Ab initio study of double oxides ZnX₂O₄ (X ₅Al, Ga, In) having spinel structure

Electronic structure and optical spectra of ZnX₂O₄ (X₅Al, Ga, and In) have been studied by density functional calculations. It is shown that ZnX₂O₄ posses a large fundamental band gap and a second gap defined as the energy difference between the two lowest conduction band (CB) energies. The second gap is found to be ̃2 eV for ZnAl₂O₄ and >2.8 for ZnGa ₂O₄ and ZnIn₂O₄. The bottommost CB for ZnX₂O₄ is well dispersive, which means that these materials possess the feature to conduct electrical current by CB electrons. Effective masses of electrons and holes are of the same order as those of the well-known ZnO and In₂O₃. Optimized equilibrium structural parameters and bulk modulus for ZnAl₂O₄ and ZnGa ₂O₄ are found to be in good agreement with the available experimental values and these values for ZnIn₂O₄ are predicted. Chemical bonding is found to be mixed ionic-covalent with predominant ionic character.