Raman spectroscopy and x-ray diffraction of sp3 CaCO3 at lower mantle pressures
Raman spectroscopy and x-ray diffraction of sp3 CaCO3 at lower mantle pressures
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DOI：
10.1103/physrevb.96.104101
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发表时间：
2017-09-01
影响因子：
3.7
通讯作者：
Goncharov, Alexander F.
中科院分区：
文献类型：
--
作者：
Lobanov, Sergey S.;Dong, Xiao;Goncharov, Alexander F.
碳在非环境条件下形成sp(2)和sp(3)键态的特殊能力导致了含碳相的结构和化学多样性。在这里，我们使用激光加热的金刚石砧细胞，结合同步加速器x射线衍射，拉曼光谱和第一线原理计算来探索pbbb40gpa下CaCO3的相变。我们发现，在105 GPa（比理论预测的交叉压力高30 GPa）下，sp(3)杂化碳的后钙石CaCO3转变为先前预测的P2(1)/c CaCO3。我们的可变单元微推弹性带模拟显示，P2(1)/c CaCO3的最低焓转变路径包括重复出现sp(2)和sp3 CaCO3中间相和过渡态。根据经验和第一性原理计算，P2(1)/c CaCO3的拉曼光谱在1025 cm（-1）处显示出一个强烈的波段，这是对称的-O拉伸振动。由于C-O键长度在sp(2)-sp(3)转变过程中增加，该拉曼带的频率比sp(2) CaCO3中对称C-O拉伸的频率低20%左右，可以作为其他碳酸盐中四面体配位碳的指纹。
The exceptional ability of carbon to form sp(2) and sp(3) bonding states leads to a great structural and chemical diversity of carbon-bearing phases at nonambient conditions. Here we use laser-heated diamond-anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that postaragonite CaCO3 transforms to the previously predicted P2(1)/c CaCO3 with sp(3)-hybridized carbon at 105 GPa (similar to 30 GPa higher than the theoretically predicted crossover pressure). The lowest-enthalpy transition path to P2(1)/c CaCO3 includes reoccurring sp(2) and sp3 CaCO3 intermediate phases and transition states, as revealed by our variable-cell nudged-elastic-band simulation. Raman spectra of P2(1)/c CaCO3 show an intense band at 1025 cm(-1), which we assign to the symmetric -O stretching vibration based on empirical and first-principles calculations. This Raman band has a frequency that is similar to 20% lower than the symmetric C-O stretching in sp(2) CaCO3 due to the C-O bond length increase across the sp(2)-sp(3) transition and can be used as a fingerprint of tetrahedrally coordinated carbon in other carbonates.