Crystal and magnetic structures of the brownmillerite compound Ca2Fe1.039(8)Mn0.962(8)O5

The crystal and magnetic structures of the brownmillerite material, Ca₂Fe_1.039(8)Mn_0.962(8)O₅ were investigated using powder X-ray and neutron diffraction methods, the latter from 3.8 to 700 K. The compound crystallizes in Pnma space group with unit cell parameters of a=5.3055(5) Å, b=15.322(2) Å, c=5.4587(6) Å at 300 K. The neutron diffraction study revealed the occupancies of Fe³⁺ and Mn³⁺ ions in both octahedral and tetrahedral sites and showed some intersite mixing and a small, ∼4%, Fe excess. While bulk magnetization data were inconclusive, variable temperature neutron diffraction measurements showed the magnetic transition temperature to be 407(2) K below which a long range antiferromagnetic ordering of spins occurs with ordering wave vector k=(000). The spins of each ion are coupled antiferromagnetically with the nearest neighbors within the same layer and coupled antiparallel to the closest ions from the neighboring layer. This combination of intra- and inter-layer antiparallel arrangement of spins forms a G-type magnetic structure. The ordered moments on the octahedral and tetrahedral sites at 3.8 K are 3.64(16) and 4.23(16) μ_B, respectively.     

Electronic behavior of three oxygen non-stoichiometric Fe/Fe oxoperovskites

For comparison with the Mn⁴⁺/Mn³⁺ oxoperovskites at the crossover from localized to itinerant behavior of the σ-bonding e electrons, the electronic properties of three oxygen non-stoichiometric, mixed-valent Fe⁴⁺/Fe³⁺ oxoperovskites were explored by measuring their resistivity ρ(T), thermoelectric power α(T), and magnetic susceptibility χ(T). Oxidation of Ca₂Fe₂O₅ by annealing in ozone progresses by oxygen insertion to give conductive CaFeO₃ perovskite clusters in a localized-electron, weakly oxidized brownmillerite Ca₂Fe₂O_5+δ matrix. Removal of 0.12 oxygen per formula unit from La_1/3Sr_2/3FeO₃ lowers somewhat its cooperative disproportionation reaction, and fivefold-coordinated ions neighboring oxygen vacancies in the more ionically bonded slabs act as donors to the covalently bonded Fe(V)O₆ planes. Single-crystal SrFeO_2.83 exhibited bad-metal behavior with superparamagnetic, electron-rich fluctuations below 240 K that, on cooling below 190 K, become progressively trapped by the oxide-ion vacancies as an immobile second phase; long-range antiferromagnetic order is stabilized below a T_N≈60 K.     

Crystallographic and magnetic characterisation of the brownmillerite Sr2Co2O5

Sr₂Co₂O₅ with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) Å, b=15.6486(8) Å and c=5.5667(3) Å based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L-R-L-R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) μ_B directed along [1 0 0].