Crystal-Like Atomic Arrangement and Optical Properties of 25La2O3-75MoO3 Binary Glasses Composed of Isolated MoO42.

Transparent and brown La2O3-MoO3 binary glasses were prepared in bulk form using a levitation technique. The glass-forming range was limited, with the primary composition being approximately 25 mol % La2O3. The 25La2O3-75MoO3 glass exhibited a clear crystallization at 546 °C, while determining its glass transition temperature was difficult. Notably, despite its amorphous nature, the glass possessed a density and packing density comparable to those of crystalline La2Mo3O12. X-ray absorption fine structure and Raman scattering analyses revealed that the glass structure closely resembles La2Mo3O12 due to the presence of isolated MoO42- units, whereas disordered atomic arrangement around La atoms was confirmed. The glass demonstrated transparency ranging from 378 to 5500 nm, and the refractive index at 1.0 µm was estimated to be 2.0. The optical bandgap energy was 3.46 eV, which was slightly smaller than that of La2Mo3O12. Additionally, the glass displayed a transparent region ranging from 6.5 to 8.0 µm. This occurrence results from the decreased diversity of MoOn units and connectivity of Mo-O-Mo, which resulted in the reduced overlap of multiphonon absorption. This glass formation, with its departure from conventional glass-forming rules, resulted in distinctive glasses with crystal-like atomic arrangements.

Atomic and Electronic Structure in MgO-SiO2.

Understanding disordered structure is difficult due to insufficient information in experimental data. Here, we overcome this issue by using a combination of diffraction and simulation to investigate oxygen packing and network topology in glassy (g-) and liquid (l-) MgO-SiO2 based on a comparison with the crystalline topology. We find that packing of oxygen atoms in Mg2SiO4 is larger than that in MgSiO3, and that of the glasses is larger than that of the liquids. Moreover, topological analysis suggests that topological similarity between crystalline (c)- and g-(l-) Mg2SiO4 is the signature of low glass-forming ability (GFA), and high GFA g-(l-) MgSiO3 shows a unique glass topology, which is different from c-MgSiO3. We also find that the lowest unoccupied molecular orbital (LUMO) is a free electron-like state at a void site of magnesium atom arising from decreased oxygen coordination, which is far away from crystalline oxides in which LUMO is occupied by oxygen's 3s orbital state in g- and l-MgO-SiO2, suggesting that electronic structure does not play an important role to determine GFA. We finally concluded the GFA of MgO-SiO2 binary is dominated by the atomic structure in terms of network topology.

Structural Origin of Additional Infrared Transparency and Enhanced Glass-Forming Ability in Rare-Earth-Rich Borate Glasses without B-O Networks.

R2O3-B2O3 binary glasses (R denotes rare-earth elements or Y) were fabricated in a very wide composition region using a levitation technique. The maximum R2O3 content of light rare-earth compounds reached 63 mol % and decreased with a decrease in the ionic radius of R3+. The thermal, optical, vibrational, and structural properties were investigated, particularly for 50R2O3-50B2O3 glasses. The glass transition temperature increased with a decrease in the ionic radius of R3+, while the thermal stability was not affected by the glass composition. The packing density increased with a decrease in the ionic radius of R3+ due to lanthanoid contraction. Raman scattering and Fourier transform infrared spectra revealed that, in the rare-earth-rich glasses, no conventional three-dimensional networks consisting of corner-sharing BOn (n = 3 or 4) units existed because all B atoms were formed as isolated BO3 units. The simple environment around B atoms in the glasses led to additional IR transmittance regions, irrespective of the kinds of R. The total correlation functions obtained from high-energy X-ray diffraction measurements were analyzed using the pair-function method and compared with those of various RBO3 crystalline phases. It was suggested that the local structure around R resembles the ν-NdBO3-type crystal structure, and the O coordination number of R ranged from 6.5 to 7.7, smaller than that of the crystalline phase. The glass-forming ability depending on R was discussed based on the structural similarities between the melt, glass, and crystalline phases.

Influence of modifier cations on the local environment of aluminum in La2O3-Al2O3 and Y2O3-Al2O3 binary glasses.

