The liquid⟷amorphous transition and the high pressure phase diagram of carbon

The phase diagram of carbon is mapped to high pressure using a computationally-tractable potential model. The use of a relatively simple (Tersoff-II) potential model allows a large range of phase space to be explored. The coexistence (melting) curve for the diamond crystal/liquid dyad is mapped directly by modelling the solid/liquid interfaces. The melting curve is found to be re-entrant and belongs to a conformal class of diamond/liquid coexistence curves. On supercooling the liquid a phase transition to a tetrahedral amorphous form (ta-C) is observed. The liquid ⟷ amorphous coexistence curve is mapped onto the pT plane and is found to also be re-entrant. The entropy changes for both melting and the amorphous ⟶ liquid transitions are obtained from the respective coexistence curves and the associated changes in molar volume. The structural change on amorphization is analysed at different points on the coexistence curve including for transitions that are both isochoric and isocoordinate (no change in nearest-neighbour coordination number). The conformal nature of the melting curve is highlighted with respect to the known behaviour of Si. The relationship of the observed liquid/amorphous coexistence curve to the Si low- and high-density amorphous (LDA/HDA) transition is discussed.

利用一个计算上易于处理的势模型将碳的相图绘制到高压区域。使用一个相对简单的（Tersoff - II）势模型可以探索很大范围的相空间。通过对固/液界面进行建模，直接绘制出金刚石晶体/液体二元体系的共存（熔化）曲线。发现熔化曲线是再入型的，并且属于一类共形的金刚石/液体共存曲线。在对液体进行过冷时，观察到向四面体无定形形式（ta - C）的相变。将液体⟷无定形共存曲线绘制在pT平面上，发现它也是再入型的。熔化以及无定形⟶液体转变的熵变是从各自的共存曲线以及摩尔体积的相关变化中获得的。在共存曲线的不同点上分析了无定形化时的结构变化，包括等容和等配位数（最近邻配位数无变化）的转变。参照硅的已知行为，强调了熔化曲线的共形性质。讨论了所观察到的液体/无定形共存曲线与硅的低密度和高密度无定形（LDA/HDA）转变之间的关系。