High pressure synthesis and study of ternary ruthenates
Sinclair, Alexandra L.
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Metal oxides containing ruthenium have a surprisingly varied range of low-temperature physical properties including the superconductor Sr2RuO4 and the metallic ferromagnet SrRuO3. The exceptionally broad manifold of itinerate and localised electronic phenomena is derived from broad Ru 4d bands and a wide number of Ru and O containing crystallographic structures. High-pressure is a powerful tool for manipulating crystal structures and tuning the associated electronic and magnetic properties. In this body of work, pressure has been utilised in two roles for the study of ternary ruthenates with perovskite and pyrochlore structures; as a synthetic method and for in situ studies (crystallographic or physical) of existing compounds. Chapter 3 details pressure dependent changes in the crystal structure of the perovskite PbRuO3 by powder x-ray diffraction up to 46 GPa and down to 20 K. PbRuO3 transformed on cooling, from orthorhombic spacegroup (Pnma) to an orbitally ordered low temperature phase, which is also an orthorhombic space group (Imma) and applied pressure reduced the critical temperature totally inhibiting the transition at 5.5 GPa. Additionally PbRuO3 was found to undergo a reversible pressure induced structural phase transition at 30 GPa and 290 K with a 10 % reduction in unit-cell volume. Indexing indicated an orthorhombic symmetry with a Pnna spacegroup. Pnna is not a spacegroup associated with perovskite or related perovskite structures despite the √2 x 2 x √2 perovskite superstructure being maintained across the transition. high-pressure resistivity and Raman measurements indicated that a metal-insulator transition accompanied the structural transition. In Chapter 4 high-pressure high-temperature (HP-HT) synthesis has been used to isolate dense phases that could not be produced at ambient pressure. The ortho-perovskite LaRuO3 with space group Pnma (# 62) was synthesised by conventional solid state methods. However to extend the series by substituting the smaller rare-earth cations, Ln3+ on the A-site of the same perovskite structure HP-HT (10 GPa and 1200° C) conditions were required. A powder diffraction study confirmed the Pnma structure of LnRuO3 where, Ln = Pr, Nd, Sm, Eu, Gd, Dy and Ho, of which the later rare-earth compounds, where Ln = Sm to Ho have not been synthesised before. Neutron powder diffraction studies of LnRuO3 where Ln = La, Pr and Nd down to 7 K suggests a ~ 10 % non-stoichiometry on the Ru site, leading to the adjusted formula LnRu0.9O3 with an unusually low Ru3.3+ valency. A possible exception to the low Ru oxidation state is EuRuO3, which has a larger unit-cell, suggesting a Eu2+Ru4+O3 charge distribution with the more common Ru4+, however, this is not concordant with magnetisation measurements. Additionally neutron diffraction suggests that the RuO6 octahedra are distorted by spin-orbit coupling. Magnetometry and resistivity measurements indicate that the compounds are semiconducting paramagnets down to 7 K. Finally in Chapter 5 is presented the analysis of a high-pressure powder x-ray diffraction experiment of the pyrochlore Tl2Ru2O7. Carried out at synchrotron facilities, we have extended the pressure-temperature phase diagram to 3.7 GPa and 25 K. Previously it had been reported that, when cooled, Tl2Ru2O7 undergoes a structural phase transition from a cubic (Fd-3m) phase to a low temperature, orthorhombic (Pnma) phase that forms Haldane chains - an unusual one-dimensional orbital ordering. As for PbRuO3 high-pressure conditions are found to inhibit the orbital ordering, to reduce the critical temperature and to suppress the transition at pressures exceeding 3.0 GPa.