Oxygen Triclusters Research Articles

Topological Disorder and Reactivity of Borosilicate Glasses: Quantum Chemical Calculations and 17O and 11B NMR Study

We present 17O and 11B NMR and quantum chemical calculations based on Hartree−Fock (HF) and density functional theory to investigate the site connectivity, the degree of chemical short-range order (CSRO), and the topological short-range order in binary borosilicate glasses, which can serve as a simple model system for more complex borosilicates and other covalent oxide glasses and melts. We also calculate the enthalpy of hydrolysis for each oxygen cluster in such glasses. The stability of [4]Si−O−[3]B was evaluated from the relative energy differences among the bridging oxygen clusters ([4]Si−O−[4]Si, [4]Si−O−[3]B, and [3]B−O−[3]B) calculated at the B3LYP/6-311+G(2d,p)//HF/3-21G and B3LYP/6-311+G(2d,p) levels of theory as well as data from 17O 3QMAS NMR. The fraction of [4]Si−O−[3]B at a boron mole fraction of 0.57 is about 37% (±1), suggesting a significant inter-dispersion of [3]B and [4]Si, where a random distribution of [4]Si and [3]B and complete phase separation lead to 50 and 0% of [4]Si−O−[3]B, respectively. 11B MAS NMR shows that the fraction of boroxol rings increases with increasing boron content. The bond angle distribution function G(α) for each cluster was obtained from the lattice energy variation with angle. The calculation for [4]Si−O−[4]Si shows close similarity to that obtained by recent high energy photon diffraction experiments. We also predicted the effects of composition, fictive temperature and degree of disorder among framework cations on G(α). The calculated enthalpies of hydrolysis of [4]Si−O−[4]Si, [4]Si−O−[3]B, and [3]B−O−[3]B are −16.98, −3.03, and 16.61 kJ/mol, respectively, implying differential reactivity with water and that [4]Si−O−[3]B clusters should have intermediate kinetic stability. Quantum chemical simulations also suggest a possible mechanism for the formation of the oxygen tricluster, related to the presence of five-membered rings.

Direct observation of multiple oxygen sites in oxide glasses: recent advances from triple-quantum magic-angle spinning nuclear magnetic resonance

Triple-quantum, magic-angle spinning nuclear magnetic resonance (3QMAS NMR) on 17O has demonstrated remarkable improvements in resolution among signals from different oxide species in glasses. As many as four separate types of oxygen anion environments can be observed simultaneously in some compositions; Si–O–Si, Al–O–Al, B–O–B, Si–O–Al, Si–O–B, and Al–O–B bridging oxygens can be distinguished and at least roughly quantified; non-bridging oxygens bonded to Si vs. B can in some cases be separated, and there are hints of oxygen tricluster species. These types of data are already providing important new constraints and tests of models of glass structure and properties.

Oxygen triclusters in crystalline CaAl4O7(grossite) and in calcium aluminosilicate glasses:17O NMR

We present 17 O MAS NMR data for crystalline calcium dialuminate (grossite), CaAl 2 O 4 and monoaluminate, CaAl 4 O 7 . The first of these contains an oxygen tricluster site and serves as a model compound for sites of this type in aluminosilicate glasses. Tricluster site NMR parameters are distinct from those of bridging O atoms (Al-O-Al), allowing partial resolution in triple quantum MAS NMR spectra. Such spectra for calcium aluminosilicate glasses are consistent with the presence of a small fraction of tricluster sites. Observed chemical shifts for non-bridging oxygen (NBO) atoms in an impurity phase in the CaAl 2 O 4 sample are distinct from those for NBO in Ca-aluminosilicate glasses, indicating that the latter are primarily bonded to Si, not Al.

Structure of Sodium Aluminosilicate Glasses

A series of sodium aluminosilicate glasses composed of varying ratios (R) of Al2O3/Na2O (0.25 R 2.0) has been simulated with the molecular dynamics technique using a tetrahedral form of a three‐body interaction potential. Extrema in the activation energies for sodium diffusion and in the diffusion constants for all of the atomic species were observed for glasses with equal concentrations of Al2O3 and Na2O (R= 1.0). These changes corresponded to the minimum observed experimentally in the activation energy for electrical conductivity and to the maximum observed in the viscosity for glasses with compositions of R= 1.0. The coordination of aluminum remained 4 over the entire compositional range, negating the need to invoke a coordination change of aluminum to explain the changes in the physical properties. The changes to the simulated physical properties as R passed through the equivalence point were attributed to the elimination of nonbridging oxygen, to the introduction of oxygen triclusters, and to changes in the distribution of ring structures within the glass networks.