Tomotaka Nakatani

Temperature dependence of pre-edge features in Ti K-edge XANES spectra for ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase

XANES (X-ray absorption near-edge structure) spectra of the Ti K-edges of ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase were measured in the temperature range 20-900 K. Ti atoms for all samples were located in TiO6 octahedral sites. The absorption intensity invariant point (AIIP) was found to be between the pre-edge and post-edge. After the AIIP, amplitudes damped due to Debye-Waller factor effects with temperature. Amplitudes in the pre-edge region increased with temperature normally by thermal vibration. Use of the AIIP peak intensity as a standard point enables a quantitative comparison of the intensity of the pre-edge peaks in various titanium compounds over a wide temperature range.

High-temperature single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type PbTiO3 phases.

A high-temperature single-crystal X-ray diffraction study of a synthetic PbTiO3 perovskite was carried out over the wide temperature range 298-928 K. A transition from a tetragonal (P4mm) to a cubic (Pm \bar 3 m) phase has been revealed near 753 K. In the non-centrosymmetric P4mm symmetry group, the difference in relative displacement between Pb and O along the c-axis is much larger than that between Ti and O. The Pb and Ti cations contribute sufficiently to polarization being shifted in the opposite direction compared with the shift of O atoms. Deviation from the linear changes in Debye-Waller factors and bonding distances in the tetragonal phases can be interpreted as a precursor phenomenon before the phase transition. Disturbance of the temperature factor Ueq for O is observed in the vicinity of the transition point, while Ueq values for Pb and Ti are continuously changing with increasing temperature. The O site includes the clear configurational disorder in the cubic phase. The polar local positional distortions remain in the cubic phase and are regarded as the cause of the paraelectricity. Estimated values of the Debye temperature ΘD for Pb and Ti are 154 and 467 K in the tetragonal phase and decrease 22% in the high-temperature phase. Effective potentials for Pb and Ti change significantly and become soft after the phase transition.

Variable-temperature single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type barium titanate phases.

A variable-temperature single-crystal X-ray diffraction study of a synthetic BaTiO3 perovskite has been performed over the temperature range 298-778 K. A transition from a tetragonal (P4mm) to a cubic (Pm3m) phase has been revealed near 413 K. In the non-centrosymmetric P4mm symmetry group, both Ti and O atoms are displaced along the c-axis in opposite directions with regard to the Ba position fixed at the origin, so that Ti(4+) and Ba(2+) cations occupy off-center positions in the TiO6 and BaO12 polyhedra, respectively. Smooth temperature-dependent changes of the atomic coordinates become discontinuous with the phase transition. Our observations imply that the cations remain off-center even in the high-temperature cubic phase. The temperature dependence of the mean-square displacements of Ti in the cubic phase includes a significant static component which means that Ti atoms are statistically distributed in the off-center positions.

Temperature dependence of Zr and Ti K-edge XANES spectra for para- and ferro-electric perovskite-type PbZrO3, PbTiO3 and BaTiO3

Zr and Ti K-edge XANES spectra of PbZrO3, PbTiO3 and BaTiO3 perovskite-type compounds were measured in the temperature range from 10K to 850K. Quantitative comparisons for the near-edge spectra were performed in a wide temperature range using the absorption intensity invariant point (AIIP) standardization. Clear temperature dependence for pre-edge shoulder is identified by the calculating the temperature difference of the XANES spectrum intensity. Decrease of pre-edge shoulder and peak intensity is observed only in the para- and ferro-electric phases and draw curves, not straight lines. The gradients for shoulder and pre-edge peak intensity are rich in a variety. The decrease in absorption of pre-edge peak and shoulder is speculated due to the shift from the off-centre position of the Zr atom with respect to the oxygen octahedron to center position. The Zr ion in the PbZrO3 para-electric phase has same temperature behaviors of Ti ions in the ferroelectric perovskite.

XAFS study on Ca local structure in natural glasses and tektite

The local structures of tektite and natural glasses were studied by Ca K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in order to provide quantitative data on bonding distances and coordination numbers. The pre-edge peak intensities of tektites are 10.7-11.7%, and those of peudotachylite, Kirauea volcanic glass, impactite, pitchstone and perlite are 6.7–10.9%. The main peak shoulder intensities of tektites are 68.3–70.7%, and other natural glasses are 63.0–63.9%. XAFS analysis indicated all tektites possess 7-coordinated Ca, but natural glasses possess 6-, 7- and 8- coordinated Ca. This study indicated that different petrogenesis of natural glasses gives different local structures of calcium.

