Non-Monotonic Variation of the Low Lattice Thermal Conductivity with Temperature in Penta-HgO2 Sheet

Abstract

Inspired by the experimental synthesis of bulk HgO₂ with the potential of exfoliation to form a penta-HgO₂ sheet composed entirely of pentagonal motifs, a detailed theoretical study on the lattice thermal conductivity by using first-principles calculations combined with the unified theory of thermal transport is performed. It is found that the penta-HgO₂ sheet is semiconducting with an indirect bandgap of 1.18 eV and possesses a low lattice thermal conductivity of 2.07 W m⁻¹ K⁻¹ (3.28 W m⁻¹ K⁻¹) along the x (y)-direction at 300 K. More interestingly, the variation of its thermal conductivity with temperature is non-monotonic, different from most cases. The phonon dispersion, phonon scattering, and phonon coherence is further systematically investigated to understand the underlying physics. This results suggest that the strong intrinsic anharmonicity resulting from its unique atomic configuration with the buckled structure and the heavy element of Hg leads to a high scattering rate, resulting in the ultralow particle-like thermal transport of 0.20 W m⁻¹ K⁻¹ (0.01 W m⁻¹ K⁻¹) in the x (y)-direction, while the narrow average frequency interval and strong phonon linewidth are responsible for the dominant coherent thermal transport and non-monotonic variation of the low lattice thermal conductivity of the penta-HgO₂ sheet.