High Pressure Raman Spectrum Study of Na2CO3

Carbonate is one of the important carriers of carbon in the earth's interior. Therefore, its crystal chemistry under the condition of temperature and pressure corresponding to the mantle is the key to understand the carbon occurrence state and cycle process of deep earth, but structural stability and phase transition are the basic research contents of crystal chemistry. Na2CO3 is a common alkaline carbonate, which enters the earth' s interior by subduction of oceanic crust. The existence of sodium carbonate in the subducted slab can significantly reduce the melting temperature of peridotite, promote partial melting and induce mantle heterogeneity. The inclusions of sodium carbonates have been found in the diamonds from the mantle transition zone and the lower mantle, providing direct mineralogical evidence that sodium carbonate can deeply subductin to the deep mantle. The lattice vibration modes of Na2CO3 at ambient condition were reported previously by Raman spectroscopy, but its stability and structural changes under high pressure are poorly reported. In this study, we used silicone oil as pressure transmitting medium and the Raman spectrum of Na2CO3 powder have been carefully ascertained in the pressure range of O. 001-27. 53 GPa and the wavelength range of 600 1 200 cm using diamond anvil cell combined with advanced confocal Raman spectroscopy. This experiment focused on the analysis of the behavior of [CO3 12 group vibration mode in the process of compression and decompression. The results showed that splitting of vibration peaks respectively appeared in symmetric stretching vibration y,, antisymmetric stretching vibration y3 and the in-plane bending vibration y4 of the [C0312 at the pressure range of O. 001-11. 88 GPa. With the increase of pressure, all peaks shift to high frequency, and the full width at half maximum (FWHM) increases gradually. The phase transition occurred at 13. 40 GPa, accompanied by a new Raman peak at 690. 08 cm and the intensity of the peak increases with the increase of pressure. At the same time, the intensity of antisymmetric stretching vibration and in-plane bending vibration continued to weaken, and the FWHM of Na2CO3 also continued to increase, indicating that the phase transition of Na2CO3 originates from the internal lattice vibration of [C0312. When the pressure is decompressed to 4. 18 GPa, it is found that the vibration mode of [C0312 is identical with that at ambient condition, and the new peak has disappeared, indicating that the phase transition is caused by the distortion of [C0312 group and is recoverable. The Raman peaks continued shifting to high frequencies when the pressure increased to 27. 53 GPa, suggesting this new phase can remain stable in this pressure range. The intensity of Raman peaks at the antisymmetric stretching vibration y3 and in-plane bending vibration y4 decreased during compression. Meanwhile, the calculated dependence coefficient of relative pressure-shift of each Raman peak showed that the response of each vibration mode to pressure is different in [C0312. This is probably related to the length of the C-0 bond. Finally, by comparison, the intensity of symmetric stretching vibration y, peak is higher than that of antisymmetric stretching vibration y3 and in-plane bending vibration y. The pressure also has little effect on the typical Raman peak y, of [C0312 and therefore can be used to distinguish different kinds of carbonates.