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This paper presents the crystal structure and the phase transitions of K x Na 1− x NbO 3 (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100–800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic–tetragonal and tetragonal–cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.

The crystal structure of the 1 + 2 mixed-layer Aurivillius phase Bi5Ti1.5W1.5O15 (SG I2cm no 46: − c b a , Z = 4 , a = 5.4092 ( 3 ) A , b = 5.3843 ( 3 ) A and c = 41.529 ( 3 ) A ) consisting of the ordered intergrowth of one and two octahedra thick perovskite-type blocks separated by [Bi2O2]2+ slabs is reported. Supported by an electron diffraction investigation and, using the Rietveld analysis, it is shown that this compound should be described using a I-centering lattice in agreement with the generalised structural model of the Aurivillius type compounds recently presented by the authors. The structure of this Bi5Ti1.5W1.5O15 phase is analyzed in comparison with the related simple members (Bi2WO6 and Bi3Ti1.5W0.5O9). The crystal structure of Bi3Ti1.5W0.5O9 is also reported.