Diffuse X-ray scattering in the lead-free piezoelectric crystals Na_(1/2)Bi_(1/2)TiO_3 and Ba-doped Na_(1/2)Bi_(1/2)TiO_3

摘要

X-ray diffuse scattering has been measured by single-crystal X-ray diffraction on the lead-free piezo-electric perovskites Na_(0.5)Bi_(0.5)TiO_3 (NBT) and (Na_(0.5)Bi_(0.5))_(0.89)Ba_(0.11)TiO_3 (NBT-Ba_(0.11)). Evidence of a pronounced local structure or included phase different from the rhombohedral average symmetry (space group R3c), or of a modulated structure with a characteristic period of 40 A, is presented for NBT from high-resolution synchrotron X-ray mapping and CCD diffractometer investigations. The additional scattering shows pronounced anisotropy and produces satellite peaks displaced by 0.2 in the h and k indices for reflections such as {640} indexed on the pseudo-cubic unit cell with a = 7.761(2) A. By contrast, weaker, less anisotropic diffuse scattering is observed for tetragonal NBT-Ba_(0.11) (space group P4mm), which is directed along the pseudo-cubic <110>. The lack of satellite peaks and pronounced anisotropic diffuse scattering in the tetragonal phase of phase NBT-Ba_(0.11) lends support to the idea that the massive tetragonal to rhombohedral phase transition in NBT at around 503-573 K, which is absent in NBT-Ba_(0.11), may be the key to understanding the local structure found repeatedly at room temperature in this complex material.