Study On The Phase Transition Of Helium Under High Pressure

As widely used calibrants, Bismuth (Bi) attracts many attentions because of its abundance phase polymorphism and particular electronic structures and physical properties. There have been many studies on solid phase transitions under high pressure, however, there are still several outstanding uncertainties and the controversies are mainly at the stability field of phase III at room temperature. Most of the previous studies concerning the crystal structures used a single method of X-ray diffraction (XRD). The understanding of the liquid Bi is quiet few. A temperature-driven liquid-liquid structural transformation under ambient pressure was reported several years ago, but the liquid-liquid phase transitions under high pressure need further research.X-ray absorption fine structure (XAFS) is also a useful tool in determining the geometric structure and is particularly suitable to study the systems in short-range order such as amorphism and liquid. XAFS provides a new idea to study the phase transitions. In this paper, we use the high-pressure XAFS as the main research method combined with XRD and the first-principle calculations to study the solid-solid, the solid-liquid and the liquid-liquid phase transitions of Bi under high pressure.According to the different shapes of the EXAFS curves, we obtain the pressure range of each different phase. Compared with the phase diagram, the sequence of phase transition at room temperature of Bi is:Ⅰ→Ⅱ→Ⅲ→Ⅴ. The pressure ranges are well conformed to the previous research. Our research focus on the most controversial phase of phase Ⅲ. We find that it is hard to detect the structure of phase III using XRD, which is also the reason for why the controversy continues for a long time. Both the calculated XANES spectra and the enthalpies as a function of pressure for three proposed structures of phase III show that the incommensurate composite structure described by McMahon et al. is the most reasonable structure. Using this structure and the structures of phase I, II and V, we calculate the XANES spectra of each phase. The calculated spectra are in excellent agreement with the experiment results, supporting that the structures including the incommensurate composite structure are appropriate. In addition, according to the radial structural functions, the nearest neighbor distance of phase III abnormally increases under compression, which may be corresponding to the highly anisotropic covalent state of the incommensurate structure. To study the electronic properties of Bi, we calculated electronic density of states (DOS) of four phases. It shows that Bi undergoes from the semimetallic phase I to the typical metallic phase V. The metallic character increases with the increasing pressure, which is responsible for its complex structural transition.Combined XANES with high temperature technique, the solid-liquid and the liquid-liquid phase transitions are studied. The average energy levels of the valence electrons show a little decrease during the beginning of melting. One possible explanation is that some valence electrons extend to more localized while part of the metallic bonding are break during melting. What’s more, a discontinuous change of the absorption edge of the XANES spectra from 820K to 830K shows a direct evidence for a liquid-liquid structural change of Bi under high pressure. The previous phase boundary may need to be redefined.In all, we use an independent source which distinguishes with XRD to study the phase transitions of Bi. It can enrich the understanding of the structural transitions of Bi under high pressure, promote the development of high pressure XAFS technology and offer some revelation and reference for the similar research.