1. 中国地震局地震预测研究所，北京 100036
2. 中国工程物理研究院流体物理研究所，四川 绵阳 621900
3. 防灾科技学院，河北 三河 065201
4. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室，北京 100029
5. School of Earth Sciences, University of Bristol, Bristol BS81RJ, United Kingdom
6. 中国工程物理研究院核物理与化学研究所，四川 绵阳 621999
High Pressure Raman Spectrum Study of Na2CO3
XU Chao-wen1, 2, 3, GAO Jing4*, LI Ying1*, QIN Fei5, LIU Hong1, YI Li1, CUI Yue-ju1, SUN Feng-xia1, FANG Lei-ming6
1. Institute of Earthquake Forecasting, China Earthquake Administration (CEA), Beijing 100036, China
2. Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900，China
3. Institute of Disaster Prevention, Sanhe 065201，China
4. State Key Laboratory of Lithospheric Evolution, and Institutions of Earth Science, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
5. School of Earth Sciences, University of Bristol, Bristol BS81RJ, United Kingdom
6. Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621999, China
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
徐超文，高 静，李 营，秦 霏，刘 红，易 丽，崔月菊，孙凤霞，房雷鸣. Na2CO3的高压拉曼光谱研究[J]. 光谱学与光谱分析, 2021, 41(07): 2087-2091.
XU Chao-wen, GAO Jing, LI Ying, QIN Fei, LIU Hong, YI Li, CUI Yue-ju, SUN Feng-xia, FANG Lei-ming. High Pressure Raman Spectrum Study of Na2CO3. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2021, 41(07): 2087-2091.
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