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Étude par dynamique moléculaire de spectres vibrationnels de verres de silice

Abstract : The structure and dynamics of model silica glasses was studied at the microscopic level by means of molecular dynamics simulations. Four potential models were tested in the calculation of structural characteristics and of the vibrational spectra of three cristalline silica polymorphs : a-quartz, a-cristobalite, and faujasite. While the structure of the solids is well reproduced by all four force fields, the generalized valence force field gives a better agreement for the vibrational spectra.
The dynamics of substructures in amorphous silica was analyzed by the calculation of the spectrum of the breathing coordinate of three- and four membered SiO4 rings (3R and 4R, respectively) and of the manifestation of this vibration in the Raman spectrum of model glasses. Results of the calculations show that the D2 feature at 625 cm-1 can be assigned to the breathing motion of the 3R rings, whereas such an assignment of the D1 peak at 496 cm-1 to the 4R rings is ambiguous.
The study of silica glasses doped by Er3+ ions indicates the formation of clusters of ions already at a low concentration. An increase of the concentration results in the formation of large cluster in which the erbium ions are connected to each other by double bridges Er-O-Er and such clusters can be considered as precursors of the formation of an Er2O3 phase. The analysis of the structure of the glasses shows an important increase of the number of 3R and 4R rings due to the formation of rings of mixed composition.
The large differences of masses between the Er and Si atoms as well as of the bond force constants lead to a decoupling of the Er-O bond vibration from the dynamics of the silica matrix. This manifests itself in an increase of the intensity at low frequencies in the density of vibrational states of the doped glasses.
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Thi Thao To. Étude par dynamique moléculaire de spectres vibrationnels de verres de silice. Analyse de données, Statistiques et Probabilités [physics.data-an]. Université des Sciences et Technologie de Lille - Lille I, 2007. Français. ⟨tel-00282362v2⟩

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