Quasi-1D electronic transport and isotropic phonon transport in the Zintl Ca5In2Sb6

Abstract

Crystals with anisotropic thermoelectric transport coefficients can yield a high figure-of-merit along the direction with the highest electronic mobility, provided the Seebeck coefficient and thermal conductivity are relatively isotropic. In this study, we combine experiment and theory to investigate the anisotropic properties of the quasi-1D Zintl phase Ca5In2Sb6. Ca5In2Sb6 is predicted by ab initio calculations to have extremely anisotropic p-type electrical conductivity and power factor, arising from light effective mass parallel to its ladder-like polyanionic chains. In contrast, the Seebeck coefficient and lattice thermal conductivity are predicted to be relatively isotropic. The latter is evidenced by the nearly isotropic computed speed of sound tensor and experimentally obtained thermal expansion coefficients. In order to characterize the anisotropic electrical conductivity, Ca5In2Sb6 single crystals were grown from an In–Sb rich molten flux and measured both parallel and perpendicular to the polyanionic chains. Due to the small crystal cross-sections, measurements perpendicular to the growth direction demanded a novel photolithography methodology whereby micro-ribbons were extracted using focused ion beam milling, and processed using laser photolithography to deposit contacts for electrical resistivity and Hall coefficient measurements. The conductivity parallel to the growth direction was found to be nearly 20x higher than the perpendicular direction, in agreement with our theoretical predictions. This study represents one of the first experimental confirmations of highly anisotropic electrical conductivity in Zintl thermoelectrics.