Solid carbon dioxide exhibits a rich phase diagram at high pressures. Metastable phase III is formed by compressing dry ice above ∼10–12 GPa. Phase VII occurs at similar pressures but higher temperatures, and its stability region is disconnected from III on the phase diagram. Comparison of large-basis-set quasi-harmonic second-order Møller–Plesset perturbation theory calculations and experiment suggests that the long-accepted structure of phase III is problematic. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure agree poorly with experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical.

中文翻译：

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理论预测表明二氧化碳相III和VII是相同的

固态二氧化碳在高压下表现出丰富的相图。亚稳态阶段III是通过压缩约10-12 GPa以上的干冰而形成的。VII相在相似的压力下出现，但温度更高，并且其相变区的稳定性区域与III相断开。比较大基集准谐波二阶Møller-Plesset微扰理论的计算结果和实验结果表明，长期以来人们所接受的III相结构存在问题。实验的III和VII相结构都松弛到相同的VII相结构。此外，对于VII期预测的拉曼光谱与在III和VII期实验观察到的光谱非常吻合，而对于声称的III期结构的拉曼光谱与实验观察结果却相差甚远。晶体结构预测被用来寻找可能占第三阶段的其他潜在结构，但没有发现。这些结果加在一起表明，第三阶段和第七阶段可能是相同的。