1 Introduction

Xenon is a noble gas, chemically inert at ambient conditions. A few xenon fluorides have been found 139; 140; 141, with Xe atoms in the oxidation states +2, +4, or +6. Upon application of high pressure, molecular phase of insulating XeF$_2$ was found to transform into two- and three-dimensional extended solids and become metallic 142. Clathrate Xe-H solids were also observed at pressures above 4.8 GPa 143. Two xenon oxides (XeO$_3$, XeO$_4$) 144 are known at atmospheric pressure, but are unstable and decompose explosively above 25$^\circ $ C (XeO$_3$) and -40$^\circ $ C (XeO$_4$) 145. A crystalline XeO$_2$ phase with local square-planar XeO$_4$ geometry has recently been synthsized at ambient conditions 146.

Growing evidence shows that noble gases, especially Xe, may become reactive under pressure 147. The formation of stable xenon oxides and silicates could explain the missing xenon paradox, i.e. the observation that the amount of Xe in the Earth atmosphere is an order of magnitude less than what it would be if all Xe were degassed from the mantle into the atmosphere 148.One possibility to explain this defect is to assume that Xe is largely retained in the Earth’s mantle. In fact, a recent experiment discovered that xenon reacts with SiO$_2$ at high pressures and temperatures 149; 150. At the same time, recent theoretical investigation showed that no xenon carbides are stable at least up to the pressure of 200 GPa 151, and an experimental and theoretical high pressure work 152 found no tendency for xenon to form a metal alloy with iron or platinum.

Here we address possible stability of xenon oxides using quantum-mechanical calculations of their energetics. Since structures of stable xenon oxides are not experimentally known, we predict them using the recently developed evolutionary algorithm for crystal structure prediction 4; 23. We also analyze chemical bonding in these exotic compounds.