Crystal structure and magnetic properties of Ca₂MnAlO₅.₅, an n=3 brownmillerite phase

The first bulk n = 3 brownmillerite structure Ca2MnAlO5.5 has been synthesized by high-pressure oxidation of the n = I brownmillerite phase, Ca2MnAlO5, such that alternate AlO4 tetrahedral layers are oxidized to AlO6 octahedra. Rietveld analysis of room-temperature neutron powder diffraction (NPD) data shows a body-centered material (Imma, a = 5.2860(1) angstrom, b = 29.5334(6) angstrom, c = 5.4027(1) angstrom). Although magnetic measurements detected an increase in antiferromagnetic ordering at around 40 K, low-temperature NPD measurements implied the presence of no long-range magnetic order.