| Abstract
| - The room-temperature crystal structures of Ba6Mn4MO15 (M = Zn, Cu) have been refined usingX-ray and neutron powder diffraction data simultaneously. Ba6Mn4ZnO15 is isostructural with Ba6Ni5O15 andcontains transition metals in chains of oxide polyhedra (trigonal prisms and octahedra); neighboring chainsare separated from each other by the Ba atoms. Zn and Mn occupy the trigonal prismatic site in equalconcentrations, with the remaining Zn and Mn partially ordered over the two crystallographically distinctoctahedral sites. The neutron diffraction data show Ba6Mn4CuO15 to adopt a very similar structure but revealthat Cu is displaced from the center of the trigonal prism to give pseudo-square-planar coordination. A neutrondiffraction experiment performed at 1.7 K showed additional intensity due to long-range magnetic order onthe octahedral sites, and this has been modeled using an unusual magnetic structure in which theantiferromagnetic superexchange within the highly frustrated crystal structure leads to a rotation of 120° betweenspins in neighboring chains. The magnetic moments refined to 0.16(2) and 0.12(3) μB per octahedral site inthe Cu and Zn compounds, respectively. The magnetic susceptibility of Ba6Mn4CuO15 is consistent with thetrigonal prismatic Cu remaining paramagnetic in the range 5 ≤ T/K ≤ 300, in which case the contribution tothe susceptibility from the octahedrally coordinated Mn is qualitatively similar to the observed susceptibilityof Ba6Mn4ZnO15, showing a broad maximum (90(5) K for M = Cu, 45(5) K for M = Zn) indicative of1-dimensional ordering. In neither compound is it possible to identify the Néel temperature, although thesusceptibility of Ba6Mn4CuO15 suggests that the Cu spins in trigonal prismatic coordination freeze at 5 K, andit is likely that this coincides with the onset of long-range magnetic ordering on the octahedral sites.
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