Based on a combination of molecular dynamics simulations, and Raman and Brillouin light scatterin... more Based on a combination of molecular dynamics simulations, and Raman and Brillouin light scattering spectroscopies, we investigate the structure and elastic properties relationship in an archetypical calcium silicate glass system.
Microstructure in silicate glasses is reviewed from the standpoint of density fluctuations, phase... more Microstructure in silicate glasses is reviewed from the standpoint of density fluctuations, phase separation and microsegregation. Recent experiments in Brillouin Scattering and Nuclear Magnetic Resonance (NMR) spin relaxation are used to characterise the length scale, magnitude and dimensionality of density fluctuations in monophasic silicate glasses and of phase separation in biphasic glasses. New Small Angle X-ray Scattering (SAXS) experiments are introduced which reveal the presence of much finer microstructure whose geometry is linked with the position and width of the First Sharp Diffraction Peak. For silica this structure resembles the quasiperiodicity recently identified in large tetrahedral model networks. For silicate glasses quasiperiodic order with a shorter spacing occurs in the microsegregation of glass modifiers from glass formers evident from Molecular Dynamic structures as well as from the empirically determined local structure.
ABSTRACT Some new crystallographic data on the cubic to tetragonal phase transition at Tc = 95 K ... more ABSTRACT Some new crystallographic data on the cubic to tetragonal phase transition at Tc = 95 K in CsCaCl3 allow us to infer the space group D184h for the low temperature phase connected with alternate rotations of the CaCl6 along 〈0 0 1〉 directions. From the relative distortion (c-a)/a we estimate the rotation angle which is the order parameter of the transition, smaller than in other chloride perovskites as CsPbCl3 and CsSrCl3.
ABSTRACT We have examined the titled glasses^1 in the 0.14 < x < 0.32 soda range (1... more ABSTRACT We have examined the titled glasses^1 in the 0.14 < x < 0.32 soda range (10 compositions) using T-modulated DSC. The T_g(x) deduced from the inflection point of the reversing heat-flow are found to decrease with x, at first ( 0.14 < x 0.23) more rapidly. Frequency corrected non-reversing heat flow term, DeltaH_nr(x),in these glasses is found to reveal a global minimum in the 0.19 < x < 0.23 composition range( thermally reversing window). The change in the specific heat jump at Tg accessed from the reversing heat flow, DeltaC_p(x), increases monotonically with x. A possible interpretation^2 of these results is that glasses are stressed rigid at x 0.23 in analogy to trends seen in the chalcogenides. 1.Y.Vaills, et al. Mater.Science and Engg.B40,199(1993). 2.M.Zhang and P.Boolchand , Science 266,1355(1994). Supported by NSF grant DMR 97-02189
ABSTRACT The EPR spectra of Gd3+ doped single crystal of LiYF4 are analyzed. The Gd3+ ion substit... more ABSTRACT The EPR spectra of Gd3+ doped single crystal of LiYF4 are analyzed. The Gd3+ ion substitutes for Y3+. The spin Hamiltonian parameters are determined by assuming a D2d point symmetry at the Gd3+ site. The values of the crystal field terms are tentatively discussed within the framework of a point charge calculation and of the superposition model.
(1 - x)SiO2-xNa2O glasses have been studied by Brillouin scattering when sodium oxide molar conce... more (1 - x)SiO2-xNa2O glasses have been studied by Brillouin scattering when sodium oxide molar concentration x, varies in the range 0.05 < x < 0.44. Non-linear dependence of elastic constants on sodium oxide concentration is shown unambiguously for the first time and the compressive elastic constant C11 experienced a non-monotonic behaviour with a minimum value around x = 0.15. These results are compared with those obtained previously in the case of other alkali silicate glasses (Li, K). The variations of elastic constants of these glasses with the alkali metal nature and concentration are discussed on the basis of relations with the molar density of silicon atoms and the force constants between modifiers and non-bridging oxygens.
