Synthesis, structural and magnetic properties of some transition metal oxides

Abstract

The thesis compiles the synthesis and study of several new materials in the perovskite and corundum families. These transition-metal oxide materials have been structurally characterised and their physical properties have been examined. The new series of substituted Dion-Jacobson layered perovskites (CuCl)La1–xSrxNb2O7 with x = 0, 0.1 and 0.2 has been prepared via topotactic ion-exchange reactions. Refined powder synchrotron diffraction data confirms that all of these compounds adopt the Pbam orthorhombic structure above room temperature and the lattice parameter a becomes larger with increasing substitution of Sr2+. The Pbam (<500 K) - Pbmm (500 - 640 K) - P4/mmm (>640 K) phase transitions found in (CuCl)LaNb2O7 that have been published in a previous report shift towards room temperature as x increases. The magnetisation data suggests that the transition to the spin dimer ground state, present in (CuCl)LaNb2O7 at 14.5 K, is fully suppressed upon Sr2+ substitution. Low temperature neutron diffraction data indicates that substituted (CuCl)La1-xSrxNb2O7 phases are paramagnetic and retain the Pbam structure down to 2 K. New corundum derivatives Co2ScSbO6 and Ni2-xCoxScSbO6 solid solutions that are of interest as potential multiferroic materials have been synthesised. Ni2-xCoxScSbO6 (x = 0 – 1.5) was prepared via a conventional solid-state route at ambient pressure while Co2ScSbO6 was synthesised under high-pressure condition as the pure phase cannot be prepared at ambient pressure. All of these compounds adopt the polar Ni3TeO6-type structure and order ferrimagnetically below the Néel temperature of 60 K. Meta-magnetic transitions are observed for x = 0.5 and 1 revealing complex magnetic behaviour. Ni2ScSbO6 is reported to have a helimagnetic spin order with propagation vector k = [0 0.036 0]. A remarkable series of long-period lock-in [0 0 1/3n] helical spin structures with n = 5, 6, 8 and 10 is found in Ni2-xCoxScSbO6, coexisting with a ferrimagnetic [0 0 0] phase at high Co-contents. The presence of electrical polarisation and spontaneous magnetisation offers possibilities for multiferroic properties.Ruthenium-containing metal oxides have a diverse range of unusual physical properties as a result of electron-electron correlations within broad 4d bands. A new perovskite material YRuO3 with non-magnetic Y3+ at the A-site, that is structurally similar to the LnRuO3 perovskites, has been synthesised at 1473 K and 15 GPa, and adopts a Pnma superstructure with a = 5.835(1), b = 7.528(1) and c = 5.193(1) Å. DC and AC magnetisation data both confirm that magnetic ordering occurs at around 97 K. Apparent changes in spin or domain relaxation processes are observed at 47.2 and 47.4 K in AC magnetisation data as well. Magnetisation steps evidence an unprecedented fractional weak ferromagnetic state at ¾ which reflects extreme single-ion anisotropy resulting from the strong spin-orbit coupling of the d5 Ru3+ ion. Resistivity measurements on a ceramic pellet display a semiconducting behaviour with a relatively small activation energy of 70 meV. A neutron diffraction study of YRuO3 reveals a G-type antiferromagnetic spin structure with an ordered moment of 0.33(3) µB at 1.7 K. The refined Ru-site is fully occupied, in comparison to the ~ 10 % Ru-site deficiency found in all the other LnRuO3 compounds, suggesting the existence of the pure unusual Ru3+ oxidation state. This study elucidates the electronic and magnetic properties of YRuO3, which appears to be the first Ru3+ oxide to exhibit spin ordering phenomena.