Raman microscopy has been used to characterize the interlayer anions in synthesized hydrotalcites of formula Mg 6 Al 2 (OH) 16 (XO 4 )·4H 2 O, where X is S, Mo or Cr. The Raman spectrum shows that both the chromate and molybdate anions are not polymerized in the hydrotalcite interlayer. This lack of polymerization is attributed to the effect of pH during synthesis. A model of bonding is proposed for the interlayer anions based upon the observation of two symmetric stretching modes and symmetry lowering of the chromate, molybdate and sulphate anions. Two types of anions are present, hydrated and hydroxyl surface-bonded.
Complexes of the type [M(phen)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and phen = 1,10-phenanthroline) were found to co-crystallize to form molecular alloys (solid solutions of molecules) with general formula [MA x MB 1–x (phen)3](PF6)2·0.5H2O in which the relative concentrations of the metal complexes in the crystals closely match those in the crystallizing solution. Consequently, the composition of the co-crystals can be accurately predicted and controlled by modulating the relative concentrations of the metal complexes in the crystallizing solution. Although they are chemically and structurally similar, complexes of the type [M(bipy)3](PF6)2 (M = Ni(II), Fe(II), Ru(II) and bipy = 2,2′-bipyridine) display markedly different behavior upon co-crystallization. In this case, the resulting co-crystals of general formula [MA x MB 1–x (bipy)3](PF6)2 have relative concentrations of the constituent complexes that are markedly different from the relative concentrations of the complexes initially present in the crystallizing solution. For example, when the nickel and iron complexes are co-crystallized from a solution containing a 50:50 ratio of each, the result is the formation of some crystals with a higher proportion of iron and others with a higher proportion of nickel. The relative concentrations of the metal complexes in the crystals can vary from those in the crystallizing solutions by as much as 15%. This result was observed for a range of combinations of metal complexes (Ni/Fe, Ni/Ru, and Fe/Ru) and a range of starting concentrations in the crystallizing solutions (90:10 through to 10:90 in 10% increments). To explain this remarkable result, we introduce the concept of “supramolecular selection”, which is a process driven by molecular recognition that leads to the partially selective aggregation of like molecules during crystallization.
Raman spectroscopy has been used to study the structure of the humite mineral group ((A 2 SiO 4 ) n -A(OH, F) 2 where n represents the number of olivine and brucite layers in the structure and is 1, 2, 3 or 4 and A 2+ is Mg, Mn, Fe or some mix of these cations). The humite group of minerals forms a morphotropic series with the minerals olivine and brucite. The members of the humite group contain layers of the olivine structure that alternate with layers of the brucite-like sheets. The minerals are characterized by a complex set of bands in the 800-1000 cm −1 region attributed to the stretching vibrations of the olivine (SiO 4 ) 4− units. The number of bands in this region is influenced by the number of olivine layers. Characteristic bending modes of the (SiO 4 ) 4− units are observed in the 500-650 cm −1 region. The brucite sheets are characterized by the OH stretching vibrations in the 3475-3625 cm −1 wavenumber region. The position of the OH stretching vibrations is determined by the strength of the hydrogen bond formed between the brucite-like OH units and the olivine silica layer. The number of olivine sheets and not the chemical composition determines the strength of the hydrogen bonds.
Raman spectroscopy, complemented by infrared spectroscopy, has been used to characterise the ferroaxinite minerals of the theoretical formula Ca 2 Fe 2+ Al 2 BSi 4 O 15 (OH), a ferrous aluminium borosilicate. The Raman spectra are complex but are subdivided into sections on the basis of the vibrating units. The Raman spectra are interpreted in terms of the addition of borate and silicate spectra. Three characteristic bands of ferroaxinite are observed at 1082, 1056 and 1025 cm −1 and are attributed to BO 4 stretching vibrations. Bands at 1003, 991, 980 and 963 cm −1 are assigned to SiO 4 stretching vibrations. Bands are found in these positions for each of the ferroaxinites studied. No Raman bands were found above 1100 cm −1 showing that ferroaxinites contain only tetrahedral boron. The hydroxyl stretching region of ferroaxinites is characterised by a single Raman band between 3368 and 3376 cm −1 , the position of which is sample-dependent. Bands for ferroaxinite at 678, 643, 618, 609, 588, 572, 546 cm −1 may be attributed to the n 4 bending modes and the three bands at 484, 444 and 428 cm −1 may be attributed to the n 2 bending modes of the (SiO 4 ) 2− .
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