Temperature Dependence of Phonons in Elemental Cubic Metals Studied by Inelastic Scattering of Neutrons and X-Rays

Citation

Kresch, Max G. (2009) Temperature Dependence of Phonons in Elemental Cubic Metals Studied by Inelastic Scattering of Neutrons and X-Rays. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5FYM-3M24. https://resolver.caltech.edu/CaltechETD:etd-12082008-130722

Abstract

The vibrations of atoms in a crystal (phonons) make up the majority of its entropy (or heat capacity), and these vibrations are typically modeled by simple harmonic oscillators. Deviations from this harmonic oscillator model are responsible for such well known effects as thermal expansion and temperature dependent elastic constants. Other anharmonic effects, such as the temperature dependence of phonon energies and their linewidths, may be less well known, but also significant. Changes in phonon energies can impact the phonon entropy.

Measurements of the phonon spectra of aluminum, lead, nickel, and iron as a function of temperature are presented, and the anharmonic contributions to the entropies of these cubic metals are considered. These contributions are found to be of the same order of magnitude as those from independent electrons (discounting magnetic contributions). Trends in phonons and phonon-related properties of a wider array of face-centered- and body-centered-cubic (FCC & BCC) metals are also considered. The near-neighbor forces, spectral shapes, and anharmonic entropies of the BCC metals are shown to be far more varied than those of the FCC metals, and this is explained in terms of the crystal structures themselves. Finally, given the similarities in the FCC metals, experimental data and molecular dynamics simulations are used to investigate their phonon linewidths. Trends exist, and they imply similarities in the relative strengths of the harmonic and anharmonic forces in the FCC metals.

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