Liquid B2O3 up to 1700 K: x-ray diffraction and boroxol ring dissolution

Boron coordination change in barium borate melts and glasses and its contribution to configurational heat capacity, entropy, and fragility

High-energy x-ray diffraction from molten and glassy BaB2O4 and BaB4O7 has been performed using aerodynamic levitation and laser heating over a wide range of temperatures. Remarkably, even in the presence of a heavy metal modifier dominating x-ray scattering, it was possible to extract accurate values for the tetrahedral, sp3, boron fraction, N4, which declines with increasing temperature, using bond valence-based mapping from the measured mean B-O bond lengths while accounting for vibrational thermal expansion. These are used within a boron-coordination-change model to extract enthalpies, ΔH, and entropies, ΔS, of isomerization between sp2 and sp3 boron. The results for BaB4O7, ΔH = 22(3) kJ mol-1 boron, ΔS = 19(2) J mol-1 boron K-1, agree quantitatively with those found previously for Na2B4O7. Analytical expressions for N4(J, T) and associated configurational heat capacity, CPconf(J, T), and entropy, Sconf(J, T), contributions are extended to cover a wide composition range 0 ≤ J = BaO/B2O3 ≤ 3 using a model for ΔH(J) and ΔS(J) derived empirically for lithium borates. Maxima in the CPconf(J, Tg) and fragility index contributions are thereby predicted for J ≲ 1, higher than the maximum observed and predicted in N4(J, Tg) at J ≃ 0.6. We discuss the utility of the boron-coordination-change isomerization model in the context of borate liquids containing other modifiers and the prospect of neutron diffraction to aid in empirical determinations of modifier-dependent effects, illustrated by new neutron diffraction data on Ba11B4O7 glass, its well-known α-polymorph, and lesser-known δ-phase.

Zinc borate glasses: properties, structure and modelling of the composition-dependence of borate speciation

The short-range order of binary zinc borate glasses, xZnO-(1-x)B₂O₃, has been quantitatively described as a function of ZnO content over the entire glass forming range for the first time, to the best of our knowledge. Multiple spectroscopic techniques (¹¹B NMR, Raman, infrared) reveal detailed structural information regarding borate speciation and network connectivity, and a new model for quantifying the molar fractions of short-range order units is proposed. A consistent thermal history dependence for the fraction of tetrahedral boron (N₄) is well accounted for by the proposed model. The model predicts density within 0.1% of experimental values and N₄ to within 1% of NMR values. The intermediate character of four-coordinated zinc in borate glasses of this series is evident by the far infrared profiles and the glass transition temperature behavior, which decreases non-monotonically with increasing ZnO content.

Anomalous structure transition in undercooled melt regulates polymorphic selection in barium titanate crystallization

The crystallization processes of titanates are central to the fabrication of optical and electrical crystals and glasses, but their rich polymorphism is not fully understood. Here, we show when and how polymorphic selection occurs during the crystallization of barium titanate (BaTiO₃, BT) using in situ high energy synchrotron X-ray diffraction and ab initio molecular dynamic simulation. An anomalous structure transition is found in molten BT during cooling across the cubic-hexagonal transition temperature, which enables nucleation selection of BT by manipulating the undercooling: a cubic phase is preferred if nucleation is triggered at large undercooling, whereas a hexagonal phase is promoted at small undercooling. We further reveal that the nucleation selection between the cubic and the hexagonal phase is regulated by the intrinsic structure property of the melt, in particular, the degree of polymerization between Ti-O polyhedra. These findings provide an innovative perspective to link the polymorphic crystallization to the non-isomorphic structure transition of the melt beyond the conventional cognition of structural heredity.

Examination of structure and optical properties of Ce3+-doped strontium borate glass by regression analysis

Amorphous materials with non-periodic structures are commonly evaluated based on their chemical composition, which is not always the best parameter to evaluate physical properties, and an alternative parameter more suitable for performance evaluation must be considered. Herein, we quantified various structural and physical properties of Ce-doped strontium borate glasses and studied their correlations by principal component analysis. We found that the density-driven molar volume is suitable for the evaluation of structural data, while chemical composition is better for the evaluation of optical and luminescent data. Furthermore, the borate-rich glasses exhibited a stronger luminescence due to Ce³⁺, indicating a higher fraction of BO_3/2 ring and larger cavity. Moreover, the internal quantum efficiency was found to originate from the local coordination states of the Ce³⁺ centres, independent of composition or molar volume. The comparison of numerical data of the matrix is useful not only for ensuring the homogenous doping of amorphous materials by activators, but also for determining the origin of physical properties.

