Platelet NaNbO₃grown by single-step molten salt synthesis: Study on bismuth migration in topochemical conversion reaction

Platelet NaNbO₃grains were grown at 1150-1225 °C by single-step molten salt synthesis. The structural and compositional transformation from the precursor Aurivillius phase to perovskite NaNbO₃by the topochemical conversion reaction was studied. No compositional distribution was confirmed for the platelet grains grown at 1150 °C, whereas it was observed that the expulsion of bismuth and incorporation of sodium were simultaneously initiated in spots for the grains grown at 1170 °C. With increasing the growth temperature the topochemical conversion reaction was promoted, and single-phase NaNbO3 grains were eventually grown with heat treatment at 1225 °C for 6 h. In order to trace the structural transformation due to the topochemical conversion reaction, preconversion and postconversion platelet grains were chosen for characterizing the microstructure. It was found that the precursor Aurivillius phase is a mixed phase described as Bi_2.5Na_m-1.5Nb_mO_3m+3(m = 5, 6, and 8). In the interior of the platelet grains, migration paths vertically elongated to the principal surface are created, and bismuth is expelled via the vertical path as well as the horizontal path along the (Bi₂O₂)₂₊layer.