2013

O. L. G. Alderman, A. C. Hannon, D. Holland, S. Feller, G. Lehr, A. J. Vitale, U. Hoppe, M. v Zimmermann, A. Watenphul, Lone-Pair Distribution and Plumbite Network Formation in High Lead Silicate Glass, 80PbO.20SiO₂, Physical Chemistry Chemical Physics, 2013, 15, 8506-8519, DOI: 10.1039/C3CP51348C

Abstract: For the first time a detailed structural model has been determined which shows how the lone‐pairs of electrons are arranged relative to each other in a glass network containing lone‐pair cations. High energy x‐ray and neutron diffraction patterns of a very high lead content silicate glass (80PbO.20SiO2) have been used to build three‐dimensional models using empirical potential structure refinement. Coordination number and bond angle distributions reveal structural similarity to crystalline Pb11Si3O17 and α‐ and β‐PbO, and therefore strong evidence for a plumbite glass network built from pyramidal [PbOm] polyhedra (m~3‐4), with stereochemically active lone‐pairs, although with greater disorder in the first coordination shell of lead compared to the first coordination shell of silicon. The oxygen atoms are coordinated predominantly to four cations. Explicit introduction of lone‐pair entities into some models leads to modification of the local Pb environment, whilst still allowing for reproduction of the measured diffraction patterns, thus demonstrating the non‐uniqueness of the solutions. Nonetheless, the models share many features with crystalline Pb11Si3O17, including the O‐Pb‐O bond angle distribution, which is more highly structured than reported for lower Pb content glasses using reverse Monte Carlo techniques. The lone‐pair separation of 2.85 Å in the model glasses compares favourably with that estimated in α‐ PbO as 2.88 Å, and these lone‐pairs organise to create voids in the glass, just as they create channels in Pb11Si3O17 and interlayer spaces in the PbO polymorphs.

O. L. G. Alderman, D. Iuga, A. P. Howes, K. Pike, D. Holland and R. Dupree, Spectral assignments and NMR parameter – structure relationships in borates using high-resolution 11B NMR and density functional theory, Physical Chemistry Chemical Physics, 2013, 15, 8208-8221, DOI: 10.1039/C3CP50772F.

Abstract: High-resolution, solid-state 11B NMR spectra have been obtained for a range of polycrystalline borates using double rotation (DOR), multiple-quantum magical angle spinning and isotopic dilution, combined with high magnetic fields. DOR linewidths can be less than 0.2 ppm in isotopically diluted samples allowing highly accurate values for the isotropic shift and electric field gradient to be obtained. The experimental values are used as a test of density functional calculations using both projector augmented wave based CASTEP and WIEN2k. The CASTEP calculations of the isotropic chemical shift are generally in very good agreement with experiment, having r.m.s. deviation 0.40 ppm. WIEN2k calculations of electric field gradient magnitude, CQ, and asymmetry, η, are also in excellent agreement with experiment, with r.m.s. deviations 0.038 MHz and 0.042 respectively. However, whilst CASTEP gives a similar deviation for η (0.043) it overestimates CQ by ~ 15%. After scaling of the calculated electric field gradient by 0.842 the deviation in CQ is practically identical to that of the WIEN2k calculations. The spectral assignments that follow from the experimental and computational results allow identification of correlations between the isotropic chemical shift and (a) the average B-O-B bond angle, θ, for both three and four coordinated boron, giving δiso(BIII) = (185.1 – θ)/3.42 ppm and δiso(BIV) = (130.2 – θ)/5.31 ppm; and (b) the ring-site T3 unit trigonal planar angular deviation, Stri, giving δiso(T3(ring)) = (1.642 x 10–2 – Stri)/(8.339 x 10–4) ppm.

2012

O. L. G. Alderman, D. Iuga, A. P. Howes, D. Holland and R. Dupree, Double rotation ¹¹B NMR applied to polycrystalline barium borates, Physics and Chemistry of Glasses - European Journal of Glass Science and Technology B. June 2012, 53 (3), 132-140.

Abstract: The potential of 11B double rotation nuclear magnetic resonance to provide detailed structural information is demonstrated through the application of the technique to polycrystalline materials obtained by devitrification of barium diborate glasses. Isotropic chemical shift values and quadrupole interaction parameters for the four three-coordinated sites in α-BaB4O7 are extracted by exploiting the field dependency of the quadrupolar interaction. The technique is applied to the polymorph β-BaB4O7, and two new barium borate materials of undetermined structure, and its role as a powerful structural probe is demonstrated and discussed.