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ARTICLE IN PRESS+ MODEL22 G. Gouadec, Ph. Colomban / Progress in Crystal Growth and Characterization of Materialsxx (2007) 1e56heating [189,221,292], couplings or defect-induced diffusions [292] may add to the size effecton the Raman spectra of nanomaterials. The stress will act on the position of the peaks [186],defects (non-stoichiometry for instance) will widen them [185,186,191] and temperature willact on both position and width. Falkovsky and Camassel [293] wrote a short review on the contributionof all kinds of defects to band broadening.4. Selected case studies4.1. The structural variety of glass ceramicsCeramics and glass-ceramics obtained from liquid precursors (molten glass and salts, solutions,gels, organic precursors [1,294e296]) form a large range of nanophased materials. Thetransition from the liquid to the solid state ‘‘freezes’’ a local steric disorder and thermal annealingis necessary for long distance Coulombian interactions to stabilize the thermodynamically4 3stable crystalline phase. In these materials, strongly covalent species like the SiO 4 or PO 4tetrahedra often experience a site symmetry different from the average one derived from crystallographicstudies [296,297]. For instance, Fig. 10a compares the Raman spectrum ofa 2GeO 2 eAl 2 O 3 e2H 2 O gel prepared through alkoxides hydrolysisepolycondensation withthose of the phases obtained after thermal treatment up to 1450 C [298]. The intermediateAl 2 Ge 2 O 7 phase (stable from 1250 to 1300 C) has a low symmetry monoclinic structure butfine low frequency Raman peaks, characteristic of an ordered compound, while the bands ofcrystalline mullite are as broad as those of the glass in spite of a higher symmetry orthorhombicstructure. This relates to an orientational disorder induced by oxygen vacancies in the AlO 4 tetrahedra.Moreover, since the occurrence of a broadening indicates distorted entities, it is(a)Mullite(b)T', R'BendingModesStretchingModesIntensity / arbitrary unitAl 2 Ge 2 O 7mesoporous GlassGel1450°C1250°C800°C11610290308318548441370310708776847Ba 6 Nb 2 O 11795742Ba 4 Ca 2 Nb 2 O 11200 400 600 800 10001200 200 400 600 800 1000 1200Wavenumber / cm -1Fig. 10. (a) Germanium mullite in different physical states (adapted from Ref. [298, pp. 161e168]). (b) Perovskiterelatedbarium niobiates (Adapted from Ref. [299, pp. 339e347]).Please cite this article in press as: G. Gouadec, Ph. Colomban, Prog. Cryst. Growth Charact. Mater. (2007),doi:10.1016/j.pcrysgrow.2007.01.001
ARTICLE IN PRESS+ MODELG. Gouadec, Ph. Colomban / Progress in Crystal Growth and Characterization of Materials 23xx (2007) 1e56obvious from the figure that the GeO 4 tetrahedra as well as GeO 6 octahedra are more distortedin the mesoporous glass than in the gel phase [298]. In the example of the perovskite-relatedniobiates from Fig. 10b, the NbeO modes around 800 cm 1 are split because of the occasionalpresence of oxygen vacancies in the NbO 6 octahedra. The (T 0 ,R 0 ) modes involving the Baatoms located at the corners of the unit-cell are not modified by these vacancies in Ba 6 Nb 2 O 11(hence the fine peaks) but are split again in Ba 4 Ca 2 Nb 2 O 11 because chemical and mass decouplingisolate the Ba and Ca sub-lattices [299]. The same kind of vibrational decoupling due todefects has been observed for many silicates [296,297,300].Two silicate spectra are shown in Fig. 11a and b. Most types of glass contain nanometriccrystallites and the spectrum of silicates (SiO 2 tetrahedral network) is intermediate betweenthat of a disordered network and the superposition of contributions from definite vibrational entities:tetrahedra rings in vitreous silica (the narrow defect bands in Fig. 11a originate from thevibrations of threefold (D 2 ) and fourfold (D 1 ) rings [301e304]) and complex arrangements ofSiO 4 tetrahedra in alkali/earth-alkali silicates [301]. The deformation (d SieOeSi w 500 cm 1 )and elongation (n SieO w 800e1200 cm 1 ) modes depend on the connectivity of the SiO 4 tetrahedraand can be fitted with components called Q n and Q 0 n , respectively (n represents thenumber of SieOeSi bridges per tetrahedron) [305e307]. The position and area of these componentsthen constitute characteristic parameters of silicate glass nanostructures and can beused for garnering additional information such as the original composition or the sintering temperature[305,306].The most specific part of glass spectra is a band peaking at w50 to 100 cm 1 , which iscalled the Boson Peak (BP) because its intensity obeys a BoseeEinstein distribution. Since(a)BendingmodesStretchingmodes(b)MolecularsignatureD 1*D 2Wavenumber / cm -1Intensity / a.u.**BosonPeakBosonPeak00 75 150 500 1000Fig. 11. (a) Raman spectrum of pure glassy silica (macro-configuration, l ¼ 406 nm; D 1 and D 2 : see text). (b) Ramanspectrum of a highly depolymerised potassium-rich calcium silicate stained glass (macro-configuration, l ¼ 413 nm;stars indicate plasma lines from the laser).Please cite this article in press as: G. Gouadec, Ph. Colomban, Prog. Cryst. Growth Charact. Mater. (2007),doi:10.1016/j.pcrysgrow.2007.01.001
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