Role of Phonon Scattering in TiO2- and WO3-Embedded ZrNiSn for Reduced Thermal Conductivity and Enhanced Thermoelectric Performance: An Ab Initio Approach

Thermoelectric materials provide an effective route for harvesting waste heat, motivating the present investigation of TiO2- and WO3-embedded ZrNiSn half-Heusler alloys. Using density functional theory combined with semiclassical Boltzmann transport calculations, we investigate their structural, electronic, and thermoelectric properties with explicit consideration of the electron–phonon (e–ph) interaction. TiO2 and WO3 embedding substantially modify the electronic structure, reduce the band gap, and alter the bandgap character, thereby influencing carrier transport. Conventional electron–electron (e–e) transport calculations overestimate the electrical conductivity and underestimate the Seebeck coefficient, whereas inclusion of e–ph coupling accounts for phonon-mediated carrier scattering and provides transport coefficients in closer agreement with the experiment. The e–ph interaction further suppresses electronic thermal conductivity and enhances the Seebeck coefficient through energy-dependent carrier scattering, resulting in improved thermoelectric performance. Maximum ZT values of approximately 0.58 for TiO2:ZrNiSn at 800 K and 0.81 for WO3:ZrNiSn at 850 K are obtained when e–ph interaction is included. The Grüneisen parameter and acoustic phonon group-velocity analyses further reveal substantial modifications of phonon transport and enhanced lattice anharmonicity induced by oxide embedding, supporting the strongly suppressed lattice thermal conductivity. Phonon dispersion and ab initio molecular dynamics simulations confirm the dynamical and thermal stability of both systems up to 700 K. Overall, the results demonstrate that oxide nanoparticle embedding combined with explicit e–ph coupling provides an effective strategy for tailoring both electronic and phononic transport in Hf-free ZrNiSn-based half-Heusler alloys for high-temperature waste-heat recovery.