Computational Studies of Anharmonic Lattice Dynamics and Thermal Transport in Simple Crystals, Alloys and Argyrodites
Authors: Ouyang Niuchang 
Published: December 2024

The phonon thermal transport of many crystalline solids has been accurately predicted within the Boltzmann transport equation and the lowest-order perturbation theory. However, the applicability is still limited to reliably analyzing some materials with structural phase transitions or extremely strong anharmonicity. Therefore, it is necessary to explore the full-order lattice dynamics of these systems using molecular dynamics (MD) simulations with accurate machine learning interatomic potentials and normal-mode-decomposition techniques. Moreover, the role of high-order lattice anharmonicity and two-channel Wigner thermal transport behavior also need to be elucidated.In this thesis, we first reveal a non-monotonic temperature dependence of the linewidths of the soft phonon modes, in agreement with experimental measurements, and improve previous theoretical modeling of SnS. The zone-center optical soft mode collapses at the structural phase transition, corresponding to a second-order nature. Our results also reveal that SnS exists an abnormal decrease of the $\Gamma_{4}$ and $\rm Y_{1}$ phonon frequencies with increasing pressure at the low-temperature regime, which is ascribed to the phase transition. The temperature and pressure dependence of the anisotropic lattice thermal conductivity ($\kappa_{\rm L}$) of SnS is also studied, revealing an intrinsically strong phonon-phonon scattering at high temperatures and pressures.Our results further indicate an apparent decrease in $\kappa_{\rm L}$ with Se alloying in SnS, mainly attributed to the increased scattering rates of middle-frequency phonons and the decreased group velocities of high-frequency modes.We also observe that the impacts of the quartic anharmonicity and coherence contribution are non-negligible in describing the thermal transport of SnS and $\rm SnS_{0.75}Se_{0.25}$, especially at elevated temperatures.Our study additionally reveals a decrease of $\kappa_{\rm L}$ in $\rm SnS_{0.75}Se_{0.25}$ by randomizing Se atoms in the supercells, which is ascribed to an additional phonon scattering owing to the sublattice mass disorder. Moreover, we find that the temperature renormalization of the third-order atomic force constants is significant for increasing the calculated $\kappa_{\rm L}$ of AgX (X=Cl, Br, I), and the coherence thermal transport is also non-negligible. While in AgI, the $\kappa_{\rm L}$ considerably overestimates the experimental results even including these amendments and the four-phonon scatterings. Further, MD simulations explicitly suggest an important role of the higher-than-fourth-order lattice anharmonicity in the low-frequency phonon linewidths of AgI at room temperature, which is attributed to the simultaneous restrictions of three- and four-phonon phase spaces.Finally, we find that as the complexity of the unit cell increases, the proportion of the population terms declines while the coherence contributions become more significant, leading to $\rm Ag_{7}AsS_{6}$ and $\rm Ag_{7}PS_{6}$ displaying relatively weak temperature-dependent $\kappa_{\rm L}$, while the more complex crystalline argyrodites, $\rm Ag_{8}SnS_{6}$, $\rm Ag_{8}GeS_{6}$ and $\rm Ag_{9}GaS_{6}$, exhibiting a glass-like character in their temperature dependence of $\kappa_{\rm L}$,which is attributed to the strong phonon broadening and the dominance of wavelike phonons.Using laser flash measurements and the homogeneous non-equilibrium molecular dynamics simulations based on accurate machine learning neuroevolution potentials, the temperature-dependent $\kappa_{\rm L}$ of $\rm Ag_{8}SnS_{6}$ and $\rm Ag_{8}GeS_{6}$ also display positive behavior and agree well with the literature and our experimental values.