Na2CO3的高压拉曼光谱研究

碳酸盐是碳在地球内部的重要载体之一，其在地幔高温高压条件下的晶体化学是理解地球深部碳的赋存状态和循环过程的关键，而结构稳定性和相变是晶体化学最基本的研究内容。碳酸钠（Na₂CO₃）是一种常见的碱性碳酸盐矿物，在产自地幔过渡带-下地幔的金刚石中已发现含钠的碳酸盐矿物包裹体，这成为碳酸钠能够俯冲进入地幔深部的直接矿物学证据。前人利用拉曼光谱技术研究了Na₂CO₃在常温常压下的晶格振动模式，但其在高压下的稳定性和结构变化却鲜有报道。利用金刚石压腔装置结合先进的共聚焦拉曼光谱技术，以硅油作为传压介质，在准静水压力条件下，在0.001～27.53 GPa压力区间对Na₂CO₃粉末在600~1 200 cm-1波段的振动特征进行了细致地分析。本次实验重点分析了[CO₃]2-基团振动模式在升压和卸压过程中的行为。结果表明，在0.001～11.88 GPa压力范围内，[CO₃]2-基团对称伸缩振动γ₁（1 088.06和1 070.76 cm-1）、反对称伸缩振动γ₃（865.10和797.50 cm-1）和面内弯曲振动γ₄（720.10和696.71 cm-1）都出现了振动峰的分裂。随着压力增加，所有振动峰都向高频率漂移，半高宽也逐渐增加。在13.40 GPa时，Na₂CO₃发生结构相变，具体表现为690.08 cm-1处出现1条新的拉曼峰，并且随着压力升高该峰的强度逐渐增大。同时反对称伸缩振动峰γ₃以及面内弯曲振动峰γ₄的强度持续减弱，半高宽也继续变大。这些现象表明Na₂CO₃结构相变源于[CO₃]2-内部晶格变化。当压力卸载到4.18 GPa时，[CO₃]2-的振动模式与常温常压下的完全吻合，相变出现的新峰也已经消失，表明该相变是由[CO₃]2-基团畸变引起的并且具有可逆性。继续升压至27.53 GPa，拉曼光谱继续蓝移，Na₂CO₃的拉曼谱线再没有变化，说明高压相在这一压强范围内保持稳定。在整个加压过程中，反对称伸缩振动γ₃和面内弯曲振动γ₄处的拉曼峰出现强度减弱现象。同时也计算了各个峰频率对压力的依赖系数dγ/dP，结果显示[CO₃]2-基团内各个振动模式对压力的响应是不同的，这很可能与Ｃ-Ｏ键的键长有关。最后，对比发现，对称伸缩振动γ₁峰的强度比反对称伸缩振动γ₃和面内弯曲振动γ₄峰的强度大，并且[CO₃]2-基团对称伸缩振动γ₁受压力影响相对较小，可以用来区别不同种类的碳酸盐矿物。

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 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.