MULTINUCLEAR SOLID-STATE NMR INVESTIGATION OF TITANIUM-CONTAINING SILICATE-GLASSES

The bonding and local coordination geometry of aluminium and silicon atoms in a series of potassium aluminosilicate glasses containing 7 mol% TiO2 have been studied by Si-29 and Al-27 NMR. Although the K/Al concentration ratio is varied between 0.76 and 1.40, no simple trends are observed in either the Si-29 and Al-27 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 Si-29 T1 data confirm that titanium is present as Ti4+; the Si-29 MAS data conclude that Ti cations are located in extra-framework sites. Thus Ti4+ 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 T1 measurements.