The Brillouin elastic free energy change ∆Φ between thermally annealed and quenched (N a 2 O) x (SiO 2 ) 1−x glasses is found to decrease linearly at x > 0.23 ( floppy phase), and to nearly vanish at x < 0.18 (stressed-rigid phase).The observed ∆Φ(x) variation closely parallels the mean-field floppy mode fraction f (x) in random networks, and fixes the two (floppy, stressed-rigid) elastic phases. In calorimetric measurements, the non-reversing enthalpy near T g is found to be large at x < 0.18 and at x > 0.23 , but to nearly vanish in the 0.18 < x < 0.23 range, suggesting existence of an intermediate phase between the floppy and stressed-rigid phases.Pacs: 61.43Fs, 63.50.+x
Molecular dynamics (MD) simulations and Brillouin light scattering (BLS) spectroscopy experiments have been carried to study the structure of sodium silicate glasses (SiO)(NaO), where X ranges from 0 to 45 at room temperature. The MD-obtained glass structures have been subjected to energy minimization at zero temperature to extract the elastic constants also obtained by BLS spectroscopy. The structures obtained are in good agreement with the structural experimental data realized by different techniques. The simulations show that the values of the elastic constants as a function of X (i.e., NaO mol %) agree well with those measured by BLS spectroscopy. The variations of elastic constants C and C as a function of NaO mol % are discussed and correlated to structural results and potential energies of oxygen atoms.
The structure of binary (PbO) x (P 2 O 5 ) .1−x/ glasses was investigated using Raman scattering and infrared spectroscopy, over the compositional range x = 0.5-0.68. The distribution of the phosphate groups (PO 2 ) − , (PO 3 ) 2− , (P 2 O 7 ) 4− and (PO 4 ) 3− , determined from the simulation of Raman spectra in the 800-1500 cm −1 wavenumber range, indicates a significant disproportionation reaction near the pyrophosphate composition and shows a phosphorus connectivity scheme in the glass structure in agreement with 31 P magic angle spinning NMR results. The low-wavenumber Raman spectra of glasses are discussed in the light of the spectra of crystals. The mode laying in the far-infrared region and due to lead-oxygen vibrations exhibits a composition-dependent wavenumber that correlates with bulk glass properties such as glass transition temperature.
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