Band Anisotropy Generates Axis-Dependent Conduction Polarity of Mg3Sb2 and Mg3Bi2

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 Mg3Sb2 and Mg3Bi2 possess this property. Single crystals of electron-doped Mg3Sb2 were found to show axis-dependent conduction polarity at low charge carrier concentrations (less than 1 × 1018 cm–3) based on the contribution of holes to conduction in the cross-plane direction. Mg3Bi2 also exhibited this same characteristic but over a wider range of doping with carrier concentrations greater than 1 × 1019 cm–3. This difference was attributed to the semimetallic band structure of Mg3Bi2. 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.