IRON/MANGANESE DISTRIBUTION AND MAGNETISM IN BIXBYITE

Bixbyite, (Mn⁺³,Fe⁺³)₂O₃ has the β-Mn₂O₃ crystal structure (cubic, Ia, a = 9.412 Å), with two octahedral sites, M1 has a slight trigonal distortion with a single bond length, and M2 is distorted with 2-each, short, medium, and long bond lengths. The structure tolerates a full range of solid solution and the polymorphism and magnetic ordering have been studied by X-ray diffraction, Mossbauer spectroscopy, and magnetic susceptibility [Grant et al., Phys. Rev. 175, 686, 1968]. That work showed Mn₂O₃ to be cubic above 308K (β-Mn₂O₃) and orthorhombic Pbca (α-Mn₂O₃) at lower temperatures, and the transition is fully suppressed by >9 mol% Fe. Magnetic transitions for the Mn end-member occur at 80K and 25K. The upper transition drops to ~35K >9 mol% Fe, and the lower transition disappears altogether at >1 mol% Fe. Grant et al. proposed a noncollinear magnetic ordering model of orthorhombic Mn end-member using cubic symmetry given that the orthorhombic distortion is small. However, Regulski et al. [J. Alloys Comp. 362, 236, 2004] showed this to be incompatible with neutron powder diffraction data, and proposed an alternative collinear model. Unfortunately, the small orthorhombic distortion could not be resolved in the diffraction patterns used by Regulski et al., therefore, their refinements were made using cubic symmetry. More recently, Cockayne et al. [Phys. Rev. B 87, 184413, 2013] used a combination of representation analysis and DFT calculations to propose a magnetic structure from the orthorhombic cell. Their resulting model involves largely AFM exchanges among 5 distinct Mn sites of the orthorhombic cell, with individual moments being 3 - 4 Bohr magnetons. Using neutron diffraction and natural crystals of bixbyite, we have been exploring the more Fe-rich compositions, which should remain cubic at all temperatures and likely manifest magnetic diffuse scattering due to geometrical frustration. The scattering lengths of Mn and Fe offer high fidelity to determine the Fe-Mn distribution on the two octahedral sites. Single-crystals also enable a view of diffuse scattering and more robust determination of magnetic ordering models as compared to powdered samples. Research conducted at ORNL's HFIR and SNS was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.