Abstract
Understanding and predicting thermal transport and infrared optical properties of materials play a critical role in developing devices for energy conversion and heat management. Predictive models based on first-principles calculations have significantly deepened our microscopic understanding of heat transport. At the core of modern phonon transport modeling lies the linearized phonon Boltzmann transport equation (LPBTE), which can be solved approximately or exactly using iterative, variational, or direct diagonalization methods. However, the commonly used iterative approach lacks a general convergence guarantee, while alternative methods can be computationally intensive. This dissertation presents THERMACOND, an open-source code developed for efficient and accurate computation of phonon thermal transport in crystalline solids. THERMACOND implements both iterative and direct (non-iterative) solutions to the LPBTE, leveraging crystal symmetry to restrict calculations to the
irreducible Brillouin zone (IBZ), thereby reducing computational cost. The direct method addresses the numerical instabilities often encountered with traditional iterative solvers, making it well-suited for ab initio thermal transport calculations in large and complex materials. Written in Fortran90 and parallelized using MPI, the code is benchmarked on germanium (Ge), germanium selenide (GeSe), and diamond. The agreement of its results with previous theoretical studies, experimental measurements, and results from other packages such as ShengBTE validates the methodologies and confirms its reliability and accuracy. The dissertation also investigates how intrinsic bonding characteristics influence phonon scattering and thermal transport in materials, using lanthanum monopnictides, specifically LaP and LaBi. Despite LaBi having a larger atomic mass, lower acoustic phonon velocities, and a narrower acoustic-optical phonon gap, it exhibits higher lattice thermal conductivity than LaP. Through a detailed analysis of phonon dispersions, interatomic force constants, Grüneisen parameters, and crystal orbital Hamilton population (COHP), it is shown that stronger antibonding nature of LaP’s valence states is responsible for the stronger metavalency, and stronger anharmonicity of its bonds, leading to its lower lattice thermal conductivity despite its lower atomic mass. Lastly, the dissertation presents a predictive phonon-based model for calculating infrared optical properties, including dielectric function, reflectivity, and emissivity, based on phonon frequencies and self-energies. The model is applied to high-temperature stable materials such as MgO, BaZrO3 (BZO), and GeSe. It is inspired by recent studies on epitaxial BaZr0.5Hf0.5O3 (BZHO)/MgO heterostructures that maintain structural and optical stability above 1100 °C. These heterostructures function as high-temperature spectral filters that suppress long-wavelength thermal emission from bulk selective emitters, improving TPV efficiencies by 10-15%. This work aims to develop a first-principles phonon-based model that paves the way for the discovery of thermal emitters for photonic energy conversion systems. The model, integrated into THERMACOND and parallelized using MPI, is benchmarked against experimental data to validate its accuracy.
