Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p=0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid

Electronic and magnetic properties of layered two-dimensional materials

Two-dimensional materials have recently become an area of considerable interest in the materials science community for both having a variety of properties that can be exploited for the production of devices, and for their potential to host exotic states, especially when combined with magnetism. This work focuses on the synthesis and characterization of two different families of two-dimensional materials, the kagomé layered quantum spin liquid host candidate barlowite, Cu4-xZnx(OH)6BrF, and the Van der Waals layered trimerized kagomé paramagnets Nb3X8 (X = Cl, Br). In the case of barlowite it was found that while the parent compound was not as geometrically perfect as had been previously assumed, but zinc doping showed promise for hosting a QSL state despite the deviations from perfect hexagonal symmetry. In the Nb3X8 family it was discovered that the bromide, like the chloride underwent the same layer rearrangement transition between a paramagnet and a singlet ground state, but above room temperature. Further work revealed that the transition temperature could be tuned between those of the end members by the solid solution Nb3Cl8-xBrx

Suppression of Magnetism by Spin-Orbit Coupling and Binding-Limited Deintercalation Kinetics Revealed in New Honeycomb Iridates

Iridium oxide materials (iridates) are an exciting class of compounds for physicists and materials scientists, as they host a rare meeting between several different types of electronic interactions. In iridates, strong bonds between iridium and oxygen cooperate with electron-electron repulsion to force the highest energy electrons in the material to remain localized on the iridium atoms. These localized, unpaired electrons behave strangely due to a relativistic effect called spin orbit coupling (SOC): the interaction between an electron’s spin and the magnetic field generated by its orbit around the heavily charged iridium nucleus. This unique meeting between strong bonding, coulombic repulsion and SOC interactions has been the focus of significant theoretical and experimental attention in recent years, particularly with regard to iridates that form two dimensional honeycomb lattices. The so-called honeycomb iridates received significant attention after the proposal that they could host a rare, cutting-edge magnetic state known as a spin liquid. Despite this attention, precious few materials have been discovered in which the theory describing electronic and magnetic behavior in honeycomb iridates can be developed and investigated. This dissertation presents the results of several exploratory syntheses and detailed experimental investigations that have culminated in the discovery of three new honeycomb iridates and yielded significant insight into the influence of spin-orbit coupling in iridium compounds. It also demonstrates an important case of a solid state reaction in which diffusion is not the rate limiting step, which is a key insight that can be used to drive innovation in materials chemistry and battery technology. Chapter 3 presents the results of a detailed study on defects and disorder in a canonical honeycomb iridate, Na2IrO3, by controlled chemical doping and careful structural characterization of a new solid solution, Na3−δ (Na1−xMgx)Ir2O6. These results ultimately show how even small concentrations of sodium vacancy defects, chemical doping and structural disorder can dramatically affect the material’s magnetic properties. This work also hints at the strong influence of iridium’s spin orbit coupling. In chapter 4, the structures and basic physical properties of two new honeycomb iridates, NaIrO3 and Sr3CaIr2O9, are reported. These new iridates are the first of their kind, as all of the iridium metal centers exist in the 5+ oxidation state. Conventional crystal field theory suggests these materials should be magnetic with a total of two unpaired electrons and S = 1, but magnetic susceptibility measurements and neutron scattering show complete suppression of magnetism. We demonstrate that this non-magnetic state occurs due to a strong spin-orbit interaction in the Ir5+ ion. In the final chapter, the kinetics of the heterogeneous solid–liquid Na+ ion deintercalation reaction between Na2IrO3 and an oxidizing solution are shown to be determined by the binding affinity of the solution-phase oxidant at temperatures above T ≈ 0◦C. This result is contrary to the belief that diffusion in the solid is always the rate-limiting step in such reactions, and provides insight that can be used to synthesize new materials and potentially improve the performance of modern lithium and sodium ion batteries