Lattice Dynamics Studies of Negative Thermal Expansion Due to Two Low-temperature Lattice Instabilities.
Date of Completion
Single Crystal, Scandium trifluoride, Mercurous halide, Structural quantum phase transition, Negative thermal expansion, Incipient Ferroelastic, Coefficient of thermal expansion, Inelastic X-ray scattering, Thermal diffuse scattering, Infrared reflection
Field of Study
Doctor of Philosophy
Here I present a complete study of lattice dynamics of negative thermal expansion (NTE) material Scandium trifluoride (ScF3) and discovery of NTE in mercurous iodide (Hg2I2). An inelastic x-ray scattering (IXS) study of ScF3 reveals copious evidence of an incipient soft-mode instability indicating that the T=0 state of ScF3 is very close to a structural quantum phase transition (SQPT). Is the anomalously strong and thermally persistent NTE behavior of ScF3 a consequence of the SQPT? In order to address that, we have explored the coefficient of thermal expansion (CTE) and soft mode behavior of a second stoichiometric compound, situated near a SQPT. A detailed side-by-side comparison of the metal trifluorides and mercurous halides suggest strong similarities and a generic connection between the fluctuating ground state of incipient ferroelastic materials and SNTE. We find experimental evidences for two- dimensional nanoscale correlations exist at momentum-space regions associated with possibly rigid rotations of the perovskite octahedra of ScF3. The discussion is extended by addressing the extent to which rigid octahedral motion describes the dynamical fluctuations behind NTE by generalizing a simple model supporting a single floppy mode that is often used to heuristically describe instances of NTE. Temperature-dependent infrared reflection measurement on single crystal ScF3 is performed to understand the zone center lattice dynamics of ScF3. In addition, I also carried out an instrumentation development project in the laboratory at the Department of Physics which will be briefly discussed in the last chapter.
HANDUNKANDA, SAHAN UDDIKA, "Lattice Dynamics Studies of Negative Thermal Expansion Due to Two Low-temperature Lattice Instabilities." (2018). Doctoral Dissertations. 2021.