engleski

Structural study of Co-doped zinc aluminate

Zinc aluminate, ZnAl2O4, is a wide-band-gap semiconductor transparent for light wavelengths greater that 320 nm [1]. When doped with Co2+, Mn3+ or rare-earth ions it exhibits luminescence and can be used as a cathodoluminescent material [2]. ZnAl2O4 possesses a spinel structure, the space group Fd m. Its unit cell contains 32 oxygen atoms in cubic close packing, 16 octahedral sites occupied by Al cations and 8 tetrahedral sites occupied by Zn cations [3]. Powder samples of gahnite doped with 0-100 at% Co (on account of Zn) were prepared by a sol-gel technique and additionally annealed at 800 oC for 2 h. Structural changes due to Co incorporation in zinc aluminate lattice were studied by XRD and crystal structures were refined by the Rietveld method. The XRD patterns revealed that the samples had spinel type structure. Lattice parameter a for undoped ZnAl2O4 agreed well with the literature data [3]. In doped samples it increased with Co-doping level. Considering the ionic radii for 4-coordinated Zn2+ (0.060 nm), 4-coordinated Co2+ (0.058 nm), 6-coordinated Al3+ (0.0535 nm) and 6-coordinated Co2+ (0.065 nm) it follows that the unit cell expansion on Co-doping could be induced by cobalt substitution for octahedral aluminum. However, all prepared samples were blue powders which indicated that some amount of Co2+ should be present in tetrahedral sites (substituting for Zn), but not influencing the general trend of lattice expansion. The Rietveld structure refinement confirmed such Co-doping mechanism.

Co-doped ZnAl2O4; X-ray diffraction; Rietveld structure refinement

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