冲击压缩下过渡金属钒固-液相变的超快X光衍射研究

Melting, or solid-liquid transformation, is of crucial importance to understand physical properties and dynamic responses of condensed matter under extreme conditions. However, an apparent discrepancy in the high-pressure solid-liquid phase boundary of some transition metals between static diamond anvil cell (DAC) measurements and shock wave (SW) experiments is still a long-standing open question. Within these metals, vanadium is spectacular due to the huge discrepancy of its melting temperatures between DAC and SW data. Meanwhile, it possesses intriguing solid-solid phase transformations. But our current knowledge about these behaviors is still deficient and incomplete. For this purpose, we propose to explore and investigate the structural changes and solid-liquid transformation of vanadium under shock compression with ultrafast dynamic X-ray diffraction technique in this project. Augmented with measurements of shock temperature and sound velocity, as well as the calculation of its high-pressure high-temperature phase diagram, we will attempt to confirm the solid-liquid phase boundary of shocked vanadium, and to search for other conjectured phase transition occurring between the reported DAC melting curve(PRB, 63: 132104, 2001) and SW melting curve. It will provide useful shock compression experimental information and the essential physical basis for comprehending the mechanisms that govern static and dynamic melting in transition metals.

高压固-液相变研究对理解物质在极端压缩条件下的性质变化和响应行为具有重要的科学价值，其中部分过渡金属的动/静高压固-液相界差异一直是悬而未解的难题。作为固-液相界幅值差异最大的材料，钒同时具有令人费解的奇特相变行为，但对于这些现象目前仍缺乏充分的认识和理解。本项目拟以冲击压缩下的超快X光衍射技术为主要研究手段，辅以宏观热力学量测量（粒子速度、温度和声速）和有限压力-温度下的相图计算，探索和研究过渡金属钒在冲击压缩下的结构变化和固-液相变行为，试图确定钒在冲击压缩下固-液相边界位置，探寻冲击Hugoniot线与已知静高压熔化线（PRB, 63: 132104, 2001）交点附近及以上区域是否有其它相态转变的可能，从动高压实验角度出发为揭示动/静高压熔化线差异的物理机制提供有益的实验依据和基本物理认识。

极端压缩条件下的固-液相变研究是一个长久以来广受关注的科学问题，其中令人最为困惑的便是部分过渡金属动高压与静高压固-液相界之间的巨大差异。作为固-液相界幅值差异最大的材料，钒令人费解的独特相变行为使得上述问题变得更为复杂，一直悬而未解。本项目发展了基于激光驱动的X射线衍射技术，实现了沿冲击路径200GPa内物相结构的原位测量，并辅以冲击温度、声速等宏观热物性测量和第一性原理分子动力学计算，对钒在冲击压缩下的结构、固-液相变和相图进行了系统研究，回答了以前存在困惑的科学问题，获取了一些关于钒动态响应行为的开拓性认识。包括：（1）获取钒在动态响应过程中结构变化的直接图像，确认了在原有DAC熔化线的上方、冲击熔化线的下方区域仍为BCC相固态；（2）揭示在150 GPa以上压力区间未发现再次固-固相变引起熔化线趋势变化的可能，推断沿冲击路径BCCRHBCC液相的相变序列；（3）给出新的动高压熔化线，结合熔化曲线计算和静高压XRD数据验证，可靠地获得动/静高压相界的统一认识，确认了200GPa内钒的固-液相界。这些新的知识极大地拓展和丰富了我们的视野和思路，获得的数据、知识和所建立的物理图像对进一步理解过渡金属的高压熔化行为十分关键，具有重要的科学意义。