Фазовая диаграмма системы Mg-O

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Mg-O (Magnesium-Oxygen) H.A. Wriedt No diagram for the Mg-O system has been published. Because the phase equilibria at higher O concentrations are not known, the schematic assessed diagram (which was constructed from thermodynamic considerations, melting points, and continuity requirements) is for the range 0 to 50 at.% O only. The only stable crystal structure of (Mg) is cph. The solid solution (Mg) saturates with respect to MgO at very small O concentrations, which have not been determined. The liquidus compositions along the branch from the 650 C melting point of pure Mg to the (Mg) + L + MgO equilibrium and those along the branch for MgO saturation are undetermined. Even the nature of this three-phase equilibrium reaction is unknown. It is conceivable that, under high hydrostatic pressure at high temperature, a monotectic equilibrium L1 = L2 + MgO might exist. This region of the diagram is, however, probably inaccessible to experiment. The only stable crystal structure of MgO is fcc. It melts congruently at 2827 C. Its range of compositions is very narrow, and the deviations from stoichiometric MgO are undetermined. At its Mg-rich limit, MgO is in equilibrium with (Mg) or L with less than 50 at.% O. At lower temperatures, MgO at its O-rich limit is in equilibrium with MgO2; at higher temperatures, the conjugate phase is L with more than 50 at.% O. The peroxide MgO2 is stable in a range of O2 fugacities greater than those at which MgO is stable. Thus, MgO2 is in equilibrium with MgO at its Mg-rich limit and probably with MgO4 at its O-rich limit. The latter equilibrium has not been realized. The superoxide MgO4 is probably a stable compound at O2 fugacities greater than those at which MgO2 is stable. The MgO4 that has been prepared is unstable at atmospheric pressure and temperatures above about -30 C. 59Van: N.-G. Vannerberg, Ark. Kemi, 14(10), 99-105 (1959). 70Bak: V.M. Bakulina, S.A. Tokareva, E.I. Latysheva, and I.I. Vol'nov, Zh. Strukt. Khim., 11(1), 158-159 (1970) in Russian; TR: J. Struct. Chem., 11(1), 150-151 (1970). Published in Phase Diagrams of Binary Magnesium Alloys, 1988. Complete evaluation contains 1 figure, 2 tables, and 76 references. Special Points of the Mg-O Condensed System