Molecular Orbital Effects in Heavy Transition Metal Compounds

Heavy transition metal (TM) compounds often display highly unusual physical properties due to strong spin-orbit coupling and spatially-extended \textit{d} orbitals. The latter promotes large intersite orbital hopping \textit{t}. In many cases, these enhanced \textit{t} values become comparable or even larger than Hund's coupling $J_H$. In this case, ``molecules in solids" could be a more appropriate picture when we have heavy-TM clusters, where some inter-site covalent bonds form due to the strong orbital overlap. Therefore, molecular orbital (MO) effects may become extremely important, although they have historically been ignored when studying heavy transition metal compounds. This thesis consists of three projects that were designed to study MO effects in heavy transition metal compounds with the 6H-perovskite structure.The first chapter of this thesis describes background material required to understand the original research presented later, while the second chapter describes the experimental techniques used in the research. In Chapter 3, we discovered the realization of the orbital-selective Mott state at the molecular level in \ch{Ba3LaRu2O9}, which appears to represent the first conclusive example of this phenomenon. Chapter 4 is based on my published work of \ch{Ba3CeRu2O9}, where we found a large zero-field slitting ~85 meV that is a consequence of molecular orbital formation within the Ru dimers of this material. We explored further by extending our study to the 5$d$ iridate \ch{Ba4LiIr3O12} in Chapter 5. This material adopts the same 6H-perovskite space group as the materials discussed in Chapters 3 and 4, and appears to be a rare example of a 5\textit{d}4 system exhibiting magnetic order. All these examples together suggest that MO effects cannot be ignored in heavy transition metal molecular magnets. Instead, they need to be treated on equal footing with intra-site effects to ensure an accurate description of the magnetism in these materials.