n-Type thermoelectric properties of a hexagonal SiGe polymorph superior to a cubic SiGe

The crystal structure of a material dictates many of its properties. This study focuses on a SiGe compound with a hexagonal crystal system that demonstrates superior thermoelectric properties than that of its conventional cubic polymorph. Using first-principles density functional theory (DFT) calculations combined with the semi-classical Boltzmann transport equation (BTE), we clearly elucidate the underlying mechanisms that cause the enhanced thermoelectric performance. The hexagonal SiGe compound shows different folding behavior of the electronic band structure, leading to dissimilar density of states and enhanced Seebeck coefficient, compared to the cubic counterpart. Moreover, the phonon vibrational modes of the hexagonal SiGe shorten the phonon lifetime due to the different lattice symmetry. Based on the results, we propose hexagonal SiGe as a promising material for a highly active n-type thermoelectric material with a figure of merit that is twice of its cubic analogue. Our approach demonstrates an attractive method for substantially enhancing conventional material properties without a complicated or expensive process for developing a novel material.