However, as mentioned above, the chemical analysis revealed that the samples contain also oxygen beside the elements silicon and nitrogen. The overall oxygen content of sample SN51 measured by CHGE is about 7–8 wt%. The same oxygen content was obtained for the new sample SNg5 which was prepared under the same conditions like sample SN51 assuring the minimal possible oxygen contamination. In contrast, sample SNg4 prepared from oxygen contaminated starting material contained about 12 wt% according to the CHGE analysis. After the hydrofluoric Atazanavir treatment of SN51 the overall oxygen content measured by CHGE is reduced from 7 to 4 wt%.
Fig. 3. Comparison of the XRD patterns of the samples with the minimum oxygen content (SN51, SNg5) with the XRD pattern of the sample with the maximum oxygen content (SNg4); the diffuse scattering of the samples (dashed lines) differs obviously.Figure optionsDownload full-size imageDownload as PowerPoint slide
The following local structural studies of sample SNg4 and SNg5 by STEM investigations confirmed the differences in the microstructure referring to samples with different initial oxygen content. The lattice planes are much better visible and the fringes of crystallites are sharper in sample SNg5 (see Fig. 4a) than in sample SNg4 (see Fig. 4b). At spindle apparatus point we would like to stress that the observed degradation of crystallinity in Fig. 4b is a real effect and not an artifact of the defocusing. The HRSTEM indicates the progression of amorphization of the material: the sample SNg4 with the highest initial oxygen content shows the highest fraction of the amorphous material compared to the low-oxygen containing sample.