Local structure of the lead-free relaxor ferroelectric (K{sub x}Na{sub 1-x}){sub 0.5}Bi{sub 0.5}TiO{sub 3}

The local environment of Bi and Ti atoms in the lead-free relaxor ferroelectric solid-solution (K{sub x}Na{sub 1-x}){sub 0.5}Bi{sub 0.5}TiO{sub 3} has been studied as a function of K concentration and as a function of temperature for the x=0 end member by x-ray absorption fine structure (XAFS). It is found that the local environment of Bi is much more distorted than that determined from conventional diffraction experiments. The shortest Bi-O distances are determined to be 2.22 A, and are 0.3 A shorter than those calculated from the crystallographic data. Several possible models of the Bi coordination environment, which are consistent with the XAFS data and provide bond-valence sums for Bi that are closer to the theoretical values, are proposed. The Ti displacement from the center of the oxygen octahedron increases with K concentration while the shortest Bi-O distance shows no compositional dependence. In K{sub 0.5}Bi{sub 0.5}TiO{sub 3} the value of the Ti displacement is determined to be 0.18 A. The changes of the macroscopic symmetry at the phase transition points in Na{sub 0.5}Bi{sub 0.5}TiO{sub 3} do not lead to changes of the radial atomic distribution around Ti, which is well off-center over the whole temperature range up to and including the paraelectric cubic phase. The results can be explained by assuming the presence of structural disorder.