High Pressure Synthesis of Pr$_2$O$_5$ – A Unique Lanthanoid(IV) Oxide Peroxide

Reacting praseodymium(IV) oxide with oxygen at 27 GPa in a diamond anvil cell yielded the oxide peroxide Pr$_2$ $^{IV}$(O$_2$)O$_3$, 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. Pr$_2$($_2$)O$_3$ 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 Pr$^{4+}$. 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 Pr$^{4+}$-ions was ruled out by a control experiment in argon instead of oxygen, where no oxidation of oxide ions was observed.