SI-29 AND NA-23 SOLID-STATE MAS NMR INVESTIGATIONS OF MODIFICATIONS OF THE SODIUM PHYLLOSILICATE NA2SI2O5

The results of Si-29- and Na-23-MAS NMR investigations on four modifications of the synthetic Na2Si2O5 demonstrate that the alpha-, beta- and delta-modifications are characterized unequivocally by the parameters of the corresponding NMR spectra. The studies on gamma-Na2Si2O5 show that this sample contains a large amount of secondary compounds. For alpha- und beta-Na2Si2O5 the the structural details of the silicate sheets are reflected by the Si-29; MAS NMR spectra while from the Na-23 MAS NMR spectra conclusions about the coordination number of the sodium atoms can be derived. The Si-29 MAS NMR investigations on delta-NaSi2O5 indicate that the silicate sheet of this modification consist of identical SiO4-tetrahydra the parameter of which differ from those of alpha- and beta-Na2Si2O5. The Na-23 MAS NMR studies show that in the interlayer space of delta-Na2Si2O5 two nonidentical sodium atoms exists. The NMR results give rise to the suggestion that one of the sodium is surrounded by five and the other one by six oxygen atoms.