engleski

Inverse spinel structure in cobalt doped gahnite

Zinc aluminate, ZnAl2O4, is a wide-band-gap semiconductor transparent for light wavelengths greater that 320 nm. When doped with Co2+, Mn3+ or rare-earth ions it exhibits luminescence and can be used as a cathodoluminescent material. ZnAl2O4 possesses a normal spinel structure, the space group Fd-3m. Its unit cell contains 32 oxygen atoms in cubic close packing, 16 octahedral sites occupied by trivalent Al3+ cations and 8 tetrahedral sites occupied by divalent Zn2+ cations. It is known that spinels can also demonstrate some degree of inversion in the structure which is described by the inversion parameter, delta, defined as a fraction of trivalent metal cations on tetrahedral spinel sites (or a fraction of divalent metal cations on octahedral spinel sites). 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 crystall structures were refined by the Rietveld method. Diffraction patterns indicated that all prepared samples had a spinel type structure, no additional phases were detected. Unit-cell parameter was determined by UNITCELL program and refined by WPPF program. Considering the radii for 4-coordinated Zn2+ (r=0.074 nm) and 4-coordinated Co2+ (r=0.072 nm), the deacrease of unit-cell parameter with Co-doping should be expected, however, the different trend was observed. Unit-cell parameter a increased nonlinearly with cobalt doping level. The unit cell expansion can be explained by Co2+ (r=0.0885 nm) substitution for octahedral Al3+ (r=0.0675 nm), i.e. by the appearance of inversed spinel structure on cobalt doping. The inverse spinel structure inducement due to Co doping was confirmed by the Rietveld refinement and electron paramagnetic resonance (EPR).

Gahnite; Inverse spinel structure; Cobalt doping; X-ray diffraction; EPR

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