On the Phase Diagram of the Cr-O System at High Pressure of Oxygen and Some Properties of the Compound CrO_2+X

Abstract

The pressure versus temperature diagram of the chromium oxides (Cr₂O₅, CrO₂ and Cr₂O₃) was determined by the closed vessel method in the range of higher oxygen pressures up to 600 atms. Chromium pentaoxide Cr₂O₅ is comparatively stable below 200°C, but it dissociates into CrO₂ above this temperature. The stable ranges of CrO₂ and Cr₂O₃ in the diagram are separated by the dissociation pressure curve of CrO₂ which rises exponentially with increasing temperature through the points (450°C, 100 atms.) and (520°C, 600 atms.). The presence of Cr₃O₈, which had been reported to be stable at room temperature, was not confirmed above 200°C in the present experiments. The results of determination of oxygen contents in CrO₂ phase indicate that this phase is non-stoichiometric; their composition varies from CrO_1.92 to CrO_2.02 depending on the equilibrium conditions. The ratio of O/Cr increases with the fall of temperature and incresing oxygen pressure. The lattice constants of CrO_2+X (rutile type structure) decrease with increasing oxygen contents. The latter effect is quite consistent when we assume that the excess Cr ion are present as interstitials at the interlattice points of oxygen. The accurate lattice constants of stoichiometric compound CrO₂ are found to be a=4.408, c=2.910 a.u., a/c=1.51. The saturation magnetization of CrO_2+X at room temperature seems to increase with increasing O/Cr ratio. The ferromagnetic Curie point of them, how-ever, seems not to change with O/Cr ratio by more than 10°C.