Pressure-induced increase of coordination number of xenon in the hypervalent ternary compounds of Xe-C-O

As a member of noble gas, xenon behaves complexly, especially under pressure. Hypervalent xenon compounds under extreme conditions offer the potential to probe the intrinsic physical and chemical properties of Xe and also give answers to the missing Xe paradox. Therefore, we theoretically explored the ternary phase diagrams of the Xe-C-O system under high pressures and identified two stable hypervalent compounds, $I\overline{4}\text{\ensuremath{-}}\mathrm{XeC}{\mathrm{O}}_{4}$ and $P{6}_{3}mc\text{\ensuremath{-}}{\mathrm{Xe}}_{2}\mathrm{C}{\mathrm{O}}_{4}$ at 100 and 200 GPa, respectively, where the coordination number of the Xe atom increases from 4 to 16 as pressure increases. The bonding mechanism of ${\mathrm{Xe}}_{2}\mathrm{C}{\mathrm{O}}_{4}$ is different from the three-center--four-electron bonds observed in $\mathrm{XeC}{\mathrm{O}}_{4}$, which is the pressure-induced $s{p}^{3}$ hybridization of $\mathrm{X}{\mathrm{e}}^{2+}$ with half-filled antibonding orbitals that overlap with the nonbonding orbitals of oxygen in the $\mathrm{C}{\mathrm{O}}_{4}$ unit. The ab initio molecular dynamics calculations show that these two compounds maintain solid states under the extreme conditions corresponding to the interior regions of the Earth. These findings reveal the bonding mechanism of unexpected hypervalent compounds involving the noble element Xe and potentially provide important support for the explanation of the storage of xenon in Earth.