A Short Overview of the Present Success and Limitations of using a 3rd Generation Synchrotron Source to Measure the Phase Diagram of low-Z Systems at very High Pressure: Would an ERL X-Ray Source Enable Much Progress and How?
Paul Loubeyre
Départment de Physique Théorique et Appliquée, CEA. France
Over the past ten years, interesting results have been achieved in the determination of the structural and dynamical properties of low-Z systems by optimising the use of the diamond anvil cell in front of large facilities. More particularly, by using the x-ray beam of the 3rd generation synchrotron source. The strategy for progress has relied on improving the three components of such experiments, that is : the sample quality; the characteristic of the beam to reduce the signal to background ratio; the most adapted detector for data collection. That will be illustrated on three examples: the determination of the equation of state of hydrogen by single-crystal x-ray diffraction 1)2); the determination of the structure factor of fluid oxygen and water at high pressure3); the inelastic x-ray scattering data of the sound velocity of water in the THz frequency range4).
The determination of the equation of state of hydrogen somehow exemplifies the need to improve the performance of the synchrotron x-ray beam in order to achieve new measurements. The progress has followed a staircase evolution essentially mapping the improvements of the x-ray technique and beam: energy dispersive x-ray scattering (on ID09) , angle dispersive x-ray scattering single crystal (on ID30) , micro-focus angle dispersive single-crystal X-ray scattering (on ID27). The structure of phase I and II have been disclosed. The structure of phase III is within reach. Yet, the ultimate goal is to determine the structure of metallic hydrogen . That should certainly require a significant jump in the x-ray brightness and source size. Such a beam could be obtained by an ERL synchrotron x-ray source. Also, metal hydrogen could probably be observed more easily in a transit time mode by coupling a magnetic pulse, a temperature pulse or a light pulse. But then, the necessity to collect the X-ray data in less than a short time scale will also require the high flux of the ERL synchrotron source.The dynamical structure factor of molecular fluids is essential to understand the evolution of a molecular fluid such as polymerisation, dissociation and ionisation.
The problem here is to have enough x-ray flux to be able to collect inelastic x-ray data in a reasonable time. This is all the more important at very high P-T conditions where it is difficult to confine the sample. The use of an ERL synchrotron source will enable to perform the measurements with a reduction of at least one order of magnitude in the accumulation time and also with a better energy resolution.
References:
1. X-ray diffraction and equation of state of hydrogen at megabar pressures. P.Loubeyre, R.LeToullec, D.Hausermann, M.Hanfland, R.Hemley, H.K.Mao and L.Finger, Nature 383, 702 (1996)
2. Neutron and x-ray diffraction study of the broken symmetry phase transition in solid deuterium. I. Goncharenko and P.Loubeyre. Nature 435, 1206 – 1209 (2005)
3. Quantitative structure factor and density measurements of high-pressure fluid in diamond anvil cells by x-ray difffraction: argon and water. J.Eggert, G.Weck, P.Loubeyre and M.Mezouar, Phys. Rev. B.65, 174105 (2002)
4. Pressure Evolution of the High-Frequency Sound Velocity in Liquid Water, M.Krisch, P.Loubeyre, G.Ruocco et al , Phys. Rev. Lett. 89, 125502 (2002)
