Diffuse X-ray scattering in the lead-free piezoelectric crystals Na 1/2Bi 1/2TiO 3 and Ba-doped Na 1/2Bi 1/2TiO 3

Abstract

X-ray diffuse scattering has been measured by single-crystal X-ray diffraction on the lead-free piezoelectric perovskites Na 0.5Bi 0.5TiO 3 (NBT) and (Na 0.5Bi 0.5) 0.89Ba 0.11TiO 3 (NBT–Ba 0.11). Evidence of a pronounced local structure or included phase different from the rhombohedral average symmetry (space group R3 c), or of a modulated structure with a characteristic period of 40 Å, 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) Å. By contrast, weaker, less anisotropic diffuse scattering is observed for tetragonal NBT–Ba 0.11 (space group P4 mm), 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.