This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 11919–11928 11919 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 11919–11928 Boron environments in Pyrex s glass—a high resolution, Double-Rotation NMR and thermodynamic modelling study A. P. Howes, a N. M. Vedishcheva, b A. Samoson, ac J. V. Hanna, a M. E. Smith, a D. Holland* a and R. Dupree* a Received 15th March 2011, Accepted 3rd May 2011 DOI: 10.1039/c1cp20771g It is shown, using the important technological glass Pyrex s as an example, that 1D and 2D 11 B Double-Rotation (DOR) NMR experiments, in combination with thermodynamic modelling, are able to provide unique structural information about complex glasses. 11 B DOR NMR has been applied to Pyrex s glass in order to remove both dipolar and quadrupolar broadening of the NMR lines, leading to high resolution spectra that allow unambiguous, accurate peak fitting to be carried out, of particular importance in the case of the 3-coordinated [BO 3 ] (B3) trigonal planar environments. The data obtained are of sufficient quality that they can be used to test the distributions of borate and borosilicate superstructural units predicted by the thermodynamics- based Model of Associated Solutions. The model predicts the dominant boron-containing chemical groupings in Pyrex s glass to be those associated with B 2 O 3 and sodium tetraborate (with smaller amounts of sodium triborate, sodium diborate, sodium pentaborate, danburite and reedmergnerite). Excellent agreement is found between model and experiment provided the 11 B peaks with isotropic chemical shifts of 1.4 ppm and 0.5 ppm are assigned to B4 species from borosilicate units ([B(OSi) 4 ] and [B(OSi) 3 (OB)]) and borate superstructural units (mainly triborate rings with some pentaborate and diborate) respectively. The peaks with isotropic shifts of 14 ppm and 18.1 ppm are then assigned to B3 in borate superstructural units (mainly triborate and pentaborate along with connecting B3) and boroxol rings respectively. The assignments of the DOR NMR peaks, are supported by the presence of cross-peaks in 11 B spin-diffusion DOR NMR spectra which can be used to develop a structural model in which B 2 O 3 –like regions are linked, via borate and borosilicate superstructural units, to the majority silica network. Pyrex s is thus shown to have a heterogeneous structure, with distinct molecular groupings that are far removed from a random distribution of network polyhedra with only short-range order. Introduction Pyrex s is a commercially important borosilicate glass, exploited mainly for its high temperature stability and low thermal expansion coefficient which give it good thermal shock resistance, a valuable attribute in both domestic and industrial applications, particularly when combined with its excellent corrosion resistance. The manufacturers give its composition as (presumed wt%) 13% B 2 O 3 , 80.6% SiO 2 , 2.3% Al 2 O 3 , 4.0% Na 2 O, rest 0.1%, although this will vary to some extent between different manufacturing plants. 1 This corresponds to a mol% composition of 11.6% B 2 O 3 , 83.0% SiO 2 , 1.4% Al 2 O 3 and 4.0% Na 2 O. It is important to know and understand the structure of Pyrex s , given its extensive range of uses and the potential for fine-tuning its properties. The structural units which make up alkali borosilicate glass networks include [SiO 4 ] tetrahedra of various connectivities, Q n (where n is the number of bridging oxygens to the neighbouring network polyhedra and 4–n the number of non-bridging oxygens), [BO 4 ]  tetrahedra (B4  ), and [BO 3 ] and [BO 3 ]  trigonal planar units (B3 and B3  ), the latter with one non-bridging oxygen. The alkali ions (in this case Na + ) charge balance the non- bridging oxygens and the B4  units. In terms of structural models, Pyrex s might be expected to conform to those developed for alkali borosilicate glasses. Abe 2 studied the relaxation processes of Pyrex s glass and neighbouring compositions and inferred the presence of local structures with length scale greater than the individual polyhedral units described above. He specifically suggested the presence of [B 5 O 8 ] units containing a central B4 unit attached via oxygen to four B3 units. Dell et al. 3 produced a structural model for borosilicate glasses of general formula RM 2 O.KSiO 2 .B 2 O 3 , where M 2 O is an alkali oxide modifier, a Department of Physics, University of Warwick, Coventry CV4 7AL, UK b Institute of Silicate Chemistry of the Russian Academy of Sciences, Nab. Makarova 2, Sankt Petersburg 199034, Russia c Tallinn University of Technology, Akadeemia Tee 1, Tallinn, Estonia PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by University of Warwick on 21 June 2011 Published on 26 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CP20771G View Online