Features of a tetragonal-to-incommensurate transition in Barium-Sodium-Niobate, which is obtained in a cooling process, were examined both experimentally and theoretically. In-situ observation of the transition by means of transmission electron microscopy shows that on cooling from the tetragonal phase a characteristic mazy pattern appears in satellite dark field image just below the tetragonal-to-incommensurate transition. On further cooling to room temperature, the mazy pattern is continuously changed into a pattern showing a black line contrast due to a discommensuration. In addition, the details of a phase modulation in the incommensurate structure are discussed on the basis of the Ginzburg-Landau theory.
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