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1. Field strength effect on elastoplastic behavior of aluminoborosilicate glass: I. Elastic moduli and indentation size effect

   https://doi.org/10.1111/jace.19154  

   Fry, Aubrey L. ; Ogrinc, Andrew L. ; Kim, Seong H. ; Mauro, John C. ( May 2023 , Journal of the American Ceramic Society)

   The modifier field strength (FS) is believed to play an important role in determining the elastic–plastic responses of aluminoborosilicate (ABS) glasses, but its effect is not well understood. Three novel alkali and three alkaline earth (AE) ABS compositions were created for this study which is the first part of two studies that explored the elastoplastic responses of these glasses. Six compositions were designed using different network modifiers (NWMs) to cover a range of cation FS. The glasses were also designed such that the concentrations of NWM and Al₂O₃were similar, which maximized the three‐coordinated boron fraction in the network. It is well known that modifier FS can affect the coordination number (CN) of Al and B in an ABS glass structure, for example, a higher FS modifier can promote B³ → B⁴and higher [Al^5,6], but the degree of this depends on network former (NWF) ratios. Previous work used solid‐state NMR spectroscopic analysis on the current glasses to find that there was variation between [B⁴] and [Al⁴] between the two glass series (alkali vs. AE) but that was attributed to synthesis factors and no trend with FS was associated with the varying NWF CN. Further,²⁹Si ssNMR showed no evidence of NBOs which made sense based on composition. The conclusion, therefore, was that there was a far greater correlation with modifier FS for the increased mechanical and physical properties rather than the CN of Al and B. Part I of the current work focused on the elastic moduli, Poisson's ratio, the indentation size effect (ISE), and the bow‐in parameter. This part laid out the foundation to investigate the intersection of these elastoplastic properties with hardness and crack resistance as a function of NWM FS. Results showed that: (i) the Young's, bulk, and shear moduli increased with modifier FS, whereas Poisson's ratio did not trend with FS; (ii) the alkali glasses had a significantly higher magnitudes of ISE compared to the AE glasses; and (iii) the bow‐in parameter was load dependent and decreased with modifier FS at the highest indentation load.

    

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2. Assessment of interatomic parameters for the reproduction of borosilicate glass structures via DFT‐GIPAW calculations

   https://doi.org/10.1111/jace.16655  

   Fortino, Mariagrazia ; Berselli, Alessandro ; Stone‐Weiss, Nicholas ; Deng, Lu ; Goel, Ashutosh ; Du, Jincheng ; Pedone, Alfonso ( June 2019 , Journal of the American Ceramic Society)

   Borates and borosilicates are potential candidates for the design and development of glass formulations with important industrial and technological applications. A major challenge that retards the pace of development of borate/borosilicate based glasses using predictive modeling is the lack of reliable computational models to predict the structure‐property relationships in these glasses over a wide compositional space. A major hindrance in this pursuit has been the complexity of boron‐oxygen bonding due to which it has been difficult to develop adequate B–O interatomic potentials. In this article, we have evaluated the performance of three B–O interatomic potential models recently developed by Bauchy et al [J.Non‐Cryst. Solids, 2018, 498, 294–304], Du et al [J. Am. Ceram. Soc.https://doi.org/10.1111/jace.16082] and Edèn et al [Phys. Chem. Chem. Phys., 2018, 20, 8192–8209] aiming to reproduce the short‐to‐medium range structures of sodium borosilicate glasses in the system 25 Na₂OxB₂O₃(75 − x) SiO₂(x = 0‐75 mol%). To evaluate the different force fields, we have computed at the density functional theory level the NMR parameters of¹¹B,²³Na, and²⁹Si of the models generated with the three potentials and the simulated MAS NMR spectra compared with the experimental counterparts. It was observed that the rigid ionic models proposed by Bauchy and Du can both reliably reproduce the partitioning between BO₃and BO₄species of the investigated glasses, along with the local environment around sodium in the glass structure. However, they do not accurately reproduce the second coordination sphere of silicon ions and the Si–O–T (T = Si, B) and B‐O‐T distribution angles in the investigated compositional space which strongly affect the NMR parameters and final spectral shape. On the other hand, the core‐shell parameterization model proposed by Edén underestimates the fraction of BO₄species of the glass with composition 25Na₂O 18.4B₂O₃56.6SiO₂but can accurately reproduce the shape of the¹¹B and²⁹Si MAS‐NMR spectra of the glasses investigations due to the narrower B–O–T and Si‐O‐T bond angle distributions. Finally, the effect of the number of boron atoms (also distinguishing the BO₃and BO₄units) in the second coordination sphere of the network former cations on the NMR parameters have been evaluated.

    

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3. Origin of the 29 Si NMR chemical shift in R 3 Si–X and relationship to the formation of silylium (R 3 Si + ) ions

   https://doi.org/10.1039/D0DT02099K  

   Huynh, Winn ; Conley, Matthew P. ( November 2020 , Dalton Transactions)

   null (Ed.)

   The origin in deshielding of 29 Si NMR chemical shifts in R 3 Si–X, where X = H, OMe, Cl, OTf, [CH 6 B 11 X 6 ], toluene, and O X (O X = surface oxygen), as well as i Pr 3 Si + and Mes 3 Si + were studied using DFT methods. At the M06-L/6-31G(d,p) level of theory the geometry optimized structures agree well with those obtained experimentally. The trends in 29 Si NMR chemical shift also reproduce experimental trends; i Pr 3 Si–H has the most shielded 29 Si NMR chemical shift and free i Pr 3 Si + or isolable Mes 3 Si + have the most deshielded 29 Si NMR chemical shift. Natural localized molecular orbital (NLMO) analysis of the contributions to paramagnetic shielding ( σ p ) in these compounds shows that Si–R (R = alkyl, H) bonding orbitals are the major contributors to deshielding in this series. The Si–R bonding orbitals are coupled to the empty p-orbital in i Pr 3 Si + or Mes 3 Si + , or to the orbital in R 3 Si–X. This trend also applies to surface bound R 3 Si–O X . This model also explains chemical shift trends in recently isolated t Bu 2 SiH 2 + , t BuSiH 2 + , and SiH 3 + that show more shielded 29 Si NMR signals than R 3 Si + species. There is no correlation between isotropic 29 Si NMR chemical shift and charge at silicon. 

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4. NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by 13 C{ 1 H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

   https://doi.org/10.1002/anie.201813306  

   Cui, Jinlei ; Olmsted, David L. ; Mehta, Anil K. ; Asta, Mark ; Hayes, Sophia E. ( February 2019 , Angewandte Chemie International Edition)

   Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate¹³C–¹H distances is exploited along with DFT determinations of the¹³C tensor of carbonates (CO₃²⁻). Nearby¹H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO₃⋅Mg(OH)₂⋅4 H₂O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental¹³C{¹H} REDOR and¹³C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials.

    

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5. NMR Crystallography: Evaluation of Hydrogen Positions in Hydromagnesite by 13 C{ 1 H} REDOR Solid‐State NMR and Density Functional Theory Calculation of Chemical Shielding Tensors

   https://doi.org/10.1002/ange.201813306  

   Cui, Jinlei ; Olmsted, David L. ; Mehta, Anil K. ; Asta, Mark ; Hayes, Sophia E. ( February 2019 , Angewandte Chemie)

   Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate¹³C–¹H distances is exploited along with DFT determinations of the¹³C tensor of carbonates (CO₃²⁻). Nearby¹H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO₃⋅Mg(OH)₂⋅4 H₂O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental¹³C{¹H} REDOR and¹³C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials.

    

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