A new approach for the determination of multiple cation locations and ordering, using the example of natural and heat-treated columbites

International audienceA new approach is proposed to determine unambiguously the location of four cations within the crystal structure of a natural AB(2)O(6) columbite-type compound and derivatives, when submitted to order-disorder transitions caused by heat treatments. This method is based on the successive use of electron microprobe analysis to determine the cation concentration, Mossbauer spectroscopy to identify the Fe occupation, and a crystal structure determination of the samples combining Rietveld refinements of both neutron and X-ray diffraction. This approach is tested successfully to investigate (Fe, Mn, Nb, Ta) natural minerals as well as oxides obtained by heat treatment of the initial AB(2)O(6) columbite-type compound

Magnetic structure of the quasi-two-dimensional compound CoTa2O6

International audienceWe report on a detailed investigation of magnetic properties of CoTa2O6 using several techniques: neutron and X-ray diffraction, specific-heat, magnetic susceptibility, and magnetization measurements. The compound shows quasi-two-dimensional behavior due to its layered structure of alternating Co-O and Ta-O planes. We find that all magnetic moments lie entirely in the Co-O planes, along easy axes determined by the orientations of oxygen octahedra that surround the Co ions. The easy axes in successive magnetic planes have relative orientations that differ by 90°. Antiferromagnetic ordering is observed below 6.6 K, with propagation vectors (±1/4,1/4,1/4) associated to the two non-equivalent sets of Co2+ ions, whose magnetic moments are perpendicularly oriented