Based on a combination of molecular dynamics simulations, and Raman and Brillouin light scatterin... more Based on a combination of molecular dynamics simulations, and Raman and Brillouin light scattering spectroscopies, we investigate the structure and elastic properties relationship in an archetypical calcium silicate glass system.
Microstructure in silicate glasses is reviewed from the standpoint of density fluctuations, phase... more Microstructure in silicate glasses is reviewed from the standpoint of density fluctuations, phase separation and microsegregation. Recent experiments in Brillouin Scattering and Nuclear Magnetic Resonance (NMR) spin relaxation are used to characterise the length scale, magnitude and dimensionality of density fluctuations in monophasic silicate glasses and of phase separation in biphasic glasses. New Small Angle X-ray Scattering (SAXS) experiments are introduced which reveal the presence of much finer microstructure whose geometry is linked with the position and width of the First Sharp Diffraction Peak. For silica this structure resembles the quasiperiodicity recently identified in large tetrahedral model networks. For silicate glasses quasiperiodic order with a shorter spacing occurs in the microsegregation of glass modifiers from glass formers evident from Molecular Dynamic structures as well as from the empirically determined local structure.
ABSTRACT Some new crystallographic data on the cubic to tetragonal phase transition at Tc = 95 K ... more ABSTRACT Some new crystallographic data on the cubic to tetragonal phase transition at Tc = 95 K in CsCaCl3 allow us to infer the space group D184h for the low temperature phase connected with alternate rotations of the CaCl6 along 〈0 0 1〉 directions. From the relative distortion (c-a)/a we estimate the rotation angle which is the order parameter of the transition, smaller than in other chloride perovskites as CsPbCl3 and CsSrCl3.
ABSTRACT We have examined the titled glasses^1 in the 0.14 &lt; x &lt; 0.32 soda range (1... more ABSTRACT We have examined the titled glasses^1 in the 0.14 &lt; x &lt; 0.32 soda range (10 compositions) using T-modulated DSC. The T_g(x) deduced from the inflection point of the reversing heat-flow are found to decrease with x, at first ( 0.14 &lt; x 0.23) more rapidly. Frequency corrected non-reversing heat flow term, DeltaH_nr(x),in these glasses is found to reveal a global minimum in the 0.19 &lt; x &lt; 0.23 composition range( thermally reversing window). The change in the specific heat jump at Tg accessed from the reversing heat flow, DeltaC_p(x), increases monotonically with x. A possible interpretation^2 of these results is that glasses are stressed rigid at x 0.23 in analogy to trends seen in the chalcogenides. 1.Y.Vaills, et al. Mater.Science and Engg.B40,199(1993). 2.M.Zhang and P.Boolchand , Science 266,1355(1994). Supported by NSF grant DMR 97-02189
ABSTRACT The EPR spectra of Gd3+ doped single crystal of LiYF4 are analyzed. The Gd3+ ion substit... more ABSTRACT The EPR spectra of Gd3+ doped single crystal of LiYF4 are analyzed. The Gd3+ ion substitutes for Y3+. The spin Hamiltonian parameters are determined by assuming a D2d point symmetry at the Gd3+ site. The values of the crystal field terms are tentatively discussed within the framework of a point charge calculation and of the superposition model.
(1 - x)SiO2-xNa2O glasses have been studied by Brillouin scattering when sodium oxide molar conce... more (1 - x)SiO2-xNa2O glasses have been studied by Brillouin scattering when sodium oxide molar concentration x, varies in the range 0.05 < x < 0.44. Non-linear dependence of elastic constants on sodium oxide concentration is shown unambiguously for the first time and the compressive elastic constant C11 experienced a non-monotonic behaviour with a minimum value around x = 0.15. These results are compared with those obtained previously in the case of other alkali silicate glasses (Li, K). The variations of elastic constants of these glasses with the alkali metal nature and concentration are discussed on the basis of relations with the molar density of silicon atoms and the force constants between modifiers and non-bridging oxygens.
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