Chen H

Theoretical study of the local structure and Raman spectra of CaO-SiO2 binary melts

A procedure for the Raman spectra calculation of vitreous and molten silicates was presented in this paper. It includes molecular dynamics MD simulation for the generation of equilibrium configurations, Wilsons GF matrix method for the calculations of eigenfrequencies and corresponding vectors, electro-optical parameters method (EOPM) for the Raman intensity calculations, and the bond polarizability model (BPM) for the determination of polarizability and polarizability derivative. One of the most important characteristics of this procedure is the achievement of the partial Raman spectra of five tetrahedral units, as well as the total spectral envelope. In this paper, the calculation was carried out for the vitreous and molten calcium silicates with different compositions and at various temperatures. It is worthwhile to note that the calculation is based on statistical configurations distribution in the space and so it is not needed to artificially adjust the full width at half maximum (FWHM) of spectra. It was also tested through the good agreement of the calculated spectra with the experimental, including some regularity of spectral properties. According to the calculation, the symmetrical stretching of whole tetrahedral units, to which the stretching of Si-O(nb) bond gives the main contribution to intensity, is proven to be the dominance in the high-frequency range (800-1200 cm(-1)) and the symmetrical bending of Si-O(b)-Si, to which the stretching of Si-O(b) bond exhibits the main contribution, is the dominance in the medium-frequency range (400-700 cm(-1)). As the first theoretical results, the Raman scattering coefficient of each Q(i) was found little change along with the variation of composition and temperature.

Isomorphic representations of hyperfine structure of binary silicates by interior stress, vibrational wavenumber and spacial fractional dimension

Stress index of tetrahedron (SIT) was defined to describe the topological connectivities among various silicon-oxygen tetrahedra (SiOT) in anionic clusters of binary silicate crystals, glasses and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. And also the spatial fractional dimension of hyperfine structure was introduced while comparative analysis being made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cation were investigated. It was demonstrated that alkali cation has little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen (NBO) of SiOT, but remarkably affects its Raman active optical cross section, as was consensus resulted from ab initio calculation. It also can be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cation, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides.

Raman Spectra and Structure Study of Silicate Glasses and Their Liquids

Stress index of tetrahedron (SIT) was defined to describe the topological connectivities among various silicon-oxygen tetrahedra (SiOT) in anionic clusters of binary silicate crystals, glasses, and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. The spatial fractional dimension of hyperfine structure was introduced while comparative analysis was made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber, and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cations were investigated. It was demonstrated that alkali cations have little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen (NBO) of SiOT, but remarkably affect its Raman active optical cross section, as was consensus resulted from ab initio calculation. It can also be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cations, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides. And Raman optical activity extinct effect of isolated SiOT at high basicity should be considered while being applied to quantitatively analysis.

The Raman coupling coefficients of calcium and sodium silicate melts in high-frequency region - SiOT model study

The Raman coupling coefficient (CC), which was defined by Shuker and Gammon, of silicate melt in high-frequency region, is still an untouched subject but the key parameter for quantitatively processing the Raman spectra (RS) to achieve the abundance of microstructural units. In this paper, we present the results of a newly constructed model - SiOT model (Wu et al) about the Raman coupling coefficients of calcium and sodium silicate melts, especially in the high-frequency region. This new model combines classical MD simulation with decomposition of simulated configurations and vibrational analysis including Wilsons GF matrix method, electro-optical parameter method (EOPM) and bond polarizability model (BPM). The displacement dependence of the cluster polarizability through the combination of EOPM and BPM connotes the description of the frequency dependence of CC. This allows us to make a direct comparison for the first time between the calculated VDOS (vibrational density of states) and RS within the entire frequency range. A strong conclusion was given that the partial VDOS, RS and CC are all the intrinsic properties of respective Qi species, and the only variable inducing the change in total VDOS, RS and CC is the change of microstructure, i.e. the distribution of the Qi.

A new method for the Raman spectra calculation of binary silicate melts: application to the calcium silicate

To interpret the experimental Raman spectra of vitreous and molten silicate in depth, theoretical understanding of the vibrational properties of silicate is indispensable. Up to now many researchers have concentrated to this field, e.g. Sen and Thorpe [1], Garofalini [2], Gaskell [3], Bell and Hibbins-Butle [4], Brawer [5], Furukawa & Fox & White [6], Dowty [7], and Zotov et al.[8]. Summary of their works can achieve a uniform character when applying the calculation to the disordered system that the broadening of all calculated bands was artificial but not authentic. As we know, the full widths of half maximum (fwhm) of the bands are directly related with the disorder degree of system, i.e. the distribution of the structural properties. Thus, we developed a new special method or called routine for the Raman spectra calculation of amorphous silicates, which broadens the calculated bands automatically. Firstly, classical molecular dynamics (MD) simulation is performed to obtain thousands of equilibrium configurations, which will be decomposed into five kinds of Si-O tetrahedra as defined in Fig. 1, which are denoted as Qi which subscript i represents the number of bridging oxygen.