Probing silicon and aluminium chemical environments in silicate and aluminosilicate glasses by solid state NMR spectroscopy and accurate first-principles calculations

Silicon and aluminium chemical environments in silicate and aluminosilicate glasses with compositions 60SiO(2)center dot 20Na(2)O center dot 20CaO (CSN), 60SiO(2)center dot 20Al(2)O(3)center dot 20CaO (CAS), 78SiO(2)center dot 11Al(2)O(3)center dot 11Na(2)O (NAS) and 60SiO(2)center dot 10Al(2)O(3)center dot 10Na(2)O center dot 20CaO (CASN) have been investigated by Al-27 and Si-29 solid state magic angle spinning (MAS) and multiple quantum MAS (MQMAS) nuclear magnetic resonance (NMR) experiments. To interpret the NMR data, first-principles calculations using density functional theory were performed on structural models of these glasses. These models were generated by Shell-model molecular dynamics (MD) simulations. The theoretical NMR parameters and spectra were computed using the gauge including projected augmented wave (GIPAW) method and spin-effective Hamiltonians, respectively. This synergetic computational-experimental approach offers a clear structural characterization of these glasses, particularly in terms of network polymerization, chemical disorder (i.e. Si and Al distribution in second coordination sphere) and modifier cation distributions. The relationships between the local structural environments and the Si-29 and Al-27 NMR parameters are highlighted, and show that: (i) the isotropic chemical shift of both Si-29 and Al-27 increases of about +5 ppm for each Al added in the second sphere and (ii) both the Al-27 and Si-29 isotropic chemical shifts linearly decrease with the reduction of the average Si/Al-O-T bond angle. Conversely, Al-27 and Si-29 NMR parameters are much less sensitive to the connectivity with triple bridging oxygen atoms, precluding their indirect detection from Al-27 and Si-29 NMR. (C) 2013 Elsevier Ltd. All rights reserved.