Effect of cobalt doping on the gahnite structure

Abstract

Zinc aluminate (ZnAl2O4) is known by mineral name gahnite. It is a semiconductor with a wide energy band gap of 3.8 eV, transparent for wavelengths greater than 320 nm which makes it useful in ultraviolet photoelectronic devices. When doped with Co2+, Mn3+ or rare-earth cations, it exhibits luminescence and can be used as a cathodoluminescent material. Gahnite is cubic with the normal spinel structure, space group Fd-3m. Powder gahnite samples doped with 0-100 at% Co were prepared by a sol-gel technique. XRD patterns of the prepared samples indicated that all gahnite samples had a characteristic cubic spinel structure. The unit-cell parameter a increased with Co-doping level, the increase being nonuniform. However, with Co doping the decrease of unit-cell parameter should be expected considering that ionic radius of 4-coordinated Co2+ is smaller than that of 4-coordinated Zn2+. The observed lattice expansion is an indication of a considerable level of structure inversion: a part of Co2+ substitutes for Zn2+ on tetrahedral cation sites, and remaining Co2+ substitutes for Al3+ on octahedral cation sites in the ZnAl2O4 lattice. The location of Co2+ in the structure was determined by EPR spectroscopy. The structure of Co-doped gahnite samples was refined by the Rietveld method, which confirmed that Co doping induced the inverse spinel structure. It was found that metal-oxide distances in the (Al, Co)O6 octahedra dominantly influenced the unit-cell parameter of Co-doped gahnite.