Structural relaxation in tetrahedrally-coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution. (Articolo in rivista)

Type
Label
  • Structural relaxation in tetrahedrally-coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution. (Articolo in rivista) (literal)
Anno
  • 2012-01-01T00:00:00+01:00 (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#doi
  • 10.2138/am.2012.4093 (literal)
Alternative label
  • Ardit M., Cruciani G., Dondi M. (2012)
    Structural relaxation in tetrahedrally-coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution.
    in The American mineralogist (Online)
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Ardit M., Cruciani G., Dondi M. (literal)
Pagina inizio
  • 1394 (literal)
Pagina fine
  • 1401 (literal)
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  • 97 (literal)
Rivista
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  • 8 (literal)
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  • Earth Science Department, University of Ferrara, via Saragat 1, 44100 Ferrara, Italy - Institute of Science and Technology for Ceramics (ISTEC-CNR), via Granarolo 64, 48018 Faenza, Italy (literal)
Titolo
  • Structural relaxation in tetrahedrally-coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution. (literal)
Abstract
  • The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ~3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ? = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility. (literal)
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