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| Titel |
Structural relaxation and colour in the spinel-magnesiochromite (MgAl2O4-MgCr2O4) and gahnite-zincochromite (ZnAl2O4-ZnCr2O4) solid solution series |
| VerfasserIn |
Ulf Hålenius, G. B. Andreozzi, H. Skogby |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028026
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| Zusammenfassung |
| Recent studies on binary mineral solid solution series utilising synchrotron based x-ray
absorption spectroscopies have indicated strong structural relaxation. For instance, it has been
suggested that the real Cr-O bond distances remain nearly constant (relaxation parameter (É)
of 0.85, where É=1 equals full relaxation) over the entire compositional range of the
MgAl2O4-MgCr2O4 series (Juhin et al. 2007).
In the present study we have measured room temperature optical absorption
spectra of synthetic single crystals of the ZnAl2-2xCr2xO4 (0.03-¤x-¤1) and
MgAl2-2xCr2xO4(0.02-¤x-¤1) series with the aim to explore the real architecture of the
structure and in particular the Cr-O distance as function of composition.
Our crystals were synthesized by means of flux-growth methods under atmospheric
pressure and temperature profiles resulting in an estimated cation ordering temperature of ca
850-C. Crystals close to the spinel (sensu stricto) and gahnite end-member compositions
were faintly red in colour. With increasing Cr-content the crystals become more intensely
red-coloured and at the higher Cr-contents there is a distinct shift towards a dark greenish
colouration. These colour changes are reflected in the measured optical spectra by the
position and intensity of the two spin-allowed electronic d-d transitions in octahedrally
coordinated Cr3+ at ca 18000 (4A2g -4T2g (4F) transition) and 25000 cm-1(4A2g -4T1g
(4F) transition). The energy of the first transition (ν1-band) is ca 1200 cm-1 lower in
magnesiochromite than in weakly Cr-doped spinel (x=0.02) and ca 1400 cm-1 lower in
zincochromite than in gahnite with the lowest Cr-content (x=0.03). Concomitantly the energy
of the second transition (ν2-band) decreases with increasing Cr-content in both series by ca.
1800 cm-1.
From the position of the ν1-band, a decrease in crystal field splitting, 10Dq,
for six-coordinated Cr3+ with increasing Cr-content in the MgAl2-2xCr2xO4 and
ZnAl2-2xCr2xO4 series of 6.5 and 7.5 %, respectively, is determined. Based on a Cr-O bond
distance for the CrO6 polyhedron in magnesiochromite and zincochromite of 1.995 and 1.991
Å respectively (O’Neill and Dollase, 1994) and applying the ligand field relationship
10Dq-C-
R-5 (R equals the M-O distance of the MO6-polyhedron), Cr-O bond distances in
gahnite and spinel with Cr-contents at trace levels are determined to 1.959 and 1.969 Å,
respectively. These M-O bond distances are considerably longer than the M-O distances
determined for end member gahnite and spinel by XRD-methods (1.9137 and 1.9280 Å,
respectively; O’Neill and Dollase, 1994) and shows that there is considerable structural
relaxation of M-O bonds in the two present spinel series. The relaxation parameter, É,
determined from the optical absorption spectra is 0.59 and 0.63 for the ZnAl2-2xCr2xO4 and
MgAl2-2xCr2xO4 and series, respectively. These values are lower than those suggested
from X-ray absorption spectroscopy (Juhin et al. 2007), which may be explained
by second nearest neighbour interactions. In contrast to what may be expected,
the interelectronic repulsion parameter, B, for V ICr3+ decreases with increasing
Cr-content and apparent Cr-O bond length in both of the present spinel series . This
indicates that interactions between Cr-atoms in neighbouring octahedra become
important at increasing Cr-content and result in more covalent Cr-O bonds. This in turn
suppresses the energy of 4A2g -4T2g (4F) transition (and calculated 10Dq-values) in
octahedrally coordinated Cr3+. Consequently, the values of structural relaxation
parameters determined from the optical absorption spectra must be regarded as minimum
numbers.
Literature
Juhin, A., Calas, G., Cabaret, D. and Galoisy, L. (2007): Structural relaxation around
Cr3+ in MgAl2O4. Physical Review, B76, 054105.
O’Neill, H.St.C. and Dollase, W.A. (1994): Crystal structures and cation distributions in
simple spinels from powder XRD structure refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the
temperature dependence of cation distribution in ZnAl2O4. Physics and Chemistry of
Minerals, 20, 541-555. |
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