Phase Transitions, Metallization, Superconductivity and Magnetic Ordering in Dense Carbon Disulfide and Chemical Analogs

LIYANAGAMAGE PRABASHWARA DIAS

Dissertation revised, September 2023 – The original dissertation is available as a separate file. Under high pressure, simple molecular solids transform into non-molecular (extended) solids as compression energies approach those of strong covalent bonds in constituent chemical species, often with advanced mechanical, optical, electronic, and magnetic properties. The primary goal of this research is to investigate the pressure-induced molecular to nonmolecular solids, via discoveries of new states, structures, fundamental properties, and novel phenomena in carbon disulfide and its chemical analogs under extreme conditions of pressure and temperature. Spectral, structural, resistance, and theoretical evidences show simple molecular CS2 undergoes transformations to an insulating black polymer with three-fold carbon atoms at ~9 GPa, to a semiconducting polymer above 30 GPa, and finally to a metallic solid above 50 GPa. The metallic phase is a highly disordered 3D network structure with four-fold carbon atoms. Based on first-principles calculations, we consider two plausible structures for the metallic phase: α-chalcopyrite and tridymite, both exhibiting metallic ground states. Remarkably, low-temperature, dense CS2 not only becomes metallic, but also shows the coexistence of superconductivity and spin-fluctuations. This is the first such observation of superconductivity in simple diamagnetic molecular solids like CS2 at high pressure. The superconductivity in CS2 arises from a highly disordered state at a relatively high transition temperature of ~6.2 K and is, interestingly, preceded by a magnetic ordering transition at ~45.2 K. Based on the x-ray scattering data, we suggest that the local structure changes from tetrahedral to octahedral and the associated spin-fluctuations are responsible for the observed magnetic ordering and superconductivity. A number of related molecular analogs and main group IV disulfides were also studied at high pressure and revealed systematic trends. The above-mentioned findings are important for understanding novel properties of 3D extended solids, the nature of interactions and chemical bonds, and fundamental rules of high-pressure physics and chemistry. The discovery of superconductivity in CS2 is significant for applications and justifies a search for other potential high Tc superconductors composed of low-Z 3D network structures with high phonon frequencies. This is unlike other, more typical, organic superconductors of charge-transferred salts or metal-doped carbons and, most certainly, will stimulate future experimental and theoretical studies.

Phase Transitions, Metallization, Superconductivity and Magnetic Ordering in Dense Carbon Disulfide and Chemical Analogs

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