Oxygen impurities reduce the metallization pressure of xenon

Xenon-oxygen mixtures, with low oxygen contents below 3 mol.$%$ O₂, have been studied under pressure by x-ray diffraction, Raman, and infrared absorption spectroscopies. The presence of O₂ at substitutional sites affects only marginally xenon equation of state---measured here up to 259 GPa---but impedes the face-centered cubic to hexagonal close packed martensitic phase transformation. On the other hand, a small amount of O₂ impurities drastically reduces the Xe metallization pressure: from 130 GPa for pure xenon to 49 GPa for Xe-0.6 mol.$%$ O₂. A significant softening of the O₂ intramolecular vibration frequency is observed in the same pressure range, which suggests a redistribution of the electronic density between O₂ and xenon.