Structures and magnetic ordering in the brownmillerite phases, Sr₂MnGaO₅ and Ca₂MnAlO₅

The crystal and magnetic structures of the two related phases, Sr₂MnGaO₅ and Ca₂MnAlO₅ are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr₂MnGaO₅ being best described by the space group Icmm (a = 5.4888(2) Å, b = 16.2256(6) Å, c = 5.35450(2) Å at 2 K) while that of Ca₂MnAlO₅ is best described by Ibm2 (a = 5.46258(9) Å, b = 14.9532(3) Å, c = 5.23135(8) Å at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture of the magnetic order within these phases.