Osmates at the extremes of octahedral connectivity : synthesis, structure and physical properties

Osmium is an understudied platinum group metal that offers unique advantages in the study of transition metal oxides. It is flexible in its oxidation state, varied in the structure types it can adopt, and often yields unexpected physical properties when incorporated into compounds due to the presence of strong spin-orbit coupling. This latter consideration is important in the physical properties of chiral osmates, chirality being a property of objects that are non-superimposable with their mirror image. Chiral inorganic compounds can have potentially useful effects such as spin filtering, but well characterised model systems are needed. The first part of this work studies the synthesis routes, structures and physical properties of chiral, KSbO₃-type compounds incorporating a three-dimensional network of corner linked octahedral osmium dimers. Laboratory X-rays are used for initial structural characterisation, with basic physical properties characterised by heat capacity and magnetic measurements. Further measurements of electron transport are made on La₄Os₆O₁₉ which shows an unusual phase transition. Hydrostatic pressure is tested as a potential tuning parameter for chirality in the KSbO₃-type structure, finding that it strengthens one chiral distortion while weakening another. Condensed matter physics is concerned in part with the study of the interesting, and quintessentially quantum phases which can emerge at low temperature such as superconductivity and frustrated magnetism. The second part of this work studies magnetism in a new layered osmate incorporating isolated osmium octahedra. La₇Os₃O₁₈ shows unusual magnetic properties, and has significant residual entropy, determined by heat capacity measurements, along with a complex field dependent phase diagram including the suppression of a thermodynamic crossover. Neutron diffraction is used to determine the magnetic structure, which shows orthogonal arrangement of moments on neighbouring layers. With this context, the measured phase diagram may be rationalised in terms of the behaviour of the underlying magnetic sublattices.