Exploring the origin of ultralow thermal conductivity in layered BiOCuSe

Using first-principles density functional theory calculations, a systematic study of the lattice dynamics and related (e.g., dielectric and anharmonic) properties of BiOCuSe (bismuth-copper oxyselenide), along with a comparison with its isostructural analog LaOCuSe, is performed to find the origin of the ultralow thermal conductivity $\ensuremath{\kappa}$ in BiOCuSe. From the marked differences in some of these properties of the two materials, the reasons why BiOCuSe is a better thermal insulator than LaOCuSe are elucidated. For this class of oxychalcogenide thermoelectrics, phonon frequencies with symmetries, characters, spectroscopic activities, displacement patterns, and pressure coefficients of different zone-center modes, dielectric constants, dynamical charges, and phonon and Gr\"uneisen dispersions are also determined.