Synthesis and characterization of mixed-valence LuFe2O4-δ: Effect of stoichiometry δ
Presented by Mr. Adli PECK on 8 Jul 2014 from 11:10 to 11:30
Type: Oral Presentation
Track: Track A - Division for Physics of Condensed Matter and Materials
LuFe<sub>2</sub>O<sub>4</sub> exhibits a unique mixed-valence superstructure arising from charge-ordering (CO) within the lattice. The unit cell consists of a characteristic stacking of bi-layers, in which Fe is configured in a triangular network. This results in charge and spin frustration in the material, with a resulting plenitude of degenerate magnetic-electronic ground states. Furthermore the Fe is coordinated with oxygen in a trigonal bi-pyramidal polyhedron, a rather unusual co-ordination geometry for inorganic compounds. This mixed-valence characteristic within the bi-layers has been claimed to give rise to a dipole moment, i.e., ferroelectric effects arising from the CO. However such claims of electronically driven CO are under dispute . Recent work has also demonstrated remarkable oxygen storage capacities in this compound . Previous studies have indeed shown that the Fe<sup>2+</sup>/Fe<sup>3+</sup> ratio and magnetic ordering temperature T<sub>N</sub> are influenced by the oxygen stoichiometry [3-4]. We will report our investigations of the effect of oxygen stoichiometry on the CO and magnetic-electronic properties of LuFe<sub>2</sub>O<sub>4-δ</sub>, from a comparison of stoichiometric and oxygen deficient samples. Such samples of LuFe<sub>2</sub>O<sub>4-δ</sub> of varying oxygen stoichiometry, δ, have been synthesised by solid state reaction as a polycrystalline powder. These have been characterised by x-ray diffraction, <sup>57</sup>Fe Mössbauer-effect spectroscopy (MES), SQUID magnetometry and TGA chemical analysis. Using different masses for the overall starting mixture has a radical effect on the purity of the as-synthesised LuFe<sub>2</sub>O<sub>4-δ</sub> sample. Magnetisation measurements show that T<sub>N</sub> is confined to 245-250 K for the synthesised samples, albeit with quite significant differences in the magnetisation-temperature envelopes. Variable cryogenic temperature MES measurements are used to compare the effect of oxygen content on both the CO and magnetic hyperfine structure. . Angst M, Phys. Status Solidi RRL 7, 375 (2013). . Hervieu M et al., Nature Materials 13, 74 (2013). . Yang HX et al., J. Phys. Condens. Matter 24, 435901 (2012). . Wang F et al., J. Appl. Phys. 113, 06 (2013).
Prof. Giovanni Hearne (firstname.lastname@example.org) UJ Dept. Physics