Structural Disorder in the Key Lead-Free Piezoelectric Materials, and

Using electron diffraction, trends in the local structural behaviour of the N Nb (KNN x) and the (NBT-( )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 and directions 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 directions 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. 1. Introduction (KNN x) became known as a promising lead-free piezoelectric ceramic system when relatively high piezoelectric coefficients (up to ~300？pC/N) along with Curie temperatures of up to 400°C were reported by Saito et al. in 2004 [1]. ( )Na0.5Bi0.5TiO3 + xBaTiO3 (NBT-xBT) is similarly high performing. Its raw piezoelectric performance is not as high as in KNN, ~160？pm/V [2], but it can display huge electrostrictive strains, up to 0.48% [3], which show promise for electrostrictive actuators with zero strain, as long as they are designed to account for their nonlinear response. These materials are among the few lead-free piezoelectric materials that can compete with lead-based materials in specialized applications. Due to their performance, the structure of materials such as KNN and NBT-BT is of considerable interest in order to establish structure-property relationships and to engineer improvements. At room temperature in the KNN system, there is long range ferroelectric order in the plane perpendicular to a particular parent perovskite direction (usually taken to be the direction in KNN). As long range ferroelectric order is established along both the and directions, the direction of the spontaneous polarization is then along a parent perovskite direction, in the case of the end-member KNbO3, and it might be expected to remain largely unchanged across the KNN phase diagram unless there is an unexpectedly strong coupling between