Treatment of Layered Structures Using a Semilocal meta-GGA Density Functional

Density functional theory calculations on solids consisting of covalently bonded layers held together by dispersive interactions are presented. Utilizing the kinetic energy density in addition to the density and its gradients gives the meta-generalized gradient approximation (MGGA) M06-L enough flexibility to treat correctly both the covalent and the dispersive interactions in layered solids, thus making it a significant step forward compared to the local density and generalized gradient approximations. We show how the MGGA can take advantage of the extra information in the kinetic energy density to discriminate between dispersive and covalent interactions and thereby prove that the performance of M06-L for dispersive interactions, as opposed to that for the local density approximation, is not based on an accidental cancellation of errors

Ab initio Calculations of Intrinsic Point Defects in ZnSb

Several efficient thermoelectric materials have been found among the ternary Zintl antimonides. If the band structure is highly asymmetric around the band gap, the efficiency as either n- or p-type may differ significantly. The Zintl antimonides have generally been found to be p-type. Surprisingly, this also holds true for the narrow band gap binary ZnSb and Zn₄Sb₃. Using ab initio calculations, we investigate intrinsic point defects in ZnSb as a possible origin of the p-type conductivity. Only Zn vacancies are found to be present in significant amounts at room temperature. The low formation energy of negatively charged Zn defects pins the electronic chemical potential to the lower part of the band gap leading to intrinsic ZnSb being p-type. We discuss this finding as a general explanation of p-type conductivity in Zintl antimonides, and how to overcome the doping limits in these materials