The local environment of aluminum in xLa2O3-(100 -x)Al2O3 (LA) and yY2O3-(100 -y)Al2O3 (YA) glasses is investigated using 27Al MAS NMR and Raman scattering spectroscopy in the glass forming range, i.e., 27 ≤x≤ 50 and 26 ≤y≤ 37.5, respectively. The results suggest that four-fold Al ([4]Al) is predominant in both glasses, with a content of approximately 90% in LA glasses and 60% in YA glasses; the remaining percentages are made up of five- ([5]Al) and six-fold ([6]Al) Al. In LA glasses, the fraction of [4]Al increases slightly with an increase in the La2O3 content, whereas, in YA glasses, the distribution of Al species does not change. Raman scattering spectroscopy reveals that, in LA glasses, the amount of non-bridging oxygen increases with an increase in x; moreover, [5]Al and [6]Al both increase with a decrease in x for x≤ 40. Comparing with alkali earth aluminate glasses, we discuss the relationship between the fundamental structure of the AlO4 network and two parameters (the ratio of the number of oxygen atoms to that of Al and field strength).

Fluorescence characterization of heavily Eu3+-doped lanthanum gallate glass spheres with high quenching concentration.

Highly Eu2O3-doped (up to 30 mol%) La2O3-Ga2O3 glasses were synthesized by an aerodynamic levitation technique. The red emission associated with D05 level of Eu3+was most intense at 20 mol% Eu2O3, indicating a small effect of concentration quenching even at high Eu concentration. The fluorescence decay curves indicated that the dominant transition mechanism at high Eu concentration was fast energy migration among the Eu3+ ions, which averaged the environment of the Eu, yielding a nearly single-exponential decay of D05→F27 emission. Lifetime of the D05 level gradually decreased with Eu2O3 over the 5-25 mol% and rapidly decreased at 30 mol%, very consistent with the observed emission spectra.

Structural analysis of sulfuric acid solutions containing Ti and Mn using x-ray diffraction, x-ray absorption fine structure, and molecular dynamics simulation.

Structural analyses have been performed for sulfuric acid (H2SO4) aqueous solutions containing Ti and/or Mn. These solutions are used as the positive electrolyte in the Ti-Mn redox flow (RF) battery, in which it had been found that adding Ti4+ in the positive electrolyte is very effective to reduce the MnO2 precipitation at a high state of charge. X-ray diffraction and x-ray absorption fine structure (XAFS) measurements were employed in order to obtain total correlation functions, T(r)'s, and coordination numbers around Mn and Ti in the solutions, respectively. The T(r)'s showed some peculiar peaks that were assigned to correspond to S-O, O-O, Mn-O, and Ti-O pairs in the solutions. The XAFS analysis demonstrated that both Mn and Ti have 6-coordinating oxygen atoms in the solutions. The classical molecular dynamics simulation was also carried out to obtain structural models of the solutions. By tuning the Born-Mayer type potential parameters, the T(r)'s calculated from the models showed good agreement with the experimental ones. Regarding the coordination number, the 6-coordinated Mn-O was reproduced successfully, while we need further investigation to find parameters that can reproduce the 6-coordinated Ti-O in the solutions. The simulation results also indicated the existence of Ti-SO4 bonds, which should promote the H+ dissociation from HSO4- and increase the H+ concentration in the solutions. This may be effective to suppress the MnO2 precipitation at a high state of charge in the RF battery.

Network topology for the formation of solvated electrons in binary CaO-Al2O3 composition glasses.

Glass formation in the CaO-Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO-Al2O3 glasses using combined density functional theory-reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O-Ca and O-Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al-O is stronger than that of Ca-O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71-74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

Expansion of the hexagonal phase-forming region of Lu1-xScxFeO3 by containerless processing.

Hexagonal Lu1-xScxFeO3 (0 ≤ x ≤ 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 (x ∼ 0.5). Synchrotron X-ray diffraction measurements revealed that the crystal structure of the hexagonal phase was isomorphous to hexagonal ferroelectric RMnO3 (R = a rare earth ion) with a polar space group of P63cm. As x increased, the a-axis lattice constant decreased linearly, strengthening the antiferromagnetic interaction between the Fe(3+) ions on the a-b plane. Accordingly, the weak ferromagnetic transition temperature increased from 150 K for x = 0 to 175 K for x = 0.7. These transition temperatures were much higher than those of hexagonal Lu1-xScxMnO3. The results indicate that hexagonal Lu1-xScxFeO3 is a suitable alternative magnetic dielectric for use at higher temperatures.

