Structural disorder in the key lead-free piezoelectric materials, K _xNa_1-x NbO₃ and (1 - X) Na_0.5 Bi_0.5 TiO₃ + x BaTiO₃

Using electron diffraction, trends in the local structural behaviour of the K_x Na_1-x NbO₃ (KNN x) and the (1 - x) Na _0.5 Bi_0.5 TiO₃ + x BaTiO₃ (NBT-(x)BT) systems are investigated. In KNN, electron diffraction shows a single plane of diffuse intensity perpendicular to [010] across the entire phase diagram, indicating the existence of ferroelectric disorder along this axis. An additional characteristic pattern of diffuse scattering is also observed, involving rods of diffuse intensity running along the 100 pand 001 pdirections of the perovskite substructure and indicative of octahedral tilt disorder about these axes. Similarly, in the NBT-xBT system, rods of diffuse intensity running along the 100 pdirections of the perovskite substructure are observed, again indicating octahedral tilt disorder. Ferroelectric-like disorder is also observed in highly BT doped samples, and a continuous change from the "rhombohedral" structure of NBT to the "tetragonal" structure of NBT-12BT is seen from characteristic variation in observed superstructure reflections. A crystal chemical rationalisation of these results is performed, and the implications for structure and properties are discussed.