Local structure of the lead-free relaxor ferroelectric (KxNa1-x)0.5Bi0.5TiO3 -: art. no. 174114

The local environment of Bi and Ti atoms in the lead-free relaxor ferroelectric solid-solution (KxNa1-x)(0.5)Bi0.5TiO3 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 angstrom, and are 0.3 angstrom 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 K0.5Bi0.5TiO3 the value of the Ti displacement is determined to be 0.18 angstrom. The changes of the macroscopic symmetry at the phase transition points in Na0.5Bi0.5TiO3 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.