Fig. 2

Factors influencing peak positions of Q-species in 29Si NMR spectra. Grey vertical bars are the peak frequencies observed in the present study. The number of non-bridging oxygens (number of Si–O–Na interactions) causes a positive shift in peak frequency, such that Si in Q² speciation (2 non-bridging oxygens) is at a higher frequency than Si in Q⁴ speciation (0 non-bridging oxygens). For Si in Q⁴ speciation, Al as a network-forming cation nearest neighbor has been shown to cause a shift toward higher frequency. Q⁴ Si with one Al nearest neighbor (Al1) is at a lower frequency than Q⁴ Si with 4 Al nearest neighbors (Al4). Variations on Si–O–Si bond angle can also induce large shifts in the Q⁴ peak position, shown here from lower frequency (− 122 ppm at 170°) to higher frequency (− 93 ppm at 130°). These variations in bond angle are the cause of the Al nearest neighbor effects (Lippmaa et al. 1981)