Reacting praseodymium(IV) oxide with oxygen at 27 GPa in a diamond anvil cell yielded the oxide peroxide Pr₂IV(O₂)O₃, which was characterized by single crystal X‐ray diffraction on multi‐grain samples, Raman spectroscopy and quantum theoretical calculations at various pressure points. The presence of tetravalent praseodymium ions is supported by electronic structure calculations, showing a band gap of ca. 1.2 eV, which is consistent with the anticipated chemical model of an ionic solid. Pr₂(O₂)O₃ thus far represents the most oxygen rich phase of any binary compound of a lanthanoid and oxygen and is the first example of a peroxide anion next to Pr⁴⁺. Additionally, these results demonstrate that instead of oxidizing the praseodymium ions past their +IV oxidation state, oxygen undergoes a comproportionation to form peroxide anions. Direct oxidation of the oxide anions by Pr⁴⁺‐ions was ruled out by a control experiment in argon instead of oxygen, where no oxidation of oxide ions was observed.
The reaction of PrO₂ and oxygen at 27 GPa in a diamond anvil cell afforded the unique oxide peroxide Pr₂(O₂)O₃ in which praseodymium is found in its rare oxidation state of +IV. Pr₂(O₂)O₃ constitutes the most oxygen rich binary phase of any lanthanoid and oxygen. It could be characterised by X-ray single crystal diffraction analysis at various pressure points, Raman spectroscopy and theoretical calculations.
