Structure and properties of densified silica glass: characterizing the order within disorder

世界一構造秩序のあるガラスの合成と構造解析に成功 --ガラスの一見無秩序な構造の中に潜む秩序を抽出--. 京都大学プレスリリース. 2021-12-25.The broken symmetry in the atomic-scale ordering of glassy versus crystalline solids leads to a daunting challenge to provide suitable metrics for describing the order within disorder, especially on length scales beyond the nearest neighbor that are characterized by rich structural complexity. Here, we address this challenge for silica, a canonical network-forming glass, by using hot versus cold compression to (i) systematically increase the structural ordering after densification and (ii) prepare two glasses with the same high-density but contrasting structures. The structure was measured by high-energy X-ray and neutron diffraction, and atomistic models were generated that reproduce the experimental results. The vibrational and thermodynamic properties of the glasses were probed by using inelastic neutron scattering and calorimetry, respectively. Traditional measures of amorphous structures show relatively subtle changes upon compacting the glass. The method of persistent homology identifies, however, distinct features in the network topology that change as the initially open structure of the glass is collapsed. The results for the same high-density glasses show that the nature of structural disorder does impact the heat capacity and boson peak in the low-frequency dynamical spectra. Densification is discussed in terms of the loss of locally favored tetrahedral structures comprising oxygen-decorated SiSi4 tetrahedra

Er3+ infrared fluorescence affected by spatial distribution synchronicity of Ba2+ and Er3+ in Er3+-doped BaO–SiO2 glasses

Glasses with the composition xBaO–(99.9 − x)SiO2–0.1ErO3/2 (0 ≤x ≤ 34.9) were fabricated by a levitation technique. The glasses in the immiscibility region were opaque due to chemical inhomogeneity, while the other glasses were colorless and transparent. The scanning electron microscope observations and electron probe microanalysis scan profiles revealed that more Er3+ ions were preferentially distributed in the regions where more Ba2+ ions existed in the chemically inhomogeneous glasses. The synchronicity of the spatial distributions of the two ions initially increased with increasing x and then decreased when the Ba2+ concentration exceeded a certain value. The peak shape and lifetime of the fluorescence at 1.55 μm depended on x as well as the spatial distribution of both ions. These results indicate that although ErOn polyhedra are preferentially coordinated with Ba2+ ions and their local structure is affected by the coordination of Ba2+, there is a maximum in the amount of Ba2+ ions that can coordinate ErOn polyhedra since the available space for Ba2+ ions is limited. These findings provide us with efficient ways to design the chemical composition of glasses with superior Er3+ fluorescence properties for optical communication network systems

Theoretical Insights into the ²⁷Al NMR Parameters of Binary Aluminosilicate Glass and Their Relationship to the Atomic and Electronic Structure

Al-rich 60Al2O3–40SiO2 glass is a candidate for technological applications in electronic and optical devices. Though the amorphous structure of the glass has been studied using solid-state NMR and simulation approaches, the atomic and electronic structure have not been fully revealed. Solid-state 27Al NMR spectra reflect the 27Al environment, though a comprehensive understanding of the spectra and local structure is challenging when interpreting the broadened peak shapes of the amorphous state. Here, an accurate atomic structure of 60Al2O3–40SiO2 glass was modeled using ab initio molecular dynamics (AIMD) simulations containing 418 atoms and employing the melt-quenching route with 15 K/ps. This simulation approach reproduced X-ray diffraction data better than classical molecular dynamics (CMD) simulations. The structure of the polyhedra formed by O bonded to Al was quantitatively analyzed by evaluating bond-angle distributions and the degree of symmetry using spherical harmonic functions. The relationship between chemical shifts and charge-balancing mechanisms was explored through the analysis of electronic structures obtained from AIMD-derived structures. Interestingly, the Al partial charge and the spatial electron distribution of Al–O bonds were independent of the Al coordination number, implying that valence electrons are not localized to specific atoms but are rather distributed throughout the glass network. The theoretical distribution of 27Al NMR parameters was obtained through statistical analysis of theoretically calculated NMR parameters for 100 AIMD-derived structures. By comparing the experimental 27Al NMR data with the theoretical distribution, the previously unclear relationship between 27Al NMR parameters and local structure was elucidated

Theoretical Insights into the ²⁷Al NMR Parameters of Binary Aluminosilicate Glass and Their Relationship to the Atomic and Electronic Structure

Al-rich 60Al2O3–40SiO2 glass is a candidate for technological applications in electronic and optical devices. Though the amorphous structure of the glass has been studied using solid-state NMR and simulation approaches, the atomic and electronic structure have not been fully revealed. Solid-state 27Al NMR spectra reflect the 27Al environment, though a comprehensive understanding of the spectra and local structure is challenging when interpreting the broadened peak shapes of the amorphous state. Here, an accurate atomic structure of 60Al2O3–40SiO2 glass was modeled using ab initio molecular dynamics (AIMD) simulations containing 418 atoms and employing the melt-quenching route with 15 K/ps. This simulation approach reproduced X-ray diffraction data better than classical molecular dynamics (CMD) simulations. The structure of the polyhedra formed by O bonded to Al was quantitatively analyzed by evaluating bond-angle distributions and the degree of symmetry using spherical harmonic functions. The relationship between chemical shifts and charge-balancing mechanisms was explored through the analysis of electronic structures obtained from AIMD-derived structures. Interestingly, the Al partial charge and the spatial electron distribution of Al–O bonds were independent of the Al coordination number, implying that valence electrons are not localized to specific atoms but are rather distributed throughout the glass network. The theoretical distribution of 27Al NMR parameters was obtained through statistical analysis of theoretically calculated NMR parameters for 100 AIMD-derived structures. By comparing the experimental 27Al NMR data with the theoretical distribution, the previously unclear relationship between 27Al NMR parameters and local structure was elucidated

Drastic Connectivity Change in High Refractive Index Lanthanum Niobate Glasses

The highly ionic, high refractive index La₂O₃–Nb₂O₅ system has a La-rich glass forming region and another Nb-rich glass forming region. The La-rich and Nb-rich regions have markedly different structural and physical properties. Structural analyses using diffraction and spectroscopic measurements combined with structural modeling show that the Nb-rich glass, which has unusually high oxygen packing density, is a network of distorted NbO_<i>n</i> polyhedra with mainly corner-sharing, and LaO_<i>x</i> polyhedra with both corner-sharing and edge-sharing. Contrastingly, in the La-rich glass, small-sized symmetrical NbO_<i>n</i> polyhedra with a large amount of edge-sharing are inhomogenously distributed in the network of LaO_<i>x</i> polyhedra. The drastic connectivity change of cation–oxygen polyhedra and the dense oxygen packing due to edge-sharing polyhedra contravene long-established rules of oxide glass formation. These results raise the possibility that novel higher refractive index with lower wavelength dispersion glasses, which contain highly ionic heavy elements at the lower left in the periodic table, may be synthesized