Phase Diagram and Dynamic Transformation Pathways of Carbon under Extreme Compression
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摘要: 碳在极端压缩条件下表现出复杂的结构转变、熔化行为和电子性质演化。这些过程与富碳行星内部物态、惯性约束聚变中高密度碳烧蚀层响应以及后金刚石相形成密切相关。为此，以平衡压力-温度（p-T）相图为热力学参照，综述了石墨、金刚石、液态碳和 BC8 等结构，比较了单冲击、斜波压缩、双冲击或多冲击等加载路径及其原位诊断方法，重点讨论了石墨向立方金刚石和六方堆垛相关结构的转变、金刚石熔化与液态碳结构，以及BC8碳的动力学稳定性。已有研究表明，动态压缩中的可观测结构不仅取决于压力和温度，还受到加载路径、应力状态、初始结构和诊断窗口的共同影响。因此，理解碳的动态相变需要把平衡相界放回具体的加载路径中，形成p-T-path框架，该分析框架可为极端条件下碳材料的实验路径设计、原位结构诊断和状态方程约束提供参考。Abstract: Carbon exhibits complex structural transformations, melting behavior, and electronic-property evolution under extreme compression. This behavior is relevant to carbon-rich planetary interiors, the dynamic response of high-density carbon ablators in inertial confinement fusion, and the formation of post-diamond phases. This review focuses on the dynamic phase diagram of carbon. We summarize the equilibrium reference states of graphite, diamond, liquid carbon, and BC8 carbon, and discuss typical dynamic loading paths under shock, ramp, and multiple-shock compression together with their corresponding in situ diagnostics. We focus on the graphite-to-diamond/lonsdaleite-like transformation, diamond melting and liquid-carbon structure, and the kinetic accessibility of BC8 carbon. Existing studies show that the phases observed under dynamic compression are governed not only by pressure and temperature, but also by loading time, stress state, initial structure, and diagnostic window. Therefore, a dynamic phase diagram of carbon should extend the conventional pressure-temperature (p-T) description by incorporating loading path, leading to a p-T-path representation. This perspective may provide guidance for experimental design, in situ structural diagnostics, and equation-of-state model constraints for carbon under extreme conditions.
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Key words:
- carbon /
- dynamic phase transition /
- shock compression /
- ramp compression /
- diamond melting /
- BC8 carbon /
- liquid carbon
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图 1 动态压缩下碳的p-T 相图（相界和路径均为示意。Benedict 等[18]和 Correa 等[10]的结果用于展示理论相界差异。斜波路径及阴影区表示金刚石在 BC8 理论稳定区内的亚稳保持；HD/lonsdaleite 窗口表示静态高温高压和冲击实验中与路径相关的转变结果。液态碳位于熔化线以上；BC8 和 SC1 为理论候选后金刚石相，仍需原位结构诊断验证。）
Figure 1. Schematic p-T phase diagram of carbon under dynamic compression (The phase boundaries and loading paths are schematic. Results from Benedict, et al.[18] and Correa, et al.[10] are shown to compare theoretical phase-boundary differences. The ramp-compression path and shaded region indicate metastable retention of diamond within the theoretical BC8 stability field. The HD/lonsdaleite window marks path-dependent transformation products observed in static high-pressure high-temperature (HPHT) and shock experiments. Liquid carbon lies above the melting curves. BC8 and SC1 are candidate post-diamond phases that still require in situ structural verification.)
图 2 金刚石高压p-T区域中的动态加载路径示意图（冲击 Hugoniot 和近等熵斜波路径分别表示高温冲击与低温高压加载的典型方向；双冲击或多冲击可由中间状态出发，进一步进入液态碳或 BC8 理论稳定区。）
Figure 2. Schematic diagram of dynamic loading paths in the high-pressure p-T region of diamond (The shock Hugoniot and quasi-isentropic ramp path represent typical directions of high-temperature shock loading and lower-temperature compression, respectively. Double- or multiple-shock loading may proceed from an intermediate state toward liquid carbon or the theoretical BC8 stability field.)
图 3 后金刚石碳形成的定性动力学能垒示意图（金刚石-BC8 的直接固-固重构具有较高的势垒，类液体中间态可能提供通向 BC8 的较低势垒路径；过度压缩则会增强SC1 竞争。坐标轴为示意，BC8 和 SC1 仍需动态原位结构证据确认。）
Figure 3. Schematic diagram of qualitative kinetic-barrier landscape for post-diamond carbon (Direct diamond-BC8 solid-solid reconstruction crosses a high barrier, whereas a liquid-like intermediate may provide a lower-barrier route to BC8; over-compression favors SC1 competition. The axes are schematic, and BC8/SC1 remain candidate endpoints requiring direct in situ structural confirmation.)
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