Synthesis, structure and properties of high pressure and ambient pressure ternary vanadium oxides
Transition metal oxides have been extensively studied during past decades. The purpose of this research was to synthesize new or little characterised transition metal oxides using high-pressure/high-temperature (HPHT) techniques. Various ternary vanadium oxides have been synthesised at ambient and high pressure conditions. All compounds have been studied by neutron and laboratory X-ray powder diffraction and magnetisation measurements. In some cases resistivity and synchrotron X-ray powder diffraction measurements were also carried out. The MnVO3 perovskite containing localized 3d5 Mn2+ and itinerant 3d1 V4+ states has been synthesised at 8 GPa and 1100°C. MnVO3 crystallises in Pnma space group (a = 5.2741(6) Å, b = 7.4100(11) Å, and c = 5.1184(8) Å at 300 K) and is metallic at temperatures of 2 – 300 K and at pressures of up to 67 kbar. Synchrotron X-ray powder diffraction study on the combined sample of several high pressure products showed slight variation in the stoichiometry of MnVO3. Incommensurate Mn spin order was discovered in the neutron powder diffraction measurements, which reveal a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (kx 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions. The new compound CoVO4 has been discovered in a high pressure synthesis experiment. Magnetic susceptibility measurement, synchrotron X-ray and neutron powder diffraction studies were carried out. Refinements of the synchrotron X-ray and neutron data show CoVO4 to crystallise in space group Pbcn (a = 4.5012(2) Å, b = 5.5539(3) Å, and c = 4.8330(2) Å at 300 K (synchrotron X-ray data)). The magnetic susceptibility measurement reveals that Co3+ is most likely in a low spin state in CoVO4. Monoclinic brannerite type CoV2O6 was synthesised in ambient pressure. Neutron powder diffraction measurements were carried out and an antiferromagnetic order with an a x b x 2c supercell was observed below TN = 15 K. High spin Co2+ moments of magnitude 4.77(4) μB at 4 K lie in the ac plane and are ferromagnetically coupled within chains of edge-sharing CoO6 octahedra parallel to b axis. No structural transition is observed down to 4 K, but a magnetostriction accompanying antiferromagnetic order at TN = 15 K was discovered. A field-induced 1/3 magnetisation plateau and corresponding changes in the magnetic structure were studied by carrying out neutron powder diffraction measurements at 2 K in applied magnetic fields of 0, 2.5 and 5.0 T. Three collinear magnetic phases were observed as field increases; the above antiferromagnetic state with propagation vector (0 0 ½), a ferrimagnetic (¯⅓ 1 ⅓) phase, and a (0 0 0) ferromagnetic order. Co2+ moments of 4.4 - 5.0 μB have a large orbital component and are aligned close to the c-axis direction in all cases. Spin-lattice coupling leads to a magnetostriction and volume expansion as field increases. The ferrimagnetic phase accounts for the previously reported 1/3 magnetisation plateau, and demonstrates that monoclinic CoV2O6 behaves as an accidental triangular antiferromagnetic lattice in which further frustrated orders may be accessible. Orthorhombic columbite-type NiV2O6 and CoV2O6 compounds were synthesised at 6 GPa and 900°C. Metamagnetism and magnetic transitions were found in magnetic measurements. Powder neutron diffraction studies in zero and applied field were carried out. Both compounds were refined in space group Pbcn and the following lattice parameters were obtained at 300 K, CoV2O6: a = 13.4941(20) Å, b = 5.5736(9) Å, and c = 4.8082(8) Å and NiV2O6: a = 13.3725(17) Å, b = 5.5344(7) Å, and c = 4.8162(7) Å. Neutron powder diffraction studies in zero field did not reveal any magnetic peaks for either of the compounds but magnetic order emerges in applied fields between 1 and 4 T.