The oxygen defect perovskite Ca3Mn1.35Fe1.65O8.02: A highly frustrated antiferromagnet

A new oxygen defect perovskite Ca₃Mn_1.35Fe_1.65O_8.02 has been isolated. It crystallizes in the orthorhombic system with the following parameters: $\text{a ? a}{}_{\mathrm{<mtext>p</mtext>}}\text{√2; b ? 3a}{}_{\mathrm{<mtext>p</mtext>}}$, and $\text{c ? a}{}_{\mathrm{<mtext>p</mtext>}}\text{√2}$. X-Ray diffraction shows that it corresponds to the second member of the structural series (AMO₃)_m(AMO₂□) and thus consists of double perovskite layers separated by tetrahedral layers. This phase, related to the brownmillerite structure, differs from the latter, in that it exhibits oxygen defects in the perovskite layer and an excess of oxygen in the tetrahedral layer. These results are explained by the ability of Mn^III to adopt pyramidal coordination. Its magnetic properties have been investigated by susceptibility and magnetization measurements and Mössbauer spectroscopy in the temperature range 4–300 K. The dependence of the freezing temperature on the measuring technique (125 K with Mössbauer spectroscopy and 100 K from magnetization), the wide range of temperature where the freezing of the spins occurs, the sensitivity of χ on the cooling magnetic field and the drastic lowering of C_M characterize a highly frustrated behavior due to cationic disorder in the structure.