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The effects of cation substitution and ordering in the otavite (CdC[O.sub.3])-magnesite (MgC[O.sub.3]) solid solution have been investigated on samples synthesized at 1 GPa in the temperature range 500-800 [degrees]C for run durations up to 120 h. A complete, disordered solid solution, with R3c symmetry, was obtained at 800 [degrees]C, whereas Mg and Cd show partial ordering within the cadmium dolomite stability field, with R[bar.3] symmetry, at intermediate compositions in the temperature range 500-650 [degrees]C. Rietveld refinements for X-ray diffraction data show that variation of the a-axis is linear as a function of composition, independent of the degree of order, whereas the c-axis shows a positive deviation from linearity as a function of composition, decreasing with increasing degree of order. Octahedral bond distances of the 800 [degrees]C series vary linearly with composition. Site occupancies were used to determine the long-range order parameter, Q, for samples with R[bar.3] symmetry. [Q.sup.4] vades linearly as a function of temperature, suggesting a tricritical phase transition with a critical transition temperature [T.sub.c] of 702(10) [degrees]C. EDXTEM compositional microanalyses of samples within two-phase regions are in good agreement with Rietveld refinements, and allow better constraint of phase boundaries. Keywords: X-ray powder diffraction, otavite--magnesite solid solution, carbonates, phase transition, cation ordering

Titanium [L.sub.3,2] and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are [Ti.sup.2+], [Ti.sup.3+], and [Ti.sup.4+]. Effects of polyhedra distortions are particularly observed for two oxides with mixed [Ti.sup.3+]-[Ti.sup.4+] valence state, i.e., the Magneli phases [Ti.sub.4][O.sub.7] and [Ti.sub.5][O.sub.9]. A prominent pre-peak in the Ti [L.sub.3] edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti [L.sub.3,2] ELNES spectra we propose a new quantification technique for the determination of [Ti.sup.4+]/[Ti.sup.3+] ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K-edge is sensitive to the covalent bonding in all analyzed oxides. Keywords: Electron energy loss spectroscopy, valence state of titanium, titanium oxides, density functional theory

The position of the spinel-post-spinel phase transition in Fe3O4 has been determined in pressure-temperature space by in situ measurements using a multi-anvil press combined with white synchrotron radiation. Pressure measurement using the equation of state for MgO permitted pressure changes to be monitored at high temperature. The phase boundary was determined by the first appearance of diffraction peaks of the high-pressure polymorph (h-Fe3O4) during pressure increase and the disappearance of these peaks on pressure decrease along several isotherms. We intersected the phase boundary over the temperature interval of 700-1400 °C. The boundary is linear and nearly isobaric, with a slightly positive slope. Post-experiment investigation by TEM confirms that the reverse reaction from h-Fe3O4 to magnetite during decompression leads to the formation of microtwins on the (311) plane in the newly formed magnetite. Observations made during the phase transition suggest that the transition has a pseudomartensitic character, explaining in part why magnetite persists at conditions well within the stability field of h-Fe3O4, even at high temperatures. This study emphasizes the utility of studying phase transitions in situ at simultaneously high temperatures and pressures since the reaction kinetics may not be favorable at room temperature. [ABSTRACT FROM AUTHOR]

Titanium L3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti2+, Ti3+, and Ti4+. Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti3+-Ti4+ valence state, i.e., the Magnéli phases Ti4O7 and Ti5O9. A prominent pre-peak in the Ti L3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L3,2 ELNES spectra we propose a new quantification technique for the determination of Ti4+/Ti3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K-edge is sensitive to the covalent bonding in all analyzed oxides. [ABSTRACT FROM AUTHOR]