Microscopic mechanism of low lattice thermal conductivity in natural superlattice materials BaXYF (X = Cu, Ag; Y = Se, Te) including fully quartic anharmonicity

Layered quaternary fluorochalcogenides with natural superlattice structures have received particular attention in the thermoelectric (TE) field due to their favorable electronic properties. However, further exploration of these materials in the TE field is hindered by the lack of studies on the lattice thermal transport properties. Here, we investigate the microscopic mechanism of lattice thermal transport in natural superlattice materials $\mathrm{Ba}\mathit{XY}\mathrm{F}$ ($\mathit{X}$ = Cu, Ag; $\mathit{Y}$ = Se, Te) using first-principles calculations combined with self-consistent phonon theory, compressive sensing techniques, and Boltzmann transport equations. We consider three- and four-phonon scattering as well as temperature-driven phonon energy shifts due to quartic anharmonicity. We find that the strong quartic anharmonicity plays a crucial role in the lattice stability of BaCuSeF and BaCuTeF. Furthermore, the ${\ensuremath{\kappa}}_{\mathrm{L}}$ of the four materials exhibits significant anisotropy due to different bonding types along the $a$($b$) and $c$ axes. The calculations indicate that the four materials have low lattice thermal conductivities ${\ensuremath{\kappa}}_{\mathrm{L}}$, e.g., 0.85--$1.61\phantom{\rule{4pt}{0ex}}{\mathrm{Wm}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 K. Additionally, the results exhibit that a reasonable ${\ensuremath{\kappa}}_{\mathrm{L}}$ and corresponding temperature dependence can be obtained by considering the fully quartic anharmonicity. Our findings not only fill the gap in the lattice thermal transport research of natural superlattice materials, but also deepen a comprehensive understanding of the low ${\ensuremath{\kappa}}_{\mathrm{L}}$ of natural superlattice materials.