Containerless solidification of undercooled SrO-Al2O3 binary melts.

The solidification of the SrO-Al2O3 binary system was investigated under containerless conditions using an aerodynamic levitation furnace. Glass formation was observed in compositions with 35-45 mol% SrO and 55-75 mol% SrO. Cooling curves were obtained at a constant cooling rate in the range of 1-1000 °C s(-1). The crystallization temperature was apparently independent of the cooling rate and far below the melting point when the sample was fully crystallized, whereas it decreased when the sample was partially crystallized. The difference between the crystallization temperature and the melting point under containerless conditions is considered a good measure of the glass-forming ability when there is not much difference in the critical cooling rates between the melt compositions. Furthermore, the homogeneous nucleation theory suggests that the apparent time-independent crystallization temperature is attributed to the high glass-forming ability of the SrO-Al2O3 binary system. The results suggest that the experimentally obtained continuous cooling transformation diagrams under containerless conditions provide new insights regarding solidification from an undercooled melt.

Weak ferromagnetic transition with a dielectric anomaly in hexagonal Lu0.5Sc0.5FeO3.

Lu1-xScxFeO3 (0 ≤ x ≤ 1) was synthesized by a conventional solid-state reaction. The hexagonal phase appeared at 0.4 ≤ x ≤ 0.6, between the perovskite phase (0 ≤ x ≤ 0.3) and the bixbyite phase (0.7 ≤ x ≤ 1). Structural, magnetic, and dielectric properties of hexagonal Lu0.5Sc0.5FeO3 were investigated. Synchrotron X-ray diffraction measurements revealed that the crystal structure of Lu0.5Sc0.5FeO3 is isomorphic to hexagonal ferroelectrics RMnO3 (R = rare earth ion) with a polar space group of P63cm. A weak ferromagnetic transition with a dielectric anomaly occurred at a much higher temperature (162 K) than those in hexagonal RMnO3. Although remanent magnetization was observed below the transition temperature, it decreased to almost zero at 10 K. These results indicate a strong antiferromagnetic interaction between ground-state Fe(3+) ions on the triangular lattice.

Siliceous zeolite-derived topology of amorphous silica.

The topology of amorphous materials can be affected by mechanical forces during compression or milling, which can induce material densification. Here, we show that densified amorphous silica (SiO2) fabricated by cold compression of siliceous zeolite (SZ) is permanently densified, unlike densified glassy SiO2 (GS) fabricated by cold compression although the X-ray diffraction data and density of the former are identical to those of the latter. Moreover, the topology of the densified amorphous SiO2 fabricated from SZ retains that of crystalline SZ, whereas the densified GS relaxes to pristine GS after thermal annealing. These results indicate that it is possible to design new functional amorphous materials by tuning the topology of the initial zeolitic crystalline phases.

Densification in transparent SiO2 glasses prepared by spark plasma sintering.

Recently, spark plasma sintering (SPS) has become an attractive method for the preparation of solid-state ceramics. As SPS is a pressure-assisted low-temperature process, it is important to examine the effects of temperature and pressure on the structural properties of the prepared samples. In the present study, we examined the correlation between the preparation conditions and the physical and structural properties of SiO2 glasses prepared by SPS. Compared with the conventional SiO2 glass, the SPS-SiO2 glasses exhibit a higher density and elastic modulus, but a lower-height first sharp diffraction peak of the X-ray total structure factor. Micro-Raman and micro-IR spectra suggest the formation of heterogeneous regions at the interface between the SiO2 powders and graphite die. Considering the defect formation observed in optical absorption spectra, reduction reaction mainly affects the densification of SPS-SiO2 glass. Hence, the reaction at the interface is important for tailoring the structure and physical properties of solid-state materials prepared by the SPS technique.

Oxygen close-packed structure in amorphous indium zinc oxide thin films.

