XAFS studies of the local environment of Pb impurity atoms in barium,strontium, and calcium titanates

The local environment of Pb impurity atoms in BaTiO₃, SrTiO₃, and CaTiO₃ crystals was studied by XAFS technique. It is shown that, in both polar and nonpolar phases of BaTiO₃ and in SrTiO₃, the Pb atoms are displaced from the A lattice sites by ～0.15 Å; in CaTiO₃ this displacement is absent. Large values of Debye-Waller factors (0.05–0.10 Ų) for the atoms in the first shell of Pb observed in all the three crystals indicate the distortion of the oxygen environment of Pb atoms. The appearance of these features was explained by the fact that the Pb-O chemical bond has a noticeable covalent component and a Pb atom can form strong bonds only with four of the 12 surrounding oxygen atoms. The obtained data were used to determine the main factors responsible for the occurrence of ferroelectric phase transition in SrTiO₃ and CaTiO₃ and for the increase of the Curie temperature of BaTiO₃ when it is doped with Pb.