Effective pair potential for Ca-O bonds in CaGeO3 polymorphs

The CaGeO3 perovskite and garnet were synthesized in a cubic anvil type apparatus under high pressure. The measurements of Ca and Ge K- edge XAFS spectra were carried out in the transmission mode at temperature up to 700 K. The effective pair potentials V(u)=?u2/2+?u3/3!, for Ca-O bond in various phases of CaGeO3 have been investigated by the temperature dependence of EXAFS Debye-Waller factors. The potential coefficient ? for the Ca-O bond in perovskite-type CaGeO3 is small, 4.4 eV/?2 compared with those in garnet (6.0 eV/?2) and wollastonite (6.4 eV/?2). The potential for Ca-O bond in perovskite is broader than those in other CaGeO3 polymorphs, which is one reason for the Clausius-Clapeyrons curve for perovskite-garnet phase boundary having a negative slope. The potential coefficients for the Ca-O in perovskite are significantly smaller than those for the longer Ge-Ge distances as the framework vibration though the potential coefficient decreases usually as a result of the larger bond distance.

Precursor phenomenon before phase transitions in PbTiO3 and BaTiO3

Single-crystal X-ray diffraction study of synthetic PbTiO3 and BaTiO3 perovskite was carried out in the wide temperature range 298–928 K [1,2]. Transitions from tetragonal to cubic phases have been revealed near 753 and 413 K in PbTiO3 and BaTiO3, respectively. The Pb and Ba ions contribute sufficiently to electric polarization as well as Ti because the positions of all cations shift to the opposite direction of negatively charged O positions in ferroelectric phases. The deviation from the linear changes in Debye-Waller factors and bonding distances in the tetragonal phases can be interpreted as a precursor phenomenon before the phase transition. In PbTiO3, disturbance of temperature factor Ueq for O is observed in the vicinity of transition point while Ueq values for Pb and Ti are continuously changing with increasing temperature. The O site in PbTiO3 and Ti site in BaTiO3 include the clear configurational disorder in the cubic phase. The polar local positional distortions remain in the cubic phase and are regarded as the cause of the para-electricity. The estimated values of Debye temperature ΘD for Pb and Ti are 154 and 467 K in tetragonal PbTiO3 phase and decrease 22% in the high temperature phase. Effective potentials for Pb and Ti in PbTiO3 change significantly and become soft after the phase transition. Ti K-edge X-ray absorption near edge structure (XANES) spectra of PbTiO3 and BaTiO3 were measured in the temperature range from 10K to 900K. Quantitative comparisons for the pre-edge peaks were performed in a wide temperature range and were clarified how intensity of the pre-edge peaks and shoulders changed with temperature. In ferroelectric tetragonal and paraelectric cubic phases, some kinds of peak and shoulder intensity greatly decreases with increasing temperature and the peak top energy shifts to the higher energy side. The decrease of peak intensity in ferroelectric phases is caused by decrease of the distortion in the coordination environment (atomic movement from off-center to center positions). Precursor phenomenon is also observed in the temperature dependence for the pre-edge structure in XANES spectra.

Structure refinements of kottogite and symplesite solid-solution

Kottogite and symplesite are zinc and ferrous arsenate minerals, respectively. These minerals make the Zn3-x,Fex(AsO4)2・8H2O solid-solution and belongs to the vivianite group of minerals with the chemical formula M3(TO4)2・8H2O. The structure of vivianite and symplesite were determined firstly by Mori and Ito, (1950). The structure of kottigite was refined by Hill, (1979). The strucrure of Zn1.63Fe1.37(AsO4)2・8H2O solid-solution crystallize in space group C2/m with a= 10.342(1), b= 13.484(2), c= 4.7756(5), β=105. 306(4), and Z=2. We performed the structure refinements of (Zn,Fe)3(AsO4)2・8H2O solid-solutions, Ojuela mine, Mapimi Durango, Mexico and Kiura mine, Ohita, Japan by RIGAKU single-crystal structure analysis system RAPID. The R and S values are around 0.03 and 1.08. We determined detail atomic coordinate and hydrogen atom positions. The hydrogen bonds were revealed based on hydrogen positions and bond valence caluculations. The octahedral edge-shareing M2O6(H2O)4 dimers and insular MO2(H2O)4 octahedra are linked by AsO4 terahedra. Two H2O group bonds to (Zn,Fe). Four hydrogen atoms are in the normal hydrogen bonds. Hydrogen atom positions have a tunnel structure and there is a path of proton-conduction and we conjecture that proton conductivity has large anisotropy of one direction. The related minerals, such as paradamite, legrandite and warikahnite have tunnel structure similar to vivianite group.

Local structure of Titanium in natural glasses probed by X-ray absorption fine structure

Synchrotron radiation has been used to collect titanium K-edge absorption spectra of a suite of natural glasses (tektites, impact glasses, fault rocks and volcanic glasses). XANES and XAFS analysis provided the qualitative and quantitative information of Ti oxidation state, Ti-O distance and site geometry. Tektites possess four-, five-, six-coordinated Ti, whereas fault rock-pseudotachylite, volcanic glasses and impact glass only presented five- and six-coordinated Ti. This study indicated that different petrogenesis of natural glasses has different local structures of titanium.