高温高压下氯化钠化学反应的拉曼光谱表征

Raman spectroscopic characterization of the chemical reaction of sodium chloride at high temperature and high pressure

氯化钠(NaCl)是一种典型的离子化合物,它的高温高压行为研究对高压物理和化学具有重要的基础和应用意义。为探索NaCl在高温高压下的化学反应性,我们利用金刚石压腔高压装置和双面激光加热技术在85 GPa压力范围内对NaCl样品进行(1800±300) K高温处理,并对淬火后的样品进行拉曼光谱测试。实验结果表明,在压力大于31 GPa时, NaCl分解为斜方结构的NaCl3 (Pnma)和四方结构的Na3Cl (P4/mmm)。NaCl高温高压下分解产生非1∶1化学配比的NaCl3和Na3Cl等非常规化合物,表现出与传统认识截然不同的高温高压性质。这一化学反应表明高温高压下Cl-Cl共价键、Na-Na金属键与常规Na-Cl离子键竞争的结果,可能更有利于NaCl3和Na3Cl组合的稳定出现。金刚石压腔实验中NaCl样品层中存在较大温度梯度,使得样品体系处于一种非平衡状态,这种非平衡态可能是本研究中观察到NaCl发生分解反应的关键。另外,样品体系中用作激光吸收材料的过渡金属氧化物的催化作用可能是促进NaCl分解的另一个因素。高温高压下Pnma-NaCl3化合物的稳定出现,表明地球下地幔深处氯元素也许可以以三氯聚阴离子(Cl3^-)形式存在,这种可能的独特赋存方式可以为理解地球演化过程中Cl以及其他卤素元素的分配分异行为提供不同的视角。

The high-pressure and high-temperature behaviors of sodium chloride(NaCl)have a fundamental and applicational significance for high-pressure physics and chemistry.To explore the reactivity of NaCl at high pressure and high temperature,we compressed NaCl samples to pressures up to 85 GPa and then heated them to a temperature of(1800±300)K using a diamond anvil cell and double-sided laser heating technology.Our investigation of Raman spectroscopy showed that NaCl decomposed to NaCl3 with an orthorhombic Pnma structure plus possible Na3Cl with a tetragonal P4/mmm structure above 31 GPa.The decomposition of NaCl under high temperature and high pressure produced non-conventional compounds such as NaCl3 and Na3Cl without a 1:1 stoichiometric ratio,which exhibited properties completely different from those of conventional knowledge.This indicates that the competition of the unusual Cl-Cl covalent bond and the Na-Na metallic bond with the Na-Cl ionic bond at high pressure and high temperature may prefer the occurrence of a combination of NaCl3 and Na3Cl.We speculate that the large temperature gradient in the NaCl sample layers in the diamond anvil cell experiments,which indicated a non-equilibrium state,was critical to the decomposition reaction of NaCl observed in this study.The catalytic effect of the laser absorbing materials of transition metal oxides used here could be another factor for promoting the chemical reaction of NaCl.Our experimental observations suggest that Cl exists in the form of trichloride anion(Cl3–)in P-T conditions of the lower mantle,thus forming independent alkali metal polychloride compounds.This possible unique existence provides a different perspective for understanding the distribution of Cl and halogens during Earth’s evolution.