Synthetic Methodology and Quantum Properties of Novel Indium Subchalcogenides

Khoury, Jason Frederick

doi:https://doi.org/10.21985/n2-2fx3-zw91

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Synthetic Methodology and Quantum Properties of Novel Indium Subchalcogenides

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Subchalcogenides are rare compounds that have both metal-metal and metal-chalcogenide (sulfur, selenium, tellurium) interactions. Unlike conventional semiconductors, the metals do not obey the so-called 8-N octet rule for oxidation states, often resulting in low or zero valent metal atoms. The presence of both metal-metal and metal-chalcogenide bonding can also lead to lower dimensional metallic substructures that have drawn interest for their remarkable physical properties, such as charge density wave, superconductivity, and topological behavior. However, subchalcogenides are a poorly understood class of materials due to their synthetic difficulties, as most attempts to discover them result in binary intermetallic and chalcogenide compounds instead. Herein this thesis, the synthetic methodology and bonding principles of ternary indium subchalcogenides, where the heavy transition metal is iridium or ruthenium and the chalcogen is sulfur, selenium, or tellurium, is described. Liquid indium was utilized as a flux with these reagents, therefore mitigating the synthetic challenges of making subchalcogenide materials and stabilize these compounds as large mm scale single crystals. Ir2In8Q (Q = S, Se, Te) was discovered, a series of subchalcogenide materials that crystallize in a new structure type in the P42/mnm space group. The structure consists of a 3D IrIn8 polyhedral framework with Q atoms in the channels along the c axis. The framework is corner sharing along the ab plane and alternates between corner and edge sharing along the c axis, leading to the 42 screw axis. Ir2In8Q are also Dirac semimetal candidates, and their quantum properties are fully investigated with electrical transport measurements. Ir2In8S has a high mobility of 104 cm2/Vs, comparable to other topological semimetals, and quantum oscillations suggests topologically nontrivial character. Ir2In8Q also have structural phase transitions that are seen in transport, heat capacity, and diffraction image data, with Ir2In8S having distinct rods of diffuse scattering that disappear below 230 K. Ir2In8Se and Ir2In8Te, on the other hand, have a more complex re-entrant structural modulation, where they both have supercell ordering that appears at lower temperatures (203 and 150 K for Ir2In8Se and Ir2In8Te, respectively), but revert to the original subcell as the temperature decreases below 110 K. This re-entrant behavior may be attributable to a Charge Density Wave-like transition that is competing with structural distortions. In contrast to these semimetallic structures, the more sulfur-rich structures Ir6In32S21 and RuIn11S10 were synthesized by varying reagent ratios and composition, showcasing the generalizability of this synthetic method. Ir6In32S21 crystallizes in a new structure type in the polar P31m space group, and displays Dresselhaus spin-splitting of its band structure along with semiconducting behavior, with a band gap of approximately 1.48(2) eV. The structure has a quasi-2D motif with a hexagonal Ir-In honeycomb network connecting to extensive In-S bonding. RuIn11S10 crystallizes in a new structure type in the P21/c space group, and density functional theory calculations and photoresponse measurements suggest semiconducting behavior. Throughout the metallic substructures of these compounds, we see a trend from semimetallic to semiconducting behavior with increasing sulfur content. The trend toward semiconducting behavior is likely due to poorer overlap between the M d and In 5p orbitals compared to the stronger d-d interactions in binary transition metal subchalcogenides such as Ta6S, causing the sulfur to be more anionic in nature and insulating the metallic substructures. From this work, we have advanced the state of the art in subvalent chalcogenide synthesis, along with further understanding the design principles for their electronic properties.

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