Na<sub>2</sub>CO<sub>3</sub>的高压拉曼光谱研究

doi:10.3964/j.issn.1000-0593(2021)07-2087-05

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Abstract 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 0.001～27.53 GPa and the wavelength range of 600～1 200 cm^-1 using diamond anvil cell combined with advanced confocal Raman spectroscopy. This experiment focused on the analysis of the behavior of [CO₃]^2- group vibration mode in the process of compression and decompression. The results showed that splitting of vibration peaks respectively appeared in symmetric stretching vibration γ₁, antisymmetric stretching vibration γ₃ and the in-plane bending vibration γ₄ of the [CO₃]^2- at the pressure range of 0.001～11.88 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^-1 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 [CO₃]^2-. When the pressure is decompressed to 4.18 GPa, it is found that the vibration mode of [CO₃]^2- is identical with that at ambient condition, and the new peak has disappeared, indicating that the phase transition is caused by the distortion of [CO₃]^2- 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 γ₃ and in-plane bending vibration γ₄ 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 [CO₃]^2-. This is probably related to the length of the C—O bond. Finally, by comparison, the intensity of symmetric stretching vibration γ₁ peak is higher than that of antisymmetric stretching vibration γ₃ and in-plane bending vibration γ₄. The pressure also has little effect on the typical Raman peak γ₁ of [CO₃]^2-, and therefore can be used to distinguish different kinds of carbonates.

Key words：Sodium carbonate; Raman scattering; High pressure; Phase transition

Received: 2020-07-16 Accepted: 2020-11-08

ZTFLH:

O521+.2

Corresponding Authors: GAO Jing, LI Ying E-mail: gaojing@mail.iggcas.ac.cn; subduction6@hotmail.com

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http://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2021)07-2087-05 OR http://www.gpxygpfx.com/EN/Y2021/V41/I07/2087