Anion polarizabilities in oxynitride glasses. Establishing a common optical basicity scale

Inspired by the work of John Duffy on optical basicity of oxyfluoride glasses, we apply here the concept of optical basicity to oxynitride systems. While in the original work of Duffy and Ingram the basicity of a medium could be probed by s² ions like Pb²⁺, the low energy intrinsic absorption edge of nitride-containing systems does not allow the use of such probe ions. This study uses therefore experimental data on refractive index and density of alkaline earth and rare earth containing silicate oxynitride glasses, prepared by the authors or taken from the literature. In addition, literature reports on experimental or calculated refractive index, density and polarizability data are used to compare pure nitride systems, e.g. bulk or thin film materials that are either crystalline or glassy. We compare simple and complex nitride systems with their oxygen counterparts, by calculating their optical basicity using the chemical composition as well as the established relationship between optical basicity, Λ, and electronic polarizability in oxide systems. Our results on oxynitride systems are in good agreement with Duffy's previous work on oxyfluoride glasses and indicate that the optical basicity varies for the isoelectronic anions in nitrides, oxides and fluorides (N^3-:O^2-:F^-) of a cation M^m+ as follows: Λ(MF_m) = 1/2Λ(M₂O_m) = 1/3Λ(M₃N_m). Using this relation for CaO, for which the optical basicity was set as unity by Duffy and Ingram, one has Λ(CaF₂) = 0.50, Λ(CaO) = 1.00 and Λ(Ca₃N₂) = 1.50. The optical basicity of complex nitrides can therefore be calculated by the same method established for oxides using the equivalent fractions and the basicity of the constituent nitrides. The relationship between nitride polarizability α_N and basicity Λ(nitride) was found to be linear, with Λ(nitride) = 0.39α_N- 0.14 where α_N is given in ų.

Short-Range Disorder in TeO2 Melt and Glass

High-resolution X-ray pair distribution functions for molten and glassy TeO₂ reveal coordination numbers n_TeO ≈ 4. However, distinct from the known α-, β-, and γ-TeO₂ polymorphs, there is considerable short-range disorder such that no clear cutoff distance between bonded and nonbonded interactions exists. We suggest that this is similar to disorder in δ-TeO₂ and arises from a broad distribution of asymmetric Te-O-Te bridges, something that we observe becomes increasingly asymmetric with increasing liquid temperature. Such behavior is qualitatively consistent with existing interpretations of Raman scattering spectra, and equivalent to temperature-induced coordination number reduction, for sufficiently large cutoff radii. Therefore, TeO₂ contains a distribution of local environments that are, furthermore, temperature dependent, making it distinct from the canonical single-oxide glass formers. Our results are in good agreement with high-level ab initio cluster calculations.

Many-body effects at the origin of structural transitions in B2O3

The structural properties of glassy diboron trioxide, g-B₂O₃, are investigated from ambient to high pressure conditions using two types of atomic force-field models that account for many-body effects. These models are parameterized by a dipole- and force-fitting procedure of reference datasets created via first-principles calculations on a series of configurations. The predictions of the models are tested against experimental data, where particular attention is paid to the structural transitions in g-B₂O₃ that involve changes to both the short- and medium-range order. The models outperform those previously devised, where improvement originates from the incorporation of two key physical ingredients, namely, (i) the polarizability of the oxide ion and (ii) the ability of an oxide ion to change both size and shape in response to its coordination environment. The results highlight the importance of many-body effects for accurately modeling this challenging system.

Direct Experimental Evidence for Abundant BO4-BO4 Motifs in Borosilicate Glasses From Double-Quantum 11B NMR Spectroscopy

By using double-quantum-single-quantum correlation ¹¹B nuclear magnetic resonance (NMR) experiments and atomistic molecular dynamics (MD) simulations, we resolve the long-standing controversy of whether directly interlinked BO₄-BO₄ groups exist in the technologically ubiquitous class of alkali/alkaline-earth based borosilicate (BS) glasses. Most structural models of Na₂O-B₂O₃-SiO₂ glasses assume the absence of B^[4]-O-B^[4] linkages, whereas they have been suggested to exist in Ca-bearing BS analogs. Our results demonstrate that while B^[4]-O-B^[4] linkages are disfavored relative to their B^[3]-O-B^[3]/B^[4] counterparts, they are nevertheless abundant motifs in Na₂O-B₂O₃-SiO₂ glasses over a large composition space, while the B^[4]-O-B^[4] contents are indeed elevated in Na₂O-CaO-B₂O₃-SiO₂ glasses. We discuss the compositional and structural parameters that control the degree of B^[4]-O-B^[4] bonding.

Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO₂ by B₂O₃ in the respective phosphosilicate base compositions 24.1Na₂O-23.3CaO-48.6SiO₂-4.0P₂O₅ ("S49") and 24.6Na₂O-26.7CaO-46.1SiO₂-2.6P₂O₅ ("S46"), the latter constituting the "45S5 Bioglass" utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO₄, BO₄, and BO₃ moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P-O-Si and P-O-B^[4] motifs, whose populations were estimated by heteronuclear ³¹P{¹¹B} NMR experimentation. The high Na⁺/Ca²⁺ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO₄ < SiO₄ < BO₃ ≪ PO₄. The BO₄/BO₃ intermixing was examined by double-quantum-single-quantum correlation ¹¹B NMR experiments, which evidenced the presence of all three BO₃-BO₃, BO₃-BO₄, and BO₄-BO₄ connectivities, with B^[3]-O-B^[4] bridges dominating. Notwithstanding that B^[4]-O-B^[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B^[4]-NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si-O-Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO₄ tetrahedra tend to self-associate into small chain/ring motifs embedded in BO₃/BO₄-dominated domains.