Rattling atoms induced ultralow thermal conductivity and high thermoelectric performance in monolayer Ca3Sn2S7 chalcogenide perovskite

In the field of high-performance thermoelectric energy conversion, achieving glass-like ultralow lattice thermal conductivity in crystalline solids with high electrical conductivity remains a significant challenge. We uncovered that the monolayer Ca₃Sn₂S₇ chalcogenide perovskite exhibits an extraordinary combination of ultra-low lattice thermal conductivity and high electrical conductivity. The lattice thermal conductivity (\(\kappa_{l}\)) of monolayer Ca₃Sn₂S₇ is as low as 1.27Wm⁻¹ K⁻¹ at room temperature and decreases to 0.42Wm⁻¹ K⁻¹ at 900 K. Our in-depth analysis of the phonon spectra, phonon density of states (PDOS), potential energy variations of atoms deviating from equilibrium positions, and studies on atomic displacement parameters (ADPs) attribute this exceptionally low \(\kappa_{l}\) predominantly to the rattling model mechanism. Furthermore, monolayer Ca₃Sn₂S₇ is identified as a narrow band direct semiconductor, possessing a bandgap of 0.47 eV and exhibiting high electrical conductivity. The monolayer Ca₃Sn₂S₇ reaches its peak ZT values of 7.8 for p-type and 5.2 for n-type at 600 K. The monolayer Ca₃Sn₂S₇ is considered a potential material for high-performance thermoelectric materials.