Question to blue and red shift during Raman spectroscopy of coesite:
For me, the significant blue shift of the coesite Raman lines is a mystery, and I cannot find a simple answer on the net. Therefore I have performed Raman measurements on an ideal SOS sample. This sample is characterized by an epitaxial Si layer on a single crystal wafer of sapphire (SOS).
I have taken Raman spectra of Si at different laser power from 10 to 100% of the 532 nm laser - 100% corresponds to 50 mW.
For the Si main line, I obtained a 520.1 ± 1.8 cm-1independents of the power. That means that the used laser power does not generate the blue shift in the coesite spectra, and the blue-red shift is generally reversible. Furthermore, the SOS test has shown that the problem does not come from a lousy device adjustment.
Water determination gives for coesite very high values of about 8500 ppm or more. Can the high OH-concentration be responsible for the blue shift? This high water content is only possible if the coesite is formed at very high pressure. Another possibility is a significant Al content in the coesite lattice (only possible at high pressure, similar to stishovite which can accommodate Al2O3and H2O through coupled substitution at pressures above 30 GPa (see Tschauner 2019)). For that spokes, the very strong 330 cm-1 bands in the Raman spectrum of coesite are generally missing in the “standard coesite.” All other essential lines of Coesite are present.
Tschauner, O. (2019) High-pressure minerals. Am. Miner. 104, 1701-1731
Have you looked at crystal strain/stress as a culprit? Heteroepitaxy can cause changes in bond lengths, and, thus, red/blue shifts.
Thank you very much for this important hint. The coesite sample is thin (some microns) and shows an oriented intergrow with the substrate (prismatine). The magnitude of the Raman peak shift depends on the corresponding strains produced at the interface.
Another highly probable explanation of the significant blue shift of the prominent coesite bands:
The significant blue shift (about 18 cm-1) results from the strain generated by the lattice mismatch between primary stishovite (now only remnants!) and the resulting coesite.
Do all the Raman peaks shift by the same amount? Or do they differ in blueshift?
An estimate of the pressure/strain experienced by coesite can be obtained from the Raman mode frequency vs Pressure slopes from this work:
Article Pressure Dependence of Raman Spectra of SiO2 Polymorphs: α‐Q...
Generally, the blue shift is more or less similar for all bands. However, there are also minor differences, depending on lattice mismatch and heteroepitaxy portions.
I would also expect that if lattice mismatch and heteroepitaxy is at play, a broadening of the Raman bands should be detectable. Is this the case? The fact that the whole spectrum is shifted by the same amount is indeed strange; my first guess would be a calibration issue, but since this is covered, cationic substitution or some kind of ''universal'' structural aspect inducing compressive stress/strain on the crystal would be my next guess
The change in the band FWHMs is not significant. If I study a free coesite crystal, I see no blue shift.
Cationic substitution is also possible because the coesite's water content is unusually high – e.g., via Al and H2O by coupled substitution.
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