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The Limits of The Quasi-Harmonic Approximation: Anharmonicity in Germanium and the Entropy of Melting


Mudide, Shiva (2021) The Limits of The Quasi-Harmonic Approximation: Anharmonicity in Germanium and the Entropy of Melting. Senior thesis (Major), California Institute of Technology. doi:10.7907/9mn0-y471.


Inelastic Neutron Scattering (INS) measurements were made at the Wide Angular-Range Chopper Spectrometer (ARCS) on Germanium at temperatures higher than what has been done before, from 296 K to 1203 K. Raw data was used to calculate the dynamic structure factor. Multi-phonon and multiple scattering events were accounted for and subtracted. These dynamic structure factors were then used to calculate single phonon density of states (DOS) for temperatures throughout the said temperature range. Thermal softening of the phonon modes was observed. The softening was quantitatively characterized with several Gruneisen parameters to better understand the effects phonon anharmonicity in Germanium. We find the quasi-harmonic approximation alone cannot explain the large phonon softening. The vibrational entropy contribution to the total entropy was also determined. We find that the vibrational entropy makes up almost all of the total entropy in Germanium, even at elevated temperatures.

We also conduct melting experiments to ensure containment of Si, Bi, and Pb in quartz ampules. These metals will be heated through their melting points at ARCS in the near future in order to determine the vibrational entropy contribution to the latent heat of melting. Furthermore, we write an algorithm based on the work of Sivia to determine the number of phonon modes there is the maximum evidence for in any given phonon DOS.

Item Type:Thesis (Senior thesis (Major))
Subject Keywords:material thermodynamics, computational materials science, phonon anharmonicity, inelastic neutron scattering
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Libbrecht, Kenneth George (chair)
  • Alicea, Jason F.
  • Politzer, Hugh David
  • Fultz, Brent T.
Defense Date:1 June 2021
Non-Caltech Author Email:shivamudide628 (AT)
Record Number:CaltechTHESIS:12012021-011312125
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14438
Deposited By: Shiva Mudide
Deposited On:01 Dec 2021 17:43
Last Modified:01 Dec 2021 17:43

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