Christian Hermansen

A model for phosphate glass topology considering the modifying ion sub-network

In the present paper we establish a temperature dependent constraint model of alkali phosphate glasses considering the structural and topological role of the modifying ion sub-network constituted by alkali ions and their non-bonding oxygen coordination spheres. The model is consistent with available structural data by NMR and molecular dynamics simulations and with dynamic data such glass transition temperature (Tg) and liquid fragility (m). Alkali phosphate glasses are exemplary systems for developing constraint model since the modifying cation network plays an important role besides the primary phosphate network. The proposed topological model predicts the changing trend of the Tg and m with increasing alkali oxide content for alkali phosphate glasses, including an anomalous minimum at around 20 mol.% alkali oxide content. We find that the minimum in Tg and m is caused by increased connectivity of the modifying ion sub-network, as the alkali ions must share non-bonding oxygen to satisfy their coordinati...

Structural and topological aspects of borophosphate glasses and their relation to physical properties

We establish a topological model of alkali borophosphate and calcium borophosphate glasses, which describes the effect of both the network formers and network modifiers on physical properties. We show that the glass transition temperature (Tg), Vickers hardness (HV), liquid fragility (m), and isobaric heat capacity jump at Tg (ΔCp) of these glasses are related to the network topology, which is determined by structure of the glass. Therefore, we also demonstrate that the temperature dependent constraint theory can quantitatively explain the mixed network former effect in borophosphate glasses. The origin of the effect of the type of network modifying oxide on Tg, HV, m, and ΔCp of calcium borophosphate glasses is revealed in terms of the modifying ion sub-network. The same topological principles quantitatively explain the significant differences in physical properties between the alkali and the calcium borophosphate glasses. This work has implications for quantifying structure-property relations in complex glass forming systems containing several types of network forming and modifying oxides.

An extended topological model for binary phosphate glasses

We present a topological model for binary phosphate glasses that builds on the previously introduced concepts of the modifying ion sub-network and the strength of modifier constraints. The validity of the model is confirmed by the correct prediction of Tg(x) for covalent polyphosphoric acids where the model reduces to classical constraint counting. The constraints on the modifying cations are linear constraints to first neighbor non-bridging oxygens, and all angular constraints are broken as expected for ionic bonding. For small modifying cations, such as Li(+), the linear constraints are almost fully intact, but for larger ions, a significant fraction is broken. By accounting for the fraction of intact modifying ion related constraints, qγ, the Tg(x) of alkali phosphate glasses is predicted. By examining alkali, alkaline earth, and rare earth metaphosphate glasses, we find that the effective number of intact constraints per modifying cation is linearly related to the charge-to-distance ratio of the modifying cation to oxygen.

Structure-topology-property correlations of sodium phosphosilicate glasses

In this work, we investigate the correlations among structure, topology, and properties in a series of sodium phosphosilicate glasses with [SiO2]/[SiO2 + P2O5] ranging from 0 to 1. The network structure is characterized by (29)Si and (31)P magic-angle spinning nuclear magnetic resonance and Raman spectroscopy. The results show the formation of six-fold coordinated silicon species in phosphorous-rich glasses. Based on the structural data, we propose a formation mechanism of the six-fold coordinated silicon, which is used to develop a quantitative structural model for predicting the speciation of the network forming units as a function of chemical composition. The structural model is then used to establish a temperature-dependent constraint description of phosphosilicate glass topology that enables prediction of glass transition temperature, liquid fragility, and indentation hardness. The topological constraint model provides insight into structural origin of the mixed network former effect in phosphosilicate glasses.

Response to "Comment on 'A model for phosphate glass topology considering the modifying ion sub-network"' [J. Chem. Phys. 142, 107103 (2015)].

In our recent paper [C. Hermansen, J. C. Mauro, and Y.-Z. Yue, J. Chem. Phys. 140, 154501 (2014)], we applied temperature-dependent constraint theory to model the glass transition temperature (Tg) and liquid fragility index (m) of alkali phosphate glasses. Sidebottom commented on this paper concerning the m values obtained by differential scanning calorimetry (DSC) [D. L. Sidebottom, J. Chem. Phys. 142, ⬛ (2015)]. We have considered Sidebottoms comments carefully and conclude that the m values of phosphate liquids obtained by DSC are reliable, except for the NaPO3 and possibly P2O5 compositions. Based on his dynamic light scattering measurements, Sidebottom has found that P2O5 is a strong liquid with m ≈ 20. However, based on the heat capacity jump at Tg and the stretching exponent of the relaxation function, P2O5 should be classified as an intermediate fragile liquid with m ≈ 40. We also argue that m cannot be universally related to the average connectivity of the network and point out several inconsistencies with this view.

Composition-structure-property relation of oxide glasses: insight from an extended topological approach

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