High pressure study of metal chalcogenides [electronic resource]
Keywords: Physics.

Two-dimensional (2D) materials, topological insulators (TIs), and sesquioxides have attracted huge research interest recently due to their scientific and industrial importance. The study of these materials explores the fundamentals of condensed matter physics while also shedding light on potential applications such as optoelectronics and spintronics. Pressure is an important thermodynamic parameter in changing structural and electronic configuration of a material, which can dramatically alter material properties. The integration of multiple in situ experimental probes with a diamond anvil cell enables the investigation of structural and electronic tuning of a material, exploration of novel high pressure phases, and optimization of material performances. This dissertation focuses on the effect of pressure on three systems in metal chalcogenides. First, the high pressure behavior of a representative 2D material molybdenum diselenide (MoSe2) was studied up to 60 GPa. Upon compression, MoSe2 evolves from an anisotropic 2D layered network to a three-dimensional (3D) structure without a phase transition, which is different from the case of MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggests possible optoelectronics applications. Second, the high pressure behavior of potential TIs silver chalcogenides (Ag2X, X = S, Se, and Te) were studied up to 40 GPa. Under pressure, Ag2X exhibits a series of structural transitions and an increase in Ag to nearby X coordination number. Interestingly, a triple layer stacking pattern appears while the existence of two Ag crystallographic sites is maintained. Phase Is of Ag2Te and of Ag2Se are proposed as TIs from the band-gap increase under pressure and band structure calculations. Higher pressure induces metallization of Ag2X where large electronic evolution occur, for example, phase II of Ag2Te is found to be bulk semi-metallic with topological nontrivial nature while phase III of Ag2Te becomes bulk metallic. Strong changes in infrared transmittance and reflectivity support the changes in their electronic structures. The results demonstrate silver chalcogenides as candidate materials for study of pressure induced TI to non-TI transitions under pressure. Third, the high pressure behavior of XV group sesquioxide α-Sb2O3 was investigated up to 50 GPa. During compression, a first-order structural transition occurs at ~ 15 GPa, where the "molecular" cubic Sb2O3 phase I gradually transforms into a layered tetragonal phase II through local distortions and symmetry breaking. First-principles calculations indicate that the band-gap decreases dramatically from phase I to phase II, consistent with changes in sample color and transparency. At higher pressure, a sluggish amorphization process occurs. Our results highlight the structural connections between late XV group sesquioxides and the role of the lone electron pair in determining local structures under pressure. These results underscore pressure as a powerful tool for modifying the lattice and electronic structure of metal chalcogenides and influencing their material properties.