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Linking crystal chemistry and physical properties of natural and synthetic spinels: an UV-VIS-NIR and Raman study
Full text linkThe present work deals with the detailed study of multiple oxides with spinel structure with the aim to explore two among their variety of useful physical properties: optical and vibrational properties. Spinels represent very important minerals largely occurring in rocks as well as materials of high relevance from a gemological and technological point of view. To understand their physical properties a complete chemical, structural and spectroscopic study was performed analyzing a large amount of natural and synthetic spinels by Electron MicroProbe, Single-Crystal X-ray Diffraction, Optical Absorption and Raman spectroscopies.
Regarding the optical properties, an accurate characterization on blue synthetic spinels and on natural multicolor spinels was made to investigate the cause of the various colors shown by the spinels. The synthetic blue spinels belonging to the MgAl2O4-CoAl2O4 series were successful synthetized by flux growth method. The optical spectra of the Co-spinels show electronic transitions only due to the Co2+ in tetrahedral coordination, which causes the characteristic blue colour of cobalt spinels. The multicolor spinels owe their different colors not to the major elements but to a combination of two or more minor elements such as Cr3+, V3+, Fe2+, Fe3+ and Co2+, occupying tetrahedrally (T) and/or octahedrally (M) coordinated sites in the spinel structure as also demonstrated by the recorded optical spectra.
Regarding the vibrational properties, a systematic study of three synthetic solid solutions (Mg,Co)Al2O4, (Mg,Fe)Al2O4 and (Mg(Al,Cr)2O4 was carried out by Raman spectroscopy to determine the relationships between the Raman active modes and spinel crystal chemistry. On the basis of comparative inspection of the spectra, it follows that each studied end-member exhibits a Raman fingerprint with at least one peculiar peak in terms of Raman shift and relative intensity. In addition, for each series a good relationship between a Raman active mode and spinel principal component was established. In addition to the Raman features, a study of the fluorescence features attributed to chromophoric ions observed in the Raman spectra was made to distinguish between natural, thermally-treated, and synthetic materials.
Finally, a multianalytical investigation of a natural zincian spinel by electron microprobe, single-crystal X-ray diffraction, Raman and optical absorption spectroscopies was achieved to fully characterize a Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition
Il colore blu degli spinelli a cobalto: distribuzione cationica e assorbimento UV-VIS-NIR in cristalli (Mg,Co)Al2O4 di sintesi
No full text40A) D'Ippolito V., Andreozzi G.B. (2010). Il colore blu negli spinelli a cobalto: distribuzione cationica e assorbimento UV-VIS-NIR in cristalli di (Mg,Co)Al2O4 di sintesi. Rivista Gemmologica Italiana, 5, 120-123
The blue colour of Co-spinel: cation distribution and UV-VIS-NIR absorption spectroscopy of MgAl2O4-CoAl2O4 flux-grown single crystals.
No full textColouring mechanisms in natural spinels
No full textNatural minerals belonging to the spinel group are actively sought as gemstones because of their intense multi-colour, high mechanical resistance and high thermal and chemical stability.
Causes of colour in spinels may be different, most of them being related to transition metal ions, their valence and their coordination. However, more complex colouring mechanisms are difficult to characterize and often remain unexplained. A detailed study on the causes of colours in the spinels is lacking in literature. To fully comprehend the origin of colour, a large number of natural spinels showing colours well representative for the entire variability of colour were explored by Electron Microprobe Analysis and UV-VIS-NIR-MIR spectroscopy.
From the chemical characterization, the analysed spinels exhibit a prevalent spinel s.s. (MgAl2O4) or gahnite (ZnAl2O4) component. The different showed colours do not depend on the end-member composition because, while the samples having a gahnitic composition show colours close to the blue hue, the samples having a spinel s.s. composition show all kind of colours. Hence, the various colours are due to a combination of two or more minor transition metal cations such as Cr3+, V3+, Fe2+, Fe3+ and Co2+, occupying tetrahedrally (T) and/or octahedrally (M) coordinated sites in the spinel structure.
