Hexagonal silica platelets

All the preceding samples crystallized after 2 to 7 days from an AEROSIL silica at 170 - 200°C for template/Si =0.1 -0.5 and F/Si =0.25-1 ratios (pH 5 - 8). It was necessary to seed (1-2wt. % seeds) and to use P NBr when the synthesis was performed at 80°C. The crystals obtained after 12 days were then small (1-5 pm) quasi hexagonal platelets, showing a 90° twin. Powder X-ray diffraction showed no difference from the samples prepared between 170°C and 200°C. But the resolution of the 29Si NMR spectra is completely lost for both the as-synthesized sample and the sample calcined at 550°C. This reveals the presence of defects whose nature and number could, however, not be determined yet. Nevertheless, as they do not disap-pear on calcination, this could mean that empty tetrahedral sites exist beside bridging defects. 

COjScjTi 0gxg0.50 hexag- onal mono- clinic ) X, powder, single crystal. Appropriate mixtures of the elements heated at 500 °C in sealed silica tubes. Hexagonal platelets. 76 Am 1... 

---