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.