Optical spectra of about thirty natural spinels were recorded in the UV/VIS to NIR spectral range (32000–2000 cm-1). Red, orange and magenta coloured spinels show similar absorption spectra with the main absorption bands at ~25500 cm-1 and ~18500 cm-1 assigned to spin-allowed d-d transitions of Cr3+ and V3+ in the M sites. When the contents of Cr3+ predominate on the contents of V3+, the spinels appear as red, otherwise they appear as orange. The magenta spinels have considerable content of Cr and secondary amount of Fe showing a red shift of the band at ~ 18500 cm-1, with respect to the red coloured samples.
The pink, blue and dark green spinels, in spite of exhibiting very different colours, show similar absorption spectra characterized by a strong UV-edge absorption at energy >> 30000 cm-1 due to the O2- → Fe2+ and O2-→ Fe3+ charge transfer transitions and a series of weak absorption bands in the visible range mainly assigned to spin-forbidden d-d transitions of Fe2+ at the T sites. The peak of maximum absorption in the range 20000 and 10000 cm-1 moves from ~18000 cm-1 for the pink spinels to ~15500 cm-1 for the green spinels (with the blue spinels showing an intermediate situation) depending on the increase of the iron total content and thus also of the Fe3+ content. In fact, the latter interacts with the Fe2+ producing the Fe2+-Fe3+ intervalence charge transfer at 15500 cm-1 in the green samples. The optical absorption bands in the range 18000-15000 cm-1 of the light blue and blue coloured spinels are also influenced by the presence of Co2+ in tetrahedrally coordinated sites
Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part II. Cation ordering over short-range and long-range scales
No full textOptical absorption spectroscopy and X-ray structural refinements were used to characterize short-range and long-range structures of 10 gem-quality, blue spinet single crystals synthesized on the (Mg1-xCox) Al2O4 solid solution (x = 0.07-1.00). The site distributions of Mg, Co2+, and Al show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Co2+, with a marked preference of Co2+ for tetrahedral coordination with respect to Mg, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a certain degree of inversion, i.e., occurrence of Al at T counterbalanced by the occurrence of divalent cations at M, which decreases from 0.24 to 0.13 with increasing Co2+ content. Short-range information based on the crystal field splitting parameter Dq derived from single-crystal optical spectra suggests that the local Co2+-O bond length at the T-site may increase marginally at increasing Co2+ content. An almost constant value for the Racah B-parameter, also derived from optical spectra, for tetrahedrally coordinated Co2+ suggests that any influence of substitutional second nearest neighbor cations on the ionicity of Co2+-O bonds at the T-site is very small. Long-range information shows that variations in the unit-cell parameter from 8.084 to 8.105 angstrom along the solid-solution series are mainly related to the ordering of Al at the M site as a result of the replacement of Mg by Co2+. Therefore, the spinel structure responds to the chemical variation by ordering of Al in such a manner that M-O remains almost constant and T-O increases. In this way, the lengths of shared octahedral edges are reduced and the destabilization effect due to the increased octahedral cation-cation repulsion is minimized. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure has also been shown to be valid in the MgAl2O4-CoAl2O4 solid-solution series
Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part I. Flux growth and chemical characterization
No full textNatural blue Co-bearing spinel crystals are rare and actively sought as gemstones, while synthetic blue Co-bearing spinel powders are largely used as ceramic pigments. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl 2O 4)-cobalt spinel (CoAl 2O 4) were produced by flux growth method, with Na 2B 4O 7 as flux. Lowcooling rates (2 °C/h) and linear temperature profiles were applied in the thermal interval 1200-800 °C, followed by rapid cooling. Thermal runs were performed in reducing atmosphere (fO 2 10 -8-10 -15 bars) created by a continuous flow of a CO 2:H2 mix with a ratio of 100:4 (cm 3/min). Ten experiments were successfully carried out and hundreds of inclusion-free gem-quality single crystals (up to 1 mm large) were produced in each of them, sometimes together with crusty aggregates and microcrystalline powder. