Low Temperature Synthesis of Aegirine NaFeSi2O6: Spectroscopy (57Fe Mössbauer, Raman) and Size/Strain Analysis from X-ray Powder Diffraction

Abstract

Using a low temperature synthesis protocol, it was possible to obtain phase-pure synthetic aegirine (NaFeSi2O6) at temperatures as low as 130 °C, albeit only with rather long synthesis times of ~200 h; at 155 °C, a nano-crystallite shaped phase-pure material is formed after 24 h. These are, to the best of our knowledge, the lowest temperatures reported so far for phase-pure aegirine synthesis. Powder X-ray diffraction (PXRD) was used to characterize phase purity, structural state and microstructural properties (size and strain) of the as-synthesized (130–230 °C) and heat treated (300–900 °C) samples, via Rietveld analysis of powder patterns. Melting was observed at 999 °C. With increasing synthesis temperature, crystallite size linearly increased from 10 nm to 30 nm at 230 °C, while unit cell parameters decreased. The microstrain was very small. Additional heat treatment of as synthesized samples showed that the crystallite size remained rather unaffected up to 700 °C. The lattice parameters, however, already changed at low temperatures and successively became smaller, indicating increasing ordering towards more regular arrangements of building units. This was confirmed by 57Fe Mössbauer spectroscopy, where a distinct decrease of the quadrupole splitting with increasing synthesis temperature was found. Finally, Raman spectroscopy showed that some weakly-developed pre-ordering effects were present in the samples, which appeared to be amorphous in PXRD, while well-resolved spectra appeared as soon as the long-range ordered crystalline state could be found with X-ray diffraction.