The system SrMnO 3- x

The system SrMnO _{3- x} (0 ⩽ x ⩽ 0.5) was investigated by gravimetric and quenching experiments. Four-layer, hexagonal SrMnO ₃ ( a = 5.449 Å, c = 9.078 Å) is stable in air below 1035°C. Above 1035°C the basic structure becomes increasingly anion deficient, reaching the limiting composition SrMnO _2.89 near 1400°C. Unit cell dimensions increase and distortion to orthorhombic symmetry occurs with decreasing oxygen content. Further reduction occurs from 1400°C to a melting point at 1740°C and a "perovskite" homogeneity range exists with limits SrMnO _2.74 and SrMnO _2.62. The anion-deficient perovskite phases can be rapidly reoxidized at low temperatures to yield a metastable cubic perovskite, SrMnO ₃ ( a = 3.806 Å). Stoichiometry and stability of phases are discussed from the standpoint of variation in the coordination of the manganese cation.