Structural properties of gahnite doped with cobalt, manganese or titanium

Oxide spinels include a very large group of structurally related compounds having a considerable technological importance. Among them, zinc aluminate (ZnAl2O4), known by the mineral name gahnite, is of interest due its good combination of semiconducting and optical properties which makes it useful in various photoelectronic devices. When doped with some transition-metal cations or rear-earth cations, it exhibits luminescence and can be used as a cathodoluminescence material. Gahnite possesses a cubic spinel-type structure, space group Fd-3m. Although gahnite doped with various cations already has wide applications, some cases of doping are not completely elucidated. Three series of nanocrystalline powder gahnite samples were prepared by a sol-gel technique: (i) samples doped with 0-100 at% Co (substituted for Zn), (ii) samples doped with 0-60 at% Mn (substituted for Zn), (iii) samples doped with 0-12.5 at% Ti (substituted for Al). Structural changes due to Co, Mn and Ti incorporation in the gahnite lattice were studied by XRD. Crystal structures were refined by the Rietveld method, including the analysis of diffraction line broadening. Valence state and location of dopant cations in the gahnite lattice were determined by EPR spectroscopy. Unit-cell parameter of gahnite increased on doping with Co, Mn and Ti. Doping with cobalt and manganese induced a partial inverse spinel structure, with Co2+ and Mn2+ cations residing on both tetrahedral and octahedral cation sites of the gahnite structure respectively. On the other hand, Ti4+ dopant cations occupied only octahedral sites substituting for Al3+. In the latter case the excess Zn2+ cations (included during the synthesis procedure) were also situated on octahedral cation sites due to charge compensation. On cobalt doping as well on titanium doping, the metal-oxygen distances in gahnite structural tetrahedra decreased while in structural octahedra increased. In the case of manganese doping the situation is just opposite: metal-oxygen distances in structural tetrahedra increased and in structural octahedra decreased. Such behavior is a consequence of ionic radii of involved dopant cations and their contents on the respective cation sites.