Amorphous indium zinc oxide (IZO) thin film structures of varying amounts of Zn content were investigated using X-ray diffraction measurements and molecular dynamics (MD) simulations. The characteristic amorphous structure having high oxygen coordination number and edge-shared polyhedra were confirmed using both techniques. Detailed analysis of the structural model revealed that the oxygen close-packed structure was almost realized in the nanometer range. It was also found that the number of Zn ions occupying the tetrahedral site of the oxygen close-packed structure increased with increasing ZnO content although In ions occupied the octahedral site. We conclude that the amorphous structure stability of the indium zinc oxide thin films is enhanced by the existence of Zn ions in the tetrahedral site, which block In ions in the octahedral site ordering similar to that in an In(2)O(3) crystal.

Revealing Spatial Distribution of Al-Coordinated Species in a Phase-Separated Aluminosilicate Glass by STEM-EELS.

Structure determination of glass remains an important issue in glass science. The electron microscope with its high spatial resolution and versatile functions has been widely applied to observe phase separation and structural heterogeneity in glass. While elemental analysis such as energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) may provide local compositional information with nanometer-scale resolution, structural information in a glass network cannot be directly obtained. Here, a novel way to probe local coordination is employed using electron energy loss fine structure (ELNES) in the scanning transmission electron microscope (STEM). The method is demonstrated in a phase-separated aluminosilicate glass with multiple Al-coordinated species. With the support of ab initio calculation, two exciton-like peaks in the Al L2,3-edge at around 77 and 80 eV are attributed to 4-fold and 5,6-fold Al excitations, respectively. Mapping of the relative intensity ratio for two peaks in a phase-separated microstructure reveals a heterogeneous distribution of highly coordinated Al species in real space. The finding is in agreement with previous MD simulation that 5- and 6-fold Al species are favored to form in the Al-rich phase. This work has demonstrated that complex network structure within the phase-separated region can now be studied via STEM-EELS.

Principal Vibration Modes of the La2O3-Ga2O3 Binary Glass Originated from Diverse Coordination Environments of Oxygen Atoms.

La2O3-Ga2O3 binary glass exhibits unusual optical properties owing to its high oxygen polarizability and low vibration energy. These optical properties include high refractive indices and a wide transmittance range. In this study, we performed classical molecular dynamics simulations on La2O3-Ga2O3 glass synthesized by an aerodynamic levitation technique. We have obtained structural models that reproduce experimental results, such as NMR, high-energy X-ray diffraction, and neutron diffraction. Based on our analysis, the structural features were clarified: 5-, 6-coordinated Ga, edge-sharing GaOx-GaOx polyhedral linkages, and oxygen triclusters. Additionally, the vibrational density of states was calculated by diagonalization of the dynamical matrix derived from the structural models and the results were compared with Raman scattering spectra. The mode analysis of the Raman spectra revealed that the principal bands at 650 cm-1 were mainly attributed to the stretching modes of the bridging and nonbridging oxygens. Meanwhile, the shoulder bands at the highest frequency of 750 cm-1 were mainly attributed to the stretching modes of the bridging oxygens and oxygen triclusters. The structural models obtained in this study well describe the characteristic local structures and vibrational properties of the La2O3-Ga2O3 glass.

High refractive index La-rich lanthanum borate glasses composed of isolated BO3 units.

La2O3-B2O3 binary glasses were prepared by containerless processing using a levitation technique. The bulk glass-forming region in a B-rich composition was extended compared to that using conventional melt-quench techniques. Furthermore, additional glass formation was realized in an La-rich composition. The glass transition temperature and crystallization temperature of La-rich glasses are much higher than those of B-rich glasses. Both B- and La-rich glasses were colorless and transparent and had a high refractive index with low wavelength dispersion in the visible region. With the increase of the La2O3 content, the optical absorption edge in the ultraviolet (UV) region shifts to a long wavelength. An additional infrared (IR) transmittance window was observed in La-rich glasses, indicating that the La-rich borate glasses are expected to be used in optical components in a wide wavelength region. Local structural analyses using 11B magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and Raman scattering spectra revealed that every B-atom in La-rich glasses formed a planar trigonal BO3 unit and these BO3 units were entirely isolated. The evident difference from B-rich glasses where B-atoms formed a complex network structure with BO3 and BO4 units caused the characteristic physical and optical properties of La-rich glasses.

