Date of Degree

6-2022

Document Type

Thesis

Degree Name

M.S.

Program

Nanoscience

Advisor

Angelo Bongiorno

Committee Members

Yolanda Small

Gustavo Lopez

Subject Categories

Nanoscience and Nanotechnology

Keywords

Quasi harmonic approximation, Density functional theory, Thermodynamics

Abstract

When heated up, materials change volume, typically they expand, and they also change their elastic properties, typically by softening. Computational methods to calculate materials properties at finite temperature are needed to compensate for the lack of experimental data, as well as to predict materials properties at conditions difficult to be reached in experimental labs. In this research project, I designed a set of Python codes implementing a quasi-harmonic approximation (QHA) method to calculate thermodynamic functions at constant volume, equation of state, and the isothermal Bulk modulus of cubic materials. To validate the new computational tools, this implementation of QHA has been used in conjunction with a density functional theory (DFT) approach to calculate structural, thermodynamic, and elastic properties of MgO and CaO. Comparisons of our results with both previous computational studies and available experimental results demonstrate that our computational method is sound, efficient, and allows for the prediction (within the limits of QHA) of thermoelastic properties of cubic materials. Furthermore, our calculations show that the exchange and correlation energy functional is key to achieving an agreement with the experimental results over a wide range of temperatures. In particular, our calculations show that, in the case of MgO, local density approximation (LDA) functionals yield results that agree well with the experiment up to 1500 K, whereas generalized gradient approximation (GGA) functionals give results that start to deviate significantly from the experiment at around 1000 K. Future developments of this research project include: (i) extending our QHA methods to investigate the thermoelastic properties of materials with a symmetry lower than the cubic one, and (ii) exploring methods to account for anharmonic effects and correct the performance of QHA at high temperature, i.e. larger than the Debye temperature of the material under investigation.

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