The Physical Origin of Ultralow Lattice Thermal Conductivity in Strongly Anharmonic Thermoelectric Materials Na 2 CdSe and Na 2 HgSe
Arslan Zulfiqar, Muhammad Saqib Arslan, Farooq AhmadABSTRACT
Extremely low lattice thermal conductivity is a decisive condition for the development of high‐efficiency thermoelectric materials that can convert waste heat to electrical power. By means of first‐principles calculations in conjunction with the Boltzmann transport theory, we have carried out a systematic study of structural and thermoelectric properties of Na 2 CdSe and Na 2 HgSe, especially focusing on their low lattice thermal conductivity and its origin. Besides the thermoelectric transport, the electronic and 3D mechanical properties have also been studied. Both compounds show extremely low room‐temperature lattice thermal conductivity of 0.31 Wm −1 K −1 for Na 2 CdSe and 2.32 Wm −1 K −1 for Na 2 HgSe. Detailed studies of cumulative thermal conductivities, phonon mean free paths, Grüneisen parameter, group velocity, phonon lifetime, and scattering rates show that the ultralow thermal transport mainly originates from low phonon group velocities, short mean free paths, as well as small phonon lifetimes. HSE06 calculations show that Na 2 CdSe and Na 2 Hg Se possess direct bandgap of 0.32 and 0.67 eV, respectively. Both materials have excellent thermoelectric performance, exhibiting maximum ZT values of 1.62 for Na 2 CdSe and 1.23 for Na 2 HgSe at 460 and 800 K, respectively. These results demonstrate that Na 2 CdSe and Na 2 HgSe are promising for efficient thermoelectric energy conversion with intrinsically low lattice thermal conductivity.