29Si NMR characterisation of the crystalline-amorphous transition in ZrSiO4

The series of static and magic angle spinning NMR experiments were carried out on two zircons of low and moderate alpha-decay dose. The aim was to identify the nature of the amorphous phase in terms of the local structure around silicon and to quantify the amounts of amorphous and crystalline material for a given alpha-decay dose. The effects of paramagnetic defects produced during radiation damage were checked to verify that spectra obtained were representative of the majority of Si atoms in the sample. In the moderately dosed sample 77 +/- 8% of the total possible Si NMR signal was detected. This was divided into 34 +/- 3% amorphous and 66 +/- 3% crystalline material. Rapid relaxation delay spectra show that paramagnetic defects exist more or less equally in crystalline and amorphous regions, indicating that the absolute fraction of crystalline and amorphous material is close to these values. The amorphised region has an average peak position of similar to -90 ppm characteristic of Q(2) polymerised tetrahedral environments with signal intensity spread over the whole Q(n) range. An SiO2-like phase was not observed.