The alkali group elements are considered as textbook examples of free-electron metals because of the single s electron in the valence band. However, when these metals are subjected to compression these simple systems exhibit unexpected complexity. One of the interesting point is, in the case of Na, the existence of a crystal structure which contains more than 500 atoms in the unit cell.1 The complex phase diagrams of these "simple" metals suggests extraordinary liquid states at extreme conditions and has implications for other "simple" metals. Previously, a possible link between the maximum in the melting line and liquid–liquid phase transition (LLPT) was suggested.2 Also, changes in the short-range order of molten Na at high pressure have been correlated to the corresponding solid phase transitions. The study of the liquid structures of the light alkali metals has not been completed because of the lack of experimental data. Only X ray diffraction (XRD) data at room pressure are available for liquid alkali metals thus high pressure experiments are essential to determine the high density liquid structures of these materials.3 The major difficulty with liquid diffraction at high pressure is the large scattering background signal generated by the diamond anvil cell as Bragg scattering, Compton scattering and temperature-diffuse scattering. Although special care in collecting data allows reducing or eliminating the Bragg reflections, the Compton and the diffuse scattering are far more insidious and impossible to eliminate up to now. In the case of low Z elements the signal over background ratio could be around only 1 to 5%. All these points explain the lack of experimental data of liquid alkali metals at high pressure.
We performed the first quantitative measurements of the liquid structure factor of light alkali metals up to 100 GPa using angle dispersive XRD (ADXRD). We explored the P-T diagram in order to obtain quantitative structure factor, radial-distribution function and density of liquid alkali metals up to 100 GPa by ADXRD. We confirmed the existence of a different liquid structure at the minimum of the melting curve compared with the one at room pressure as it has been predicted theoretically for Na.4
References
[1] E. Gregoryanz , et al, Science 320, 1054 (2008).
[2] S. Falconi, et al, Phys. Rev. Lett. 94, 125507 (2005).
[3] A. J. Greenfield et al., Phys. Rev. A 4, 1607 (1971).
[4] J.-Y. Raty, et al, Nature 449, 448(2007).