Microstructure related to a low-temperature transition in Ba₂NaNb₅O₁₅

In order to investigate a microstructure in a low temperature region of Ba₂NaNb₅O₁₅. in-situ observation was made in the temperature range between room temperature and 25K by transmission electron microscopy. It was found that, in addition to satellite reflection spots due to the incommensurate phase, both new satellite reflection spots at h, k, 1 ±1/2 positions and diffuse streaks running through h. k±1/2. 1 ±1/2 positions along the [100] direction appear around 200K in diffraction patterns on cooling, together with the appearance of the microdornams elongated along the [010] direction in dark field images. It is indicated from an analysis of both the diffraction patterns and dark field images that the streak is due to an irregular arrangement of out-of phase boundaries between the microdomains. A nature of the low-temperature transition in Ba₂NaNb₅O₁₅ is briefly discussed.