Research Projects

Understanding thermal transport in crystalline materials with low thermal conductivity is critical for technologies such as thermoelectric energy conversion, thermal barrier coatings, data storage, and nuclear reactors. Many of these applications employ semiconductors, where the heat is primarily carried by atomic vibrations, i.e., phonons. The phonon thermal transport is described by the Boltzmann Transport Equation (BTE). Over the past decade, first-principles approaches using density functional theory (DFT) have successfully solved the BTE for predicting lattice thermal transport properties. These methods have shown excellent agreement—typically within 20%—with experimental measurements for various material systems, including simple crystals, compound semiconductors, two-dimensional materials, and nanostructures, without relying on fitting parameters. Despite their accuracy, DFT-based BTE calculations are computationally expensive, typically requiring 10,000 to 100,000 CPU-hours per material. As a result, they have been mostly limited to materials with relatively simple and highly symmetric structures. The computational burden increases significantly when including higher-order phonon interactions such as four-phonon scattering and phonon renormalization, both of which are essential for accurately modeling materials with very low thermal conductivity. Recent experiments have shown that materials like molecular crystals and hybrid organic–inorganic compounds exhibit ultra-low thermal conductivity, making them promising candidates for next-generation thermoelectrics. However, their thermal transport behavior is not well understood due to their large and complex unit cells, which often contain 100–200 atoms. These structures typically consist of one or more molecules weakly bonded to organic or inorganic frameworks. Within each molecule, strong covalent bonding gives rise to high-frequency vibrational modes (phonons), which are not significantly populated at room temperature. As a result, they contribute minimally to thermal transport. Given this, a useful simplification is to model these molecular components as rigid units with six degrees of freedom—three translational and three rotational. This rigid-body approximation reduces the complexity of the phonon spectrum and can significantly lower the computational cost of modeling thermal transport in such systems. In this work, we propose to (a) develop a first-principles-based BTE solver tailored for materials with complex unit cells containing molecular components, and (b) apply this solver to study thermal transport in two-dimensional hybrid perovskites,with more than 100–200 atoms in the unit cell. This approach aims to improve understanding of heat transport in these low-conductivity materials and to accelerate the discovery of efficient thermoelectric compounds.