Band gap anomalies of the ZnM2(III)O4 (M(III)=Co, Rh, Ir) spinels.

Development of high figure-of-merit p-type transparent conducting oxides has become a global research goal. ZnM(2)(III)O(4) (M(III) = Co, Rh, Ir) spinels have been identified as potential p-type materials, with ZnIr(2)O(4) reported to be a transparent conducting oxide. In this article the geometry and electronic structure of ZnM(2)(III)O(4) are studied using the Perdew-Purke-Ernzerhof generalized gradient approximation (PBE-GGA) to density functional theory and a hybrid density functional, HSE06. The valence band features of all the spinels indicate that they are not conducive to high p-type ability, as there is insufficient dispersion at the valence band maxima. The trend of increasing band-gap as the atomic number of the M(III) cation increases, as postulated from ligand field theory, is not reproduced by either level of theory, and indeed is not seen experimentally in the literature. GGA underestimates the band-gaps of these materials, while HSE06 severely overestimates the band-gaps. The underestimation (overestimation) of the band-gaps by GGA (HSE06) and the reported transparency of ZnIr(2)O(4) is discussed.