On the Phase Diagram of the Cr-O System at High Pressure of Oxygen and Some Properties of the Compound CrO2+X

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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 CrO1.92 to CrO2.02 depending on the equilibrium conditions. The ratio of O/Cr increases with the fall of temperature and incresing oxygen pressure. The lattice constants of CrO2+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 CrO2+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.
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On the Phase Diagram of the Cr-O System at High Pressure of Oxygen and Some Properties of the Compound CrO<SUB>2+X</SUB>

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