Ceramics-Silikáty 20, (2) 113 - 122 (1976)

The formation of calcium chromite was studied in equimolar mixtures MgO-CaO-Cr₂O₃; it was found that contrary to the case of neutral atmosphere, calcium chromite is not formed in air atmosphere up to 1850 °C in amounts detectable by X-ray analysis. The following temperatures of the modification transformation of β to α-CaCr₂O₄ were determined: 1695 °C in pure CaCr₂O₄ in air atmosphere, 1645 °C in pure CaCr₂O₄ in neutral atmosphere, and 1 455 °C in an equimolar MgO-CaO-Cr₂O₃ mixture in neutral atmosphere. The phase diagram of the system CaO-Cr₂O₃ was revised for air atmosphere in its subsolidus region, and a phase diagram was constructed for the same system in vacuo at p_O₂ = 0.34 ± 0.06 Pa and p_H₂O = 0.02 ± 0.004 Pa ± 0.004 Pa for up to 1050 °C. Under these conditions CaCrO₄ dissociates to CaCr₂O₄ via compounds Ca₉Cr₆O₂₄, Ca₆Cr₁₂O₂₇, [Ca₅(CrO₄)₃OH] without forming any liquid phase.