The structure of the tetrahedral backbone and the nature and time scale of the temperature-dependent structural changes in binary Ge-Se glasses and supercooled liquids with ≤33.33 atom % Ge have been investigated using ambient and high-temperature Raman spectroscopy. The composition dependence of the relative fractions of edge- and corner-shared GeSe(4) tetrahedra and that of the characteristic mean vibrational frequencies of these structural units are shown to be consistent with a structural model for these glasses based on a random interconnection between GeSe(4) tetrahedra and Se-Se chain fragments. The most prominent temperature-dependent structural change in the Ge(20)Se(80) glass and supercooled liquid involves progressive conversion of the edge-shared GeSe(4) tetrahedra into corner-shared tetrahedra, upon lowering of temperature. The time scale of this tetrahedral conversion "reaction" corresponds well with those of enthalpy and shear relaxation near glass transition. Moreover, the temperature dependence of this GeSe(4) tetrahedral speciation is shown to be the most important source for the production of configurational entropy in this supercooled liquid near the glass-transition range, signifying a direct link between structural relaxation, configurational entropy, and viscous flow.

The structural characteristics of novel alkaline-earth suborthosilicate glasses along the compositional join (1 - x)(Ca(0.5)Mg(0.5)O) - xSiO(2) with 0.28 ≤ x ≤ 0.33 are investigated using high resolution (29)Si and (17)O nuclear magnetic resonance spectroscopy. The structures of these glasses consist of isolated Q(0) and Q(1) anionic species and Mg(2+) and Ca(2+) countercations that are held together by Coulombic interactions. The concentration of the Q(1) species rapidly decreases with decreasing SiO(2) content and becomes undetectable in the glass with x = 28 mol %. The compositional variation of the physical properties of these glasses such as glass transition temperature and density can be attributed to the Q-speciation in the structure. The NBOs are associated with a random distribution of the alkaline-earth cations in their nearest neighbor coordination shell. The resulting random packing of dissimilar Ca-NBO and Mg-NBO coordination polyhedra may give rise to structural and topological frustration responsible for the unusual glass-forming ability of these suborthosilicate liquids with extremely low SiO(2) contents. Finally, the composition and the formation of Q(1) species necessitate the formation of free O(2-) ions in the structure of these glasses that are only bonded to Mg(2+) and Ca(2+) cations. The (17)O NMR results presented in this study allow for direct observation of such oxygen species.

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A fundamental mechanistic understanding of the pressure- and/or temperature-induced facile transformation of the coordination environment of boron is important for changing the physical properties of glass. We have used in situ high-pressure (up to 2 gigapascals) boron-11 solid-state nuclear magnetic resonance spectroscopy in combination with ab initio calculations to investigate the nature of the transition state for the pressure-induced BO3→ BO4 conversion in a borosilicate glass at ambient temperature. The results indicate an anisotropic elastic deformation of the BO3 planar triangle, under isotropic stress, into a trigonal pyramid that likely serves as a precursor for the subsequent formation of a BO4 tetrahedron.