Behavior of Xenon-iron system under core pressure

We have studied the behavior of Xenon-iron system up to about 155 GPa and 3000 K using high-pressure in-situ X-ray diffraction. “Missing Xenon” is a long-standing problem. Earth’s atmosphere is highly depleted in Xe and more than 90 % of the Xe expected to exist is “missing”. Since the discovery that Xe becomes metallic solid with hcp structure above about 120 GPa [Eremets, et al., 2000], strong possibility has been suggested theoretically that Xe is trapped in the Earth’s core by forming compounds or by making solid solution with iron, because iron is also an hcp metal in the same pressure range. No experiment study, however, were reported in this pressure range and various arguments were made using the data up to about 50 GPa. We have studied this system directly at the pressures of the Earth’s core using laser-heated diamond anvil cell combined with synchrotron radiation. Pure iron foil was placed in the hole of a rhenium gasket and the hole was filled with liquid Xe using cryogenic loading system. In order to prevent the contact of iron with the diamond anvil, small NaCl pellet was placed between them, which worked as a pressure marker as well. After each increment of pressure at room temperature, Fe was heated from both sides of the sample using fiber laser. Temperature was measured by spectroscopic method. Angle-dispersive X-ray diffraction measurements were made at high pressure using the BL-10XU beam line of SPring-8, Nishiharima. Three independent runs were conducted. Two runs were made to clarify the behavior of Xe-Fe system. One additional run was made by directly sandwiching Fe foil in NaCl pellets without Xe, to accurately determine the volume of Fe based on NaCl pressure scale. First run in Xe-Fe system was made up to 79 GPa and 2500 K and we found the diffractions from hcp-Fe, hcp-Xe, and B2-NaCl, as well as small amount of fcc-Xe, in agreement with the previous report up to about 50 GPa. No additional diffractions were observed at all. The result remained unchanged in the second run up to about 155 GPa and 3000 K, although the Xe has completely transformed into hcp phase. The unit cell volumes of hcp-Fe observed up to about 160 GPa in the third run were indistinguishable from those observed in the first two runs under the existence of Xe. All these experimental results clearly suggests that neither any compounds are formed between Xe and iron nor any detectable amount of dissolution of Xe into iron occurs, even after Xe has transformed into metal at about 120 GPa. This result suggests that the high-pressure alloying of iron and xenon in the core, which was proposed based on the theoretical calculation (Lee and Steinle-Neumann, 2006), is unlikely to occur and we have to find some other mechanism to explain the “missing xenon”. [1] M. I. Eremets, et al., Phys. Rev. Lett., 85 2797 (2000). [2] K. K. M. Lee and G. Steinle-Neumann, Geophys. Res. Lett., 111 B02202 (2006)