The O-V (Oxygen-Vanadium) system

The complete titanium-oxygen system from metallic titanium to gaseous oxygen including Magneli phases was critically assessed between 298 K and liquidus temperatures and at a pressure of 1 bar. All available experimental T-x phase diagram and thermodynamic data — all in all about 600 data points — were applied to model the Gibbs energies of totally eighteen phases. Of these, twelve were treated as line compounds. Gibbs energies as a function of temperature and composition using sublattice models were optimized for five condensed mixture phases. The ideal approach was accepted for the gas phase. The calculations were carried out using ChemSage.

Thermodynamic modelling of the system titanium-oxygen☆

The structures of ordered and disordered cubic Ti x O z = TiO y (y= z/x) phases containing both Ti and O vacancies were studied by x-ray diffraction. The results demonstrate that annealing TiO y with y= 0.9–1.1 below 1300 K leads to the formation of a monoclinic ordered Ti5O5phase (sp. gr. C2/m(A12m/1)). Symmetry analysis of the monoclinic Ti5O5superstructure indicates that the TiO y –Ti5O5ordering transition is defined by the {k10} Lifshits and {k4} and {k1} non-Lifshits wavevector manifolds. The ordering is shown to be a first-order transition accompanied by a decrease in the unit-cell volume of the parent lattice. The Ti and O distribution functions in titanium monoxide are calculated, and the Ti- and O-site near-neighbor environments in the monoclinic Ti5O5superstructure are identified. The region of possible long-range-order parameters is outlined.

Ordering of Cubic Titanium Monoxide into Monoclinic Ti5O5

By simultaneously solving the governing equations for defect formation and the relative conservation laws, the authors have made a general calculation of equilibrium concentrations of point defects in MgO as functions of temperature, oxygen fugacity (fO{sub 2}), and Fe content. The aim of this work is to better understand the mechanisms that control the transport processes of electrical conduction and diffusion and the oxidation state of Fe. The present model suggests the following regimes that are consistent with several kinds of experiments. Electrical conduction is dominated at low fO{sub 2} by electrons and O vacancies and at high fO{sub 2} by Fe{sup 3+} on Mg sites (Fe{sm bullet}{sub Mg}) and Mg vacancies. Defect associates involving Fe{sm bullet}{sub Mg} and Mg vacancies are required to explain experimental data on oxidation state of Fe as a function of fO{sub 2}, temperature, and Fe content. At high temperatures, intrinsic formation of Mg and O vacancies are important controls on Mg diffusivity; Fe{sup 3+} impurities at intermediate temperatures and impurity-vacancy associates at low temperatures are important. Associates involving Mg and O vacancies at low temperatures and unassociated O vacancies at higher temperatures are important for O diffusivity. The concentration of Fe at which themore » concentrations of other defects are affected strongly depends on fO{sub 2} and temperature. Even minute amounts of Fe (i.e., 1 ppm) may affect properties at temperatures below 1,273 K and fO{sub 2} above 10{sup {minus}15} MPa, whereas at temperatures above 1,873 K and fO{sub 2} below 10{sup {minus}14} MPa, material properties may be relatively insensitive up to 1% Fe content.« less

Equilibrium point defect concentrations in MgO: Understanding the mechanisms of conduction and diffusion and the role of Fe impurities

Vanadium monoxide, VOy has been studied within its homogeneity range using X-ray diffraction and pycnometric density measurements. The density and lattice parameter of VOy have been shown to vary in a complex manner with oxygen content near the stoichiometric composition VO1.0. The experimental data have been used to evaluate the vanadium and oxygen vacancy contents in VOy. We assume that two cubic phases close in lattice parameter coexist near the composition VO1.0 and form a solid solution.

T.B. Reed

Papers

STRUCTURAL, ELECTRICAL, AND MAGNETIC PROPERTIES OF VACANCY-STABILIZED CUBIC TiO AND VO.