On the nature of change in Ni oxidation state in BaTiO3-SrTiO3 system

XAFS studies of Ni-doped Ba$_{1-x}$Sr$_x$TiO$_3$ solid solution reveal that the Ni oxidation state changes from 4 in SrTiO$_3$ to 2.5 in BaTiO$_3$ when varying $x$. This change is accompanied by a noticeable change in the interatomic Ni-O distances in the first shell. The first-principles calculations show that nickel creates an impurity band in the forbidden band gap of BaTiO$_3$ and SrTiO$_3$, which explains the appearance of intense absorption of Ni-doped samples in the visible region. The analysis of the electronic structure of doped crystals and calculations of the oxygen vacancy formation energy in them show that different oxidation states of Ni in SrTiO$_3$ and BaTiO$_3$ can be explained by different formation energies of the oxygen vacancies in these compounds.Comment: 5 pages, 2 figures, 1 tabl

An experimental and theoretical study of Ni impurity centers in Ba0.8_{0.8}Sr0.2_{0.2}TiO3_3

The local environment and the charge state of a nickel impurity in cubic Ba$_{0.8}$Sr$_{0.2}$TiO$_3$ are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is $\sim$2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that Ni$^{2+}$ ions are in a high-spin state at the $B$ sites of the perovskite structure and the difference of the Ni$^{2+}$ and Ti$^{4+}$ charges is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO$_{3-\delta}$. The measurements of the lattice parameter show a decrease in the unit cell volume upon doping, which can indicate the existence of a small amount of Ni$^{4+}$ ions at the $B$ sites.Comment: 8 pages, 3 figures, 3 table

Combined first-principles and EXAFS study of structural instability in BaZrO3

Phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. Unstable phonon mode with the $R_{25}$ symmetry in the phonon spectrum indicates an instability of the cubic structure with respect to rotations of the oxygen octahedra. It is shown that the ground-state structure of the crystal is $I4/mcm$. In order to find the manifestations of the predicted instability, EXAFS measurements at the Ba $L_{\rm III}$-edge are used to study the local structure of BaZrO$_3$ at 300 K. An enhanced value of the Debye-Waller factor for the Ba-O atomic pair ($\sigma^2_1 \sim 0.015$ {\AA}$^2$) revealed in the experiment is associated with the predicted structural instability. The average amplitude of the thermal octahedra rotation estimated from the measured $\sigma^2_1$ value is $\sim$4 degrees at 300 K. The closeness of the calculated energies of different distorted phases resulting from the condensation of the $R_{25}$ mode suggests a possible structural glass formation in BaZrO$_3$ when lowering temperature, which explains the cause of the discrepancy between the calculations and experiment.Comment: 5 pages, 2 figures, 4 table

PhysSoSt5_09LebedevLO

Abstract -The local environment of Pb impurity atoms in BaTiO 3 , SrTiO 3 , and CaTiO 3 crystals was studied by XAFS technique. It is shown that, in both polar and nonpolar phases of BaTiO 3 and in SrTiO 3 , the Pb atoms are displaced from the A lattice sites by ~0.15 Å; in CaTiO 3 this displacement is absent. Large values of DebyeWaller factors (0.05-0.10 Å 2 ) 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 SrTiO 3 and CaTiO 3 and for the increase of the Curie temperature of BaTiO 3 when it is doped with Pb