PHONON ANHARMONICITY AND IONIC DIFFUSION IN EMERGENT ENERGY MATERIALS

Abstract

<p>The discovery and design of emergent energy materials require a detailed understanding of their underlying transport mechanisms, strongly associated with their intrinsic anharmonic lattice dynamics. By combining neutron/x-ray scattering spectroscopy with first-principles calculations, the intrinsic phonon properties enabling the high performance can be revealed in these complex systems. In this thesis, I present how phonons affect the transport properties, i.e. thermal and ionic conduction in materials, with increasing anharmonicity, from the nearly-instable system Mg3Sb2 to superionic conductors with sublattice melting AgCrSe2 and Li6PS5Cl.</p><p>By using a combination time-of-flight inelastic neutron scattering (INS), inelastic x-ray scattering (IXS), and ab initio molecular dynamics (AIMD), we showed that the low thermal conductivity (kl) in Mg3X2 (X = Sb, Bi) compared with isostructural ternaries CaMg2X2 and YbMg2X2 with heavier elements originates from abnormally soft phonons and extra low energy shoulders in the phonon density of states (DOS) of the binary compounds. This reflects a near-instability due to the ionic size mismatch and results in a weakened Mg-X bond. The phonon propagation in Mg3X2 was strongly disturbed by the enhanced scattering phase space.</p><p>In the superionic thermoelectric AgCrSe2, we investigated the complex atomic dynamics and the thermal conduction mechanism with combined INS, IXS measurements and theoretical calculations, providing both thermal transport and superionic diffusion mechanism. The transverse acoustic (TA) phonons breakdown selectively depending on the involvement of the mobile ions, and the spectral weight from overdamped modes transferred from vibrational to diffusive dynamics in the superionic regime. Our IXS measurements showed the persistence of long wavelength TA phonons in the superionic state, contrary to the traditional phonon-liquid picture, and we show that the ultralow kl originates from the intrinsic anharmonicity that exists even at low temperature.</p><p>In the solid-state electrolyte Li6PS5Cl, we measured both vibrational and diffusive dynamics with neutron scattering, using 7Li enriched samples to minimize neutron absorption. We combined quasiharmonic neutron scattering measurements with machine-learned molecular dynamics (MLMD) calculations to investigate the diffusion mechanism and found good agreements in diffusion constant and activation energy with reported experimental values. Our constrained MLMD showed stronger effects from the translation of PS4 to the diffusion than rotation, and the low-energy Li partial DOS below 10meV strongly couples with the diffusion. INS measurements revealed strong softening of the low energy modes and we associated the finite value in DOS at energy zero to the Li ion diffusion.</p>