High‐Temperature Phase Transitions in Na Nb O 3

An X‐ray diffraction study of a high‐purity NaNbO 3 specimen in the range 25α to 705α C is reported. The symmetry of the perovskite sub‐cell of NaNbO 3 was determined unambiguously by observing the splitting of the line groups N = 12 and N = 16 when the diffraction pattern was indexed on the basis of the perovskite cell as a unit cell for the structure. Diffraetometter studies were conducted using Fe K α radiation for the N = 12 line group and Cu K α radiation for the N = 16 line group. Phase transitions occurred at 372°, 420°, 478°, 527°, 576°, and 640°± 5°C. The symmetry of the perovskite sub‐cell changed from monoclinic to tetragonal at 372°C when heated and remained tetragonal up to 640°C when it became cubic. Two‐phase mixtures were observed at 372°, 576°, and 640°C and it was concluded that these transitions are probably of first order.