Multinuclear solid state NMR investigation of titanium-containing silicate glasses

Abstract

The bonding and local coordination geometry of aluminium and silicon atoms in a series of potassium aluminosilicate glasses containing 7 mol% TiO 2 have been studied by 29Si and 27Al NMR. Although the K/Al concentration ratio is varied between 0.76 and 1.40, no simple trends are observed in either the 29Si and 27Al MAS-NMR spectra. Both Si and Al atoms are found to be four-fold coordinated by oxygen atoms; thus, Al atoms occupy network sites in these glasses. In addition to MAS, non-spinning NMR spectra and spin-lattice relaxation times were measured. The 29Si T 1 data confirm that titanium is present as Ti 4+; the 29Si MAS data conclude that Ti cations are located in extra-framework sites. Thus Ti 4+ must undergo the rearrangements necessary to provide for charge balancing the aluminosilicate framework in glasses with K/Al ratios < 1. From chemical shift distributions it is known that the mean 〈 Si O T〉 (T = Si or Al) and 〈AlOSi〉 bond angles differ. A hypothesis of different local structures for silicate and aluminate tetrahedrons and different medium range ordering are in agreement with the bicomponent Si T 1 measurements.