Diffuse X-ray scattering in the lead-free piezoelectric crystals Na1/2Bi1/2TiO3 and Ba-doped Na1/2Bi1/2TiO3

X-ray diffuse scattering has been measured by single-crystal X-ray diffraction on the lead-free piezoelectric perovskites Na_0.5Bi_0.5TiO₃ (NBT) and (Na_0.5Bi_0.5)_0.89Ba_0.11TiO₃ (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 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 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–Ba₀.₁₁ 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–Ba₀.₁₁, may be the key to understanding the local structure found repeatedly at room temperature in this complex material.