the dot-shaped contrast. From the examination of the contrast reverse in the satellite dark field images with different reflection spots, the microdomain structure is understood to be composed of two Iq ferroelastic microdomains and can be regarded as the 2q tetragonal state in an average structure. The region of the dot-shaped contrast is, on the other hand, the lq orthorhombic state. On further cooling in the incommensurate phase, the microdomain structure disappears while large lq ferroelastic domains are formed around 220°C, In the annealed sample, however, the microdomain structure never disappears even at room temperature and appears reversibly during the heating and cooling processes. From these experimental results, the appearance of the memory effect is understood to originate from the stabilization of the microdomain structure regarded as the 2q state.
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