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula. Crystals were found to be chemically homogeneous and entirely representing the MgAl 2O 4-CoAl 2O 4 solid-solution series, with the latter component ranging from 7 to 100%. With increasing Co 2+ contents, the crystals vary in color from light blue to intensely dark blue in daylight. The unit-cell parameter a increases from 8.084 to 8.105 Å along the solid-solution series, and the observed increase is determined more by the inversion degree than by the variation in Co contents. The composition of crystal products does not correspond to the composition of the starting oxide mixture, being cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects
Cation distribution and optical properties of MgAl2O4-CoAl2O4 flux-grown single crystals
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Raman investigation of natural and synthetic gem-quality spinels
No full textNatural and synthetic gem-quality spinel single crystals were investigated by electron-microprobe analysis and Raman spectroscopy to study the dependence of Raman spectra on chemical composition.The composition of natural samples is dominanted by spinel s.s. (MgAl2O4) or gahnite (ZnAl2O4) end-member component. The remaining composition consists in hercynite (FeAl2O4) or Magnesiochromite (MgCr2O4) component, with a minor content of Mn2+, Co2+, V3+, Fe3+, which strongly control the colours assumed by these samples. The composition of synthetic samples belong to the MgAl2O4-CoAl2O4 (blue), MgAl2O4-FeAl2O4 (green) and MgAl2O4-MgCr2O4 series (red).Raman spectra of the natural and synthetic spinels display only few of the five Raman-active modes predicted for the general oxide spinel group of minerals. Natural spinels with spinel s.s. composition show four peaks at ~310, 405, 660, and 765 cm-1 attributed to the T2g (1), Eg, T2g (3) and the A1g modes, respectively. Natural spinels with gahnite composition show only three of the five Raman-active modes at ~ 415, 510, 655 cm-1 attributed to the Eg, T2g (2) and T2g (3) modes, respectively. Consequently, discrimination among spinels with different principal component is easily possible looking at the Raman shift. On the contrary, discrimination among spinels with the same principal component and different minor elements, which cause the difference in colors, is not straightforward and requests more studies. What is possible and quite successful is discrimination between natural, treated and synthetic spinels with similar composition. In fact, synthetic spinels, grown in the 1200-900°C thermal range, show a small cation disorder which cause a broadening of the bands and the occurrence of a new peak at ~ 727 cm-1, not predicted by the group theory. In the photoluminesce (PL) spectra, almost all natural and synthetic spinels show the characteristic Cr3+ bands between 5700 and 8000 cm-1, known by gemologists as 'organ pipes'. However, in the synthetic samples the disorder causes a broadening of more intense bands, a merging of the less intense ones and a shift of ~ 60 cm-1 towards IR of all of them
Raman fingerprint of spinels
No full textSynthetic and natural spinel single crystals having compositions approaching spinel end-members MgAl2O4, CoAl2O4, FeAl2O4, ZnAl2O4, MgCr2O4 and ZnCr2O4 were investigated by Raman spectroscopy in the 100-900 cm-1 range using the excitation of the blue 473.1 nm line of a diode pumped Nd:YAG laser.
Each studied end-member exhibits a Raman fingerprint with at least one peculiar peak in terms of Raman shift and relative intensity. The chromates exhibit the A1g mode at 680 cm-1, more intense than the other modes and localized at lower frequencies than the aluminates, in agreement with the heavier atomic mass of Cr with respect to Al. For aluminate spinels the more intense and diagnostic peak in the spectrum is Eg at 405 cm-1 for MgAl2O4, F2g(2) at 516 cm-1 for CoAl2O4, F2g(3) at 660 cm-1 for ZnAl2O4 and A1g at 750 cm-1 for FeAl2O4. The discrimination between natural and synthetic spinels is quite successful. Synthetic spinels usually show high cation disorder which causes a broadening of the bands and the occurrence of a new peak at ca. 720 cm-1 with an A1g mode behavior, not predicted by the group theory
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