Phase equilibria were experimentally investigated in the MgO–MnOx and the ZrO2–MgO–MnOx systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnOx system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO2–MgO–MnOx phase diagram were constructed for air conditions (bold-italicPboldObold2 = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for bold-italicPboldObold2 = 10−4 bar. The β‐spinel and halite phases of the MgO–MnOx system were found to dissolve up to 2 and 5 mol% ZrO2. A continuous c‐ZrO2 solid solution forms between the boundary ZrO2–MnOx and ZrO2–MgO systems. It stabilizes in the ZrO2–MgO–MnOx system down to at least 1613 K in air and down to 1506 K at bold-italicPboldObold2 = 10−4 bar.

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In this study, a thermodynamic description of the Mg-Mn-O system is presented.Phase diagram data, structural information, and thermochemical data are used in the assessment. All solution phases are modeled using the compound energy formalism. In addition, heat capacity measurement of the MgMn 2 O 4 and Mg 6 MnO 8 phases are reported. The thermodynamic description reproduces thermochemical, structural, and phase diagram data. K E Y W O R D SCalphad, heat capacity, metal-matrix composite, Mg

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