Computational studies of anharmonic lattice dynamics and thermal transport in simple crystals, alloys and argyrodites

Abstract

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.