37 research outputs found
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 <sup>27</sup>Al 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 <sup>27</sup>Al 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 La<sub>2</sub>O<sub>3</sub>–Nb<sub>2</sub>O<sub>5</sub> 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<sub><i>n</i></sub> polyhedra with mainly corner-sharing,
and LaO<sub><i>x</i></sub> polyhedra with both corner-sharing
and edge-sharing. Contrastingly, in the La-rich glass, small-sized
symmetrical NbO<sub><i>n</i></sub> polyhedra with a large
amount of edge-sharing are inhomogenously distributed in the network
of LaO<sub><i>x</i></sub> 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
Expansion of the Hexagonal Phase-Forming Region of Lu<sub>1–<i>x</i></sub>Sc<sub><i>x</i></sub>FeO<sub>3</sub> by Containerless Processing
Hexagonal Lu<sub>1–<i>x</i></sub>Sc<sub><i>x</i></sub>FeO<sub>3</sub> (0
≤ <i>x</i> ≤ 0.8) was directly solidified
from an undercooled melt by containerless processing with an aerodynamic
levitation furnace. The hexagonal phase-forming region was considerably
extended compared to that of the conventional solid-state reaction
(<i>x</i> ∼ 0.5). Synchrotron X-ray diffraction measurements
revealed that the crystal structure of the hexagonal phase was isomorphous
to hexagonal ferroelectric RMnO<sub>3</sub> (R = a rare earth ion)
with a polar space group of <i>P</i>6<sub>3</sub><i>cm</i>. As <i>x</i> increased, the <i>a</i>-axis lattice constant decreased linearly, strengthening the antiferromagnetic
interaction between the Fe<sup>3+</sup> ions on the <i>a–b</i> plane. Accordingly, the weak ferromagnetic transition temperature
increased from 150 K for <i>x</i> = 0 to 175 K for <i>x</i> = 0.7. These transition temperatures were much higher
than those of hexagonal Lu<sub>1–<i>x</i></sub>Sc<sub><i>x</i></sub>MnO<sub>3</sub>. The results indicate that
hexagonal Lu<sub>1–<i>x</i></sub>Sc<sub><i>x</i></sub>FeO<sub>3</sub> is a suitable alternative magnetic dielectric
for use at higher temperatures