Anharmonic phonon quasiparticles scattering via molecular dynamics simulations in crystalline solids

Abstract

Perturbative treatment has demonstrated its capacity in predicting the vibrational phonon properties of crystalline solids, their applicability is limited for strongly anharmonic systems. In this dissertation, the full lattice dynamics of these partially periodic systems are explored using an effective hybrid scheme based on molecular dynamics simulations and normal-mode-decomposition technique, which is capable of depicting atomic-resolved phonon properties with full anharmonicity.

Firstly, we have explored the temperature and pressure responses of the molecular motions and the phonon scattering in hybrid perovskite MAPbI3 through MD simulations and inelastic neutron scattering measurements. The jumping rotational mode of organic cations plays a dominant role in determining the phonon properties. The dramatic broadening of the phonon linewidth accompanying the orthorhombic-to-tetragonal phase transition is found to be mode-selective. In particular, we have demonstrated that the four-fold rotation of the MA+ cations around the CN axis is extremely sensitive to compression. A hydrostatic pressure as low as 6 kbar can lead to a dramatic change of the phonon lifetime by over 1 order of magnitude.

Secondly, the lattice dynamics across the superionic transition of AgCrSe2 has been elucidated based on ab initio molecular dynamics simulations. We unveil that the specific low-energy transverse phonons dominated by Ag atoms totally collapse, whereas longitudinal optical phonons arise from CrSe 6 cages remain largely intact during the superionic transition. The ultralow thermal conductivity originates from the atomic level structural heterogeneity can be ultimately attributed to diffusive phonon dynamics. Furthermore, we discover the extremely large selective phonon diffusive scattering can be counteracted by hydrostatic pressure-induced deactivation of the liquid-like flow of Ag atoms.

Thirdly, the lattice dynamics across the superionic transition of Cu3SbSe3 have been investigated by ab initio molecular dynamics simulations and temperature dependent Raman spectroscopy. Despite the sustain of Se-formed tetrahedrons, a superionic transition is unveiled for both structurally inequivalent Cu atoms at elevated temperatures for Cu3SbSe3. A new intermediate state of Cu3SbSe3 through the mixture of quasi-1D/2D Cu nearest-neighbor vacancy hopping is discovered below the superionic transition temperature. Our results reveal that phonon quasiparticles dominated by Cu atoms are strongly scattered across the superionic transition along the diffusion channel, while the diffusion is too slow to completely collapse the propagation of all transverse phonon modes for Cu3SbSe3.

Finally, we have constructed a first-principles-based interatomic potential for GeTe by employing the neural network techniques, which successfully reproduces the dynamical nature of GeTe phase transition. The critical mode-resolved anharmonic vibrational phonon spectra, involving structural distortion and thermal expansion contributions, have been computed at elevated temperatures, resolving the non-monotonic temperature dependences of the soft modes at zone center and zone boundary across the phase transition. Complementing the theoretical study, inelastic neutron scattering experiments on powder GeTe samples have been performed.