The system SrO-"chromium oxide" in air and oxygen

Phase relations in the system SrO-"chromium oxide" were determined in air and 1 atm O 2 (1 atm = 1.013 × 10 5 N/m 2 ) and are shown as isobaric projections on the SrO-Cr 2 O 3 pseudobinary. At both oxygen pressures, the system consists of three joins in the SrO-Cr 2 O 3 -oxygen ternary, (1) SrO-Sr 3 Cr 2 O 8 , (2) Sr 3 Cr 2 O 8 -SrCrO 4 , and (3) SrCrO 4 -Cr 2 O 3 . 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-Cr 2 O 3 starting materials). Sr 3 Cr 2 O 8 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 Sr 10 Cr 6 O 24 (OH) 2 which decomposes to SrO and Sr 3 Cr 2 O 8 above 1065 °C. In air, below 775 °C, Sr 3 Cr 2 O 8 reacts with water vapor and oxygen to form Sr 10 Cr 6 O 24 (OH) 2 and SrCrO 4 . Water vapor reactions are restricted in 1 atm O 2 . The Sr 3 Cr 2 O 8 -SrCrO 4 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 Cr 6+ occurs. Likewise, the SrCrO 4 -Cr 2 O 3 join is not binary at solidus and liquidus temperatures. In air, SrCrO 4 melts at 1251 °C to Cr 2 O 3 plus liquid with release of oxygen. In oxygen, the compound melts at 1283 °C with evolution of oxygen.