Phys. Rev. B 89, 174102 (2014)

Credit: AMERICAN PHYSICAL SOCIETY

Piezoelectric effects coupling electrical and strain fields in materials are crucial for a wide range of applications including sensors, actuators, capacitors, memories and medical diagnostic devices. As the most widespread piezoelectrics tend to be lead-based, there is at present a concerted effort to bring environmentally friendly lead-free piezoelectric materials to the market. The most promising of these is thought to be the perovskite system Na0.5Bi0.5TiO3 (NBT). Frustratingly, however, a definitive understanding of its structural properties has been elusive. The oxygen octahedra that form the backbone of its structure tend to tilt away from the ideal rhombohedral symmetry, and these tilts are notoriously difficult to detect using conventional X-ray and neutron diffraction approaches. Techniques based on electron diffraction overcome this problem, but typically only provide data over a restricted angular range. Now, thanks to a technique recently developed by Richard Beanland and colleagues, this difficulty may be a thing of the past. By capturing many electron diffraction patterns as a function of beam tilt, they are able to dramatically increase the angular range, and therefore improve the ease of interpretation of a structure. In the case of NBT, the technique provides unique insights into its symmetry, including unequivocal proof that in defect-free regions of the material, the rhombohedral structure persists down to the length-scale of a few nanometres.