Magnetic Structure and Electronic Study of Complex Oxygen‐Deficient Manganites

Abstract

Neutron diffraction and X-ray absorption near-edge structure (XANES) studies have been performed in La0.5Ca0.5MnO2.5, La0.5Sr0.5MnO2.5 and Nd0.5Sr0.5MnO2.5 oxygen-deficient perovskite compounds obtained by topotactic reduction. They all exhibit a brownmillerite structure with G-type antiferromagnetic ordering. Mn2+, Mn3+ and Mn4+ coexist at the octahedral sites, whereas only Mn2+ is placed in the tetrahedral positions. A magnetic moment of 1.6 microB has been detected at the tetrahedral layers, which can be explained by assuming Mn2+ is in a low-spin configuration.

Thermodynamic and kinetic assessments of strontium-doped lanthanum manganite perovskites for two-step thermochemical water splitting

“…18,19 In the case of strontium-doped lanthanum manganite, this phase has been observed only upon reduction with hydrides or upon exposure to 50% hydrogen gas. 20,21 Such extreme reducing conditions are not encountered under typical thermochemical cycling conditions and accordingly the Brownmillerite phase is not anticipated to form as part of the fuel generation process.…”

“…10 Moles of water vapor required to induce the indicated stoichiometry change per mole of oxide at 800 C using an input gas stream with pH 2 O ¼ 0.2 atm (balance inert), after reduction at 1400 C under 10 ppm oxygen (balance inert). Results for La 1Àx Sr x MnO 3Àd are computed based on the thermogravimetric data reported by Mizusaki et al 19,20,25 Behavior is compared to that of CeO 2Àd , computed based on the thermodynamic data reported by Panlener et al 24 Vertical lines correspond to Dd max for each composition. Where the curves are shown using dotted lines, the computation has large uncertainty due a gap in the thermogravimetric data in the vicinity of d ¼ 0. k surf on Sr doping level as the cause of the depression in kinetics with increasing Sr content.…”

Magnetoresistance in La0.5Sr0.5MnO2.5

“…Single‐phase brownmillerite type20, 21 La 0.5 Sr 0.5 MnO 2.5 exhibits an interesting temperature dependence of the magnetization measured at different magnetic fields, as shown in Figure 2.…”

“…Among the above brownmillerites, La 0.5 Sr 0.5 MnO 2.5 results especially interesting, due to the presence of FM interactions 20. Neutron diffraction and XANES studies21 indicates that Mn 2+ , Mn 3+ , and Mn 4+ coexist at the octahedral sites whereas Mn 2+ is placed at the tetrahedral ones. Although neutron diffraction reveals a G‐type AFM structure magnetization measurements detect FM interactions which must be of short‐range ordering probably related to a complex microstructure, observed by HREM, comprising domains with different orientations.…”

“…Actually, for δ =0.25 and x =0 the stabilization of an ordered sequence consisting of three octahedra and one tetrahedra has been described 18. Even more, for δ =0.5 La 0.5 A 0.5 MnO 2.5 (A=Ca and Sr)19, 20 and Nd 0.5 Sr 0.5 MnO 5 21 brownmillerite phases have been reported, which also can be synthesized with some compositional variations at the A sublattice in the La 1− x A x MnO 2.5 (A=Ba, Sr, Ca) system 22. 23 It is worth mentioning that, in comparison to the SrCaMnGaO 5+ δ compound, Mn is the only cation occupying the B position of the perovskite lattice, that is, magnetic cations are located at both octahedral and tetrahedral sites.…”