Low phonon energies and wideband optical windows of La2O3-Ga2O3 glasses prepared using an aerodynamic levitation technique.

xLa2O3-(100 - x)Ga2O3 binary glasses were synthesized by an aerodynamic levitation technique. The glass-forming region was found to be 20 ≤ x ≤ 57. The refractive indices were greater than 1.92 and increased linearly with increasing x. The polarizabilities of oxide ions were estimated to be 2.16-2.41 Å3, indicating that the glasses were highly ionic. The glasses were transparent over a very wide range from the ultraviolet to the mid-infrared region. The widest transparent window among the oxide glasses was from 270 nm to 10 µm at x = 55. From the Raman scattering spectra, a decrease in bridging oxide ions and an increase in non-bridging oxide ions were confirmed to occur with increasing La2O3 content. The maximum phonon energy was found to be approximately 650 cm-1, being one of the lowest among oxide glasses. These results show that La2O3-Ga2O3 binary glasses should be promising host materials for optical applications such as lenses, windows, and filters over a very wide wavelength range.

Transparent polycrystalline cubic silicon nitride.

Glasses and single crystals have traditionally been used as optical windows. Recently, there has been a high demand for harder and tougher optical windows that are able to endure severe conditions. Transparent polycrystalline ceramics can fulfill this demand because of their superior mechanical properties. It is known that polycrystalline ceramics with a spinel structure in compositions of MgAl2O4 and aluminum oxynitride (γ-AlON) show high optical transparency. Here we report the synthesis of the hardest transparent spinel ceramic, i.e. polycrystalline cubic silicon nitride (c-Si3N4). This material shows an intrinsic optical transparency over a wide range of wavelengths below its band-gap energy (258 nm) and is categorized as one of the third hardest materials next to diamond and cubic boron nitride (cBN). Since the high temperature metastability of c-Si3N4 in air is superior to those of diamond and cBN, the transparent c-Si3N4 ceramic can potentially be used as a window under extremely severe conditions.

Crack-resistant Al2O3-SiO2 glasses.

Obtaining "hard" and "crack-resistant" glasses have always been of great important in glass science and glass technology. However, in most commercial glasses both properties are not compatible. In this work, colorless and transparent xAl2O3-(100-x)SiO2 glasses (30 ≤ x ≤ 60) were fabricated by the aerodynamic levitation technique. The elastic moduli and Vickers hardness monotonically increased with an increase in the atomic packing density as the Al2O3 content increased. Although a higher atomic packing density generally enhances crack formation in conventional oxide glasses, the indentation cracking resistance increased by approximately seven times with an increase in atomic packing density in binary Al2O3-SiO2 glasses. In particular, the composition of 60Al2O3 • 40SiO2 glass, which is identical to that of mullite, has extraordinary high cracking resistance with high elastic moduli and Vickers hardness. The results indicate that there exist aluminosilicate compositions that can produce hard and damage-tolerant glasses.

High Elastic Moduli of a 54Al2O3-46Ta2O5 Glass Fabricated via Containerless Processing.

Glasses with high elastic moduli have been in demand for many years because the thickness of such glasses can be reduced while maintaining its strength. Moreover, thinner and lighter glasses are desired for the fabrication of windows in buildings and cars, cover glasses for smart-phones and substrates in Thin-Film Transistor (TFT) displays. In this work, we report a 54Al2O3-46Ta2O5 glass fabricated by aerodynamic levitation which possesses one of the highest elastic moduli and hardness for oxide glasses also displaying excellent optical properties. The glass was colorless and transparent in the visible region, and its refractive index nd was as high as 1.94. The measured Young's modulus and Vickers hardness were 158.3 GPa and 9.1 GPa, respectively, which are comparable to the previously reported highest values for oxide glasses. Analysis made using (27)Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy revealed the presence of a significantly large fraction of high-coordinated Al in addition to four-coordinated Al in the glass. The high elastic modulus and hardness are attributed to both the large cationic field strength of Ta(5+) ions and the large dissociation energies per unit volume of Al2O3 and Ta2O5.