The System SrO - "Chromium Oxide" in Air and Oxygen

Phase relations in the system SrO-"chromium oxide" were determined in air and 1 atm O₂ (1 atm = 1.013 × 10⁵ N/m²) and are shown as isobaric projections on the SrO-Cr₂O₃ pseudobinary. At both oxygen pressures, the system consists of three joins in the SrO-Cr₂O₃-oxygen ternary, (1) SrO-Sr₃Cr₂O₈, (2) Sr₃Cr₂O₈-SrCrO₄, and (3) SrCrO₄-Cr₂O₃. The former is binary from 1065 °C to liquidus temperatures in air and includes a eutectic near 79 mol percent SrO (in terms of SrO-Cr₂O₃ starting materials). Sr₃Cr₂O₈ melts congruently at 1453 °C. Below 1065 °C in air, phase relations are complicated by reactions with atmospheric water vapor resulting in the formation of Sr₁₀Cr₆O₂₄ (OH)₂ which decomposes to SrO and Sr₃Cr₂O₈ above 1065 °C. In air, below 775 °C, Sr₃Cr₂O₈ reacts with water vapor and oxygen to form Sr₁₀Cr₆O₂₄ (OH)₂ and SrCrO₄. Water vapor reactions are restricted in 1 atm O₂. The Sr₃Cr₂O₈-SrCrO₄ join contains a binary eutectic between 69-70 mol percent SrO but liquidus relations are ternary below 69 (air) and 68 (oxygen) mol percent SrO, as reduction of Cr⁶⁺ occurs. Likewise, the SrCrO₄-Cr₂O₃ join is not binary at solidus and liquidus temperatures. In air, SrCrO₄ melts at 1251 °C to Cr₂O₃ plus liquid with release of oxygen. In oxygen, the compound melts at 1283 °C with evolution of oxygen.