Orientation of Diamagnetic Layered Transition Metal Oxide Particles in 1-Tesla Magnetic Fields

摘要

The magnetic field-driven orientation of microcrystals of six diamagnetic layered transition metal oxides (HLaNb_2O_7, HCa_2Nb_3O_(10)· 0.5H_2O, KNaCa_2Nb_4O_(13), KTiTaO_5, KTiNbO_5, and H_(2.2)K_(1.8)Nb_6O_(17)·nH_2O) suspended in epoxy resins was studied by X-ray diffraction using permanent magnets producing a 0.8 T field. Although the degree of orientation, quantified as the Hermans order parameter, was strongly affected by the particle size distribution, in all cases microcrystals with ～1-2 μm lateral dimensions were found to orient with the magnetic field vector in the layer plane. Control of the orientation of ionically conducting layered oxides is of interest for practical applications in batteries and fuel cells. The consistent direction of orientation of the lamellar oxides studied can be rationalized in the framework of a quantitative bond anisotropy model developed by Uyeda (Phys. Chem. Miner. 1993, 20, 77-80). The asymmetry of metal-oxygen bonding at the faces of the octahedral layers results in long and short M-O bonds perpendicular to the plane of the sheets. This distortion of the M-O octahedra, which is a structural feature of almost all layered materials that contain octahedral bonding frameworks, gives rise to the diamagnetic anisotropy and results in an easy axis or plane of magnetization in the plane of the sheets.