A Caltech Library Service

Thermal Behavior of Cuprous Oxide: a Comprehensive Study of Three-Body Phonon Effects and Beyond


Saunders, Claire Nicole (2022) Thermal Behavior of Cuprous Oxide: a Comprehensive Study of Three-Body Phonon Effects and Beyond. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/mate-2v65.


Phonons, or quantized normal modes of crystal vibrations, are responsible for much of the thermophysical behavior in solid-state systems. This behavior includes properties like thermal expansion, defined as the change in material volume in response to temperature. Typically, materials expand upon heating and contract upon cooling; however, some undergo anomalous or negative thermal expansion (NTE). This study focuses on a material with NTE, cuprous oxide (Cu2O), commonly known as cuprite. Using computational and experimental methods, we identify the underlying mechanisms of the NTE and how these mechanisms relate to temperature-dependent phonon behavior with temperature, using both computational and experimental methods.

Computationally, we interpret temperature-dependent changes in phonon energies with perturbation theory. Assuming that the bonds between atoms behave like simple harmonic oscillators, we model the observed random motion of the atoms around their equilibrium positions with quasi-harmonic (QH) and anharmonic (AH) approximations. Furthermore, the perturbations in the atom position allow us to model phonon energy changes in response to temperatures.

While these models, particularly AH models, have proven accurate in predicting the phonon behavior, experimental methods, like inelastic neutron scattering (INS), remain the gold standard for validation. This study presents INS data from single-crystal cuprite measured on the Wide-Angular Range Chopper Spectrometer (ARCS) at the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS). We present INS data collected at 10 K, 300 K, 700 K, and 900 K. The post-processing workflow included: (1) binning with the software package Mantid, (2) reducing with a multiphonon background correction for polyatomic crystals, and (3) condensing into a single irreducible wedge in the first Brillouin zone (BZ). From this, we obtain a four-dimensional scattering function S(Q, E). Our AH calculations use the stochastic-Temperature Dependent Effective Potential (sTDEP) and the Machine Learning Interatomic Potential (MLIP) methods. The former method uses perturbation theory to include cubic and quartic AH contributions. The latter uses machine learning (ML), which in principle, includes all orders of AH terms.

This investigation of the NTE of cuprite demonstrates that QH and AH models successfully predict anomalous NTE behavior. However, only AH calculations show the temperature-dependent phonon behavior seen in INS results. This discrepancy likely stems from a fortuitous cancellation of cubic and quartic AH terms giving an apparent success of QH models for the NTE. Ultimately, a correct prediction of thermal expansion with incorrect phonons reinforces the need to look at the role of higher-order terms in the temperature-dependent behavior of this material.

Despite the success of sTDEP at predicting phonon frequency shifts, it could not account for the newly observed diffuse inelastic intensity (DII) in the INS phonon spectra. For this, MLIP was more effective.

This work provides complementary models to explain the origins of the DII, which is likely an emerging category of AH feature best described as a local nonlinear many-body process. We investigate phonon dissipation, the dynamics of systems coupled to their environments, Brownian motion, and discontinuities due to impulse transfer effects. We conclude by addressing the potential applications of the results and their role in future work on thermal lattice dynamics.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:materials thermodynamics, inelastic neutron scattering, phonon anharmonicity, many-body theory
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Schwab, Keith C. (chair)
  • Minnich, Austin J.
  • Goddard, William A., III
  • Fultz, Brent T.
  • Granroth, Garrett
Defense Date:19 May 2022
Record Number:CaltechTHESIS:05262022-015714738
Persistent URL:
Saunders, Claire Nicole0000-0001-7973-3722
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14625
Deposited By: Claire Saunders
Deposited On:27 May 2022 23:04
Last Modified:04 Aug 2022 19:25

Thesis Files

[img] PDF - Final Version
See Usage Policy.


Repository Staff Only: item control page