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In search of the missing xenon

​The noble gases, also called rare gases, such as xenon, are the most inert atomic group, but can become reactive under extreme conditions. An international team of scientists used a combination of several synchrotron techniques and ab initio1 modelling to investigate a possible direct reaction between xenon and oxygen at high pressure. They managed to synthesise two oxides under high pressure. This demonstrates that xenon is reactive at pressures relevant to the Earth’s interior. This study, published in Nature Chemistry, could help to resolve the so-called “missing xenon paradox” by providing evidence for a possible storage of this element in the deep Earth planet. ​

Published on 31 May 2016

​Noble gases are characterised by their high chemical stability. The high school paradigm in chemistry classes states that “nothing can force a noble gas atom into chemical bonding”. This inertness is used in geochemistry, where the quantities of noble gases contained in the Earth’s different layers is used to build models of the evolution of the deep Earth and atmosphere. 

Of the six noble gases, one especially puzzles researchers: xenon (Xe). 

The chemistry of xenon is much more complex than expected. In fact, the binding of the external electrons to the ionic core of a noble gas atom decreases when the size of the atom increases, and xenon is one of the biggest rare gases (by order of size: neon, argon, krypton, xenon, radon). Since the 1960s, compounds containing oxidized xenon (which has lost some external electrons) have been synthesised; but xenon oxides are unstable. Recently, theoretical proposals have emerged for the formation of strongly bonded and stable xenon compounds under pressure. However, experimental data for xenon oxides have not been reported previously at the high pressures at which xenon oxides might become stable. 

Xenon is a geological enigma. The atmosphere contains much less xenon than expected from the composition of the stony meteorites similar to those which have formed the Earth some 4.54 billion of years ago. One proposal for resolving this “missing xenon paradox” is that xenon is stored in the deep Earth, thus motivating a study to provide a better understanding of the chemistry of xenon under geologically relevant pressures (up to 3.6 million atmospheres at the centre of the Earth). 

The team of researchers investigated the possibility of a chemical reaction under high pressure between xenon and oxygen, which is the most abundant element in the Earth’s mantle. Mixtures of xenon and oxygen gases were loaded into diamond anvil cells (Figure 1), then compressed up to the million-atmosphere range and heated with an infrared laser to induce reactions. Indeed, reactions were observed and the products have been characterised with microfocused X-ray diffraction and X-ray absorption on beamlines ID27 and BM23 dedicated respectively to these techniques, at the ESRF, the European Synchrotron, Grenoble, France (Figure 2, Figure 3). Experimental data were interpreted with the help of ab initio modelling performed at the University of Cambridge, which also predicted new compounds stable under high pressure.
1 A calculation is said to be ab initio (or "from first principles") if it relies on basic and established laws of atomic physics without additional assumptions or special models.​

Figure 1 (left). An open diamond anvil cell. The conical diamond is visible in the center of the left disk. The metal sheet in the centre of the right disk serves as the joint surrounding the sample. © ESRF
Figure 2 (right).​ A very thin beam of synchrotron X-rays is sent to the sample compressed between two diamond anvils; the “diffraction pattern” of deflected X-rays is recorded on a bidimensional detector. A chemical reaction is detected by the appearance of a new diffraction pattern wich characterizes the products. © ESRF ​​

Figure 3. Laser-heating setup on the beamline dedicated to high-pressure X-ray diffraction (ID27). © ESRF/McBride​

Two oxides were synthesised thanks to the combination of these techniques: Xe2O5 and Xe3O2 (the latter was ab initio predicted) – Figure 4
 2 An atmophile element is a chemical element whose existence in nature in the solid state is marginal. Because of their volatility, atmophiles are very rare elements within the Earth and are mostly present in the atmosphere.

Sakura Pascarelli and Mohamed Mezouar, scientists in charge of the ESRF beamlines, explain « The structure of the new oxides has been solved thanks to the high performance of the European Synchrotron and the combination of several techniques: XRD (X-ray diffraction), at ID27, and XAS (X-ray absorption) at BM23, coupled to ab-initio modelling. The use of these complementary methods was essential for the unambiguous determination of long and short range order in these materials. It’s a complex scientific problem with many experimental challenges that have been overcome for the first time ».
These two compounds are predicted to be stable above about 0.5 million atmospheres, which is lower than previous estimates and indicates greater chemical reactivity in xenon oxides than previously thought. This is due to an unexpected role of d electrons in the bonding. This study also shows that xenon atoms adopt mixed valence states in the oxides stable at the lowest pressure (+4 to +6 oxidation state in Xe2O5, 0 to +4 in Xe3O2), yielding unusual chemical formulas for oxides. This may be a general trend in compounds formed under high compression.

As explained by Agnès Dewaele, main author, CEA, « There is increasing evidence that xenon becomes very reactive under high pressure. In addition to our study, calculations have recently predicted that xenon reacts with iron and nickel, the major components of the Earth’s deepest envelope, the core, under relevant conditions; this remains to be verified experimentally. We don’t know yet how this reactivity has affected the path of xenon atoms after the Earth’s accretion. However, these studies suggest that the geochemical definition of xenon, which is classified as a volatile and atmophile2, could be revised, as well as the use of xenon isotopes to date processes of the Earth’s differentiation3. »
 3 The differentiation of the Earth means the mechanisms that resulted in core formation, mantle, crusts, atmosphere, that is to say envelopes with very different chemical composition from a star of initially homogeneous composition.​
 3 The differentiation of the Earth means the mechanisms that resulted in core formation, mantle, crusts, atmosphere, that is to say envelopes with very different chemical composition from a star of initially homogeneous composition.​

Figure 4. Structures of the stable xenon oxides. a, Xe2O5 and b, Xe3O2. Xenon atoms are shown in blue shades and oxygen atoms in red shades. The oxygen atoms have an oxidation state of -2, and the darker shade of red indicates an oxygen atom that bonds only to one xenon atom. The oxidation states of the xenon atoms are indicated by different shades of blue. (Courtesy of Nicholas Worth, University of Cambridge)​

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