Lattice thermal transport in La3Cu3X4 compounds (X=P,As,Sb,Bi): Interplay of anharmonicity and scattering phase space

Thermal conductivities of La₃Cu₃X₄(X=P,As,Sb,Bi) compounds are examined using first-principles density functional theory and Boltzmann transport methods. We observe a trend of increasing lattice thermal conductivity (κl) with increasing atomic mass, challenging our expectations, as lighter mass systems typically have larger sound speeds and weaker intrinsic scattering. In particular, we find that La₃Cu₃P₄ has the lowest κ_l, despite having larger sound speed and the most restricted available phase space for phonon-phonon scattering, an important criterion for estimating and comparing κ_l among like systems. The origin of this unusual behavior lies in the strength of the individual anharmonic phonon scattering matrix elements, which are much larger in La₃Cu₃P₄ than in the heavier La₃Cu₃Bi₄ system. Lastly, our finding provides insights into the interplay of harmonic and anharmonic properties of complex, low-thermal-conductivity compounds, of potential use for thermoelectric and thermal barrier coating applications.