High–temperature Raman spectroscopic study of CO₂–containing melanophlogite

Abstract

CO 2-containing melanophlogite from Fortunillo, Italy was studied using a micro-Raman spectrometer with the ability to measure the low-frequency region. A very intense and broad feature was found below 100 cm −1. To clarify the origin of this feature in relation with CO 2 , heat treatment experiments and in-situ high-temperature Raman measurements were conducted up to 1100°C. As a result of the heat treatment experiments, nearly CO 2free melanophlogite was obtained at 950°C for 6 h. For shorter time duration or lower treatment temperature, CO 2 vibrational Raman peaks (Fermi diad) were still observed, and those peaks were split. The low-frequency feature also reduced its intensity in these degassed samples. For the in-situ study, the intensity of CO 2 Raman peaks started to drop at around 450°C, and simultaneously the low-frequency feature intensity decreased. The splitting of the CO 2 Raman peaks started from 450°C, and it was interpreted as redistribution of CO 2 molecules in two distinct cages in the structure. The low-frequency feature completely disappeared at 1100°C. It was concluded that the low-frequency feature is originated from CO 2 molecules. Librational and translational modes of CO 2 molecules in the cages of melanophlogite would be responsible for the low-frequency feature. The hightemperature Raman spectroscopic study thus provides us insight into CO 2 diffusion in melanophlogite structure.

Phase transitions of tridymite MC: A low frequency Raman spectroscopic study

“…As most of samples in previous studies at high temperature confirmed OS phase, the transitions regarding OS phase (i.e., OP/OS and OS/OC) should be studied. New generation Raman notch filter (Lebedkin et al, 2011) now allows us to measure the lower frequency region even using a conventional single monochromator-based Raman system as demonstrated in our previous studies (Kanzaki, 2018;Kanzaki, 2019a). Recently, Raman spectra of tridymite modifications (MC, MX-1, and PO-10) stable at room temperature including the low frequency region were reported (Kanzaki, 2019b), and the low frequency peaks were found for all phases studied.…”

“…Relative wavenumbers in reported spectra were calibrated against 10 Raman peaks of a synthetic coesite sample using a third-order polynomial equation. Detailed calibration procedure is given in Kanzaki (2019a). There was no significant background increase due to thermal radiation up to 500°C, and no correction including background subtraction was applied for all the reported spectra.…”

Raman spectra of tridymite modifications: MC, MX–1, and PO–10

“…Backscattered geometry was employed. In order to reject the Rayleigh scattering, two Ondax's SureBlock Raman notch filters (Lebedkin et al, 2011) were used, and the present configuration can measure Raman spectrum down to~15 cm −1 including the anti-Stokes side as demonstrated in our previous studies (Kanzaki, 2018(Kanzaki, , 2019. The beam passed through the filters is introduced to a 500 mm monochromator (Acton Spectra Pro 500i) with 1200 g/mm grating, and the dispersed light was detected by a liquid nitrogencooled CCD detector (Princeton Instruments' PyLoN 400BR eXcelon).…”

“…Relative wavenumbers in reported spectra were calibrated against 10 Raman peaks of a synthetic coesite standard. The detailed calibration procedure is given in Kanzaki (2019). Reported peak positions are obtained by fitting Raman spectrum with Voigt profile Figure 1.…”

“…The low frequency region contains rich information such as valuable structural information of tridymite modifications as suggested by Kihara et al (2005), but the region is rarely reported. New generation Raman notch filter (Lebedkin et al, 2011) now allows us to measure the lower frequency region even using a conventional single monochromator-based system as demonstrated in our previous studies (Kanzaki, 2018(Kanzaki, , 2019…”