Award Date

12-15-2025

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Physics and Astronomy

First Committee Member

Ashkan Salamat

Second Committee Member

Keith V. Lawler

Third Committee Member

Joshua Island

Fourth Committee Member

Chern Chuang

Number of Pages

142

Abstract

Pressure is powerful tool by which one can manipulate the properties of materials. In this work we utilize pressure to explore binary oxide systems and characterize their structural and electronic properties. The observation of anomalous structural and electronic behavior in the rutile to CaCl2 phase transition in SnO2 led to the prediction that such behavior is inherent to all oxides experiencing such a phase transition sequence.[1] Here, the ultra-wide band gap semiconductor GeO2 is confirmed to exhibit anomalous behavior during the rutile to CaCl2 phase transition. A phase pure rutile GeO2 sample synthesized under high-pressure, high-temperature, conditions is probed using synchrotron diffraction and X-ray and optical spectroscopy under high pressure conditions. Density functional theory calculations show that the enthalpic barrier to displacing an oxygen decreases with pressure leading up to the rutile to CaCl2 phase transition. The band structure of the distorted state indicates the displacements form small polarons providing another explanation for the conductivity seen in other rutile materials. We then extend this methodology to study binary oxide systems with a similar sequence of phase transitions. Transport measurements, and synchrotron x-ray diffraction data are used in conjunction with first-principles calculations show that RuO2 undergoes a large, low-temperature resistivity increase above 30–40 GPa, accompanied by abrupt pressure drops consistent with a first-order transition. By using Density Functional Theory (DFT) we find that rutile, CaCl2, and HP-PdF2 structures are metallic, but fluorite-type (Fm=3m) insulating; isochoric thermodynamics indicate this denser fluorite polymorph can be favored at low temperature for volumes corresponding to 40–60 GPa, offering a consistent origin for the observed loss of metallicity. The results show how targeted P–T pathways stabilize otherwise inaccessible phases, including an insulating fluorite-type RuO2.

Controlled Subject

Pressure; Thermodynamics; Oxides

Disciplines

Condensed Matter Physics | Geophysics and Seismology | Other Physics | Physics

File Format

PDF

File Size

9200 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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