Wrinkled layers lead to high in-plane zT values in hexagonal CaAgSb

Abstract

Layered thermoelectric materials (LTMs) have attracted great attention due to their anisotropic transport behaviors that provide an opportunity to disentangle the interrelated electrical and thermal conductivities. In this study, we found that hexagonal CaAgSb (h-CaAgSb) possesses a lower lattice thermal conductivity and a higher electrical conductivity simultaneously along the in-plane direction when compared with the out-of-plane direction. The low in-plane lattice thermal conductivity mainly originates from the low group velocity of longitudinal acoustic phonon modes. Meanwhile, strong anharmonicity is discovered for the low-lying optical phonon modes. On the other hand, the high in-plane electrical conductivity relies on the small effective mass. Thus, both p-type and n-type h-CaAgSb exhibit a high zT over 2.0 along the in-plane direction at the optimal carrier concentrations. The anisotropic transport properties of h-CaAgSb reported in this work may provide guidance to the experiments. More importantly, the physical insights revealed for the disentangled electrical and thermal transport properties may pave the way for finding other excellent LTMs and optimizing the thermoelectric performance through structure engineering.