Band Anisotropy Generates Axis-Dependent Conduction Polarity of Mg<sub>3</sub>Sb<sub>2</sub> and Mg<sub>3</sub>Bi<sub>2</sub>
Yosuke Goto, Hidetomo Usui, Masayuki Murata, Joshua E. Goldberger, Joseph P. Heremans, Chul‐Ho Lee
Abstract
Materials that exhibit axis-dependent conduction polarity, meaning simultaneous p- and n-type conduction along different crystallographic directions, could be used to develop novel electronic and energy harvesting technologies, such as transverse thermoelectric devices. The present work demonstrates that layered Zintl-phase Mg 3 Sb 2 and Mg 3 Bi 2 possess this property. Single crystals of electron-doped Mg 3 Sb 2 were found to show axis-dependent conduction polarity at low charge carrier concentrations (less than 1 × 10 18 cm –3 ) based on the contribution of holes to conduction in the cross-plane direction. Mg 3 Bi 2 also exhibited this same characteristic but over a wider range of doping with carrier concentrations greater than 1 × 10 19 cm –3 . This difference was attributed to the semimetallic band structure of Mg 3 Bi 2 . First-principles calculations established that axis-dependent conduction polarity appeared in these compounds as a consequence of band anisotropy that arises from the isotropic conduction band minimum and the anisotropic valence band maximum. Specifically, electron bands were primarily responsible for carrier conduction in the in-plane direction, whereas hole bands were dominant in the cross-plane direction. It is evident from these results that 122-type Zintl phases represent a new platform for the exploration of axis-dependent polarity based on band anisotropy engineering.