Pressure-induced yttrium oxides with unconventional stoichiometries and novel properties

The oxygenic motifs (e.g., O2-, O22- O22-, and O2-) that are present in compounds have a substantial effect on their electronic structure and behavior. Herein, first-principles swarm-intelligence crystal structural searches reveal that the reaction between Y and O under high pressure leads to the formation of novel compounds with unique properties. Several O-rich Y-O compounds (e.g., YO2, Y2O5, and YO3) emerge as being stable. It is shown that the oxygenic motifs found within the stable species depend upon the oxygen content and pressure (e.g., O2- in YO and Y2O3, the coexistence of O2- and O22- in YO2 and Y2O5, O22- in Pm-3 YO3, and O2- in Cmcm YO3), and are accompanied by a transition in the electronic structure from superconducting to metallic to semiconducting. Notably, the Cmcm symmetry YO3 phase, consisting of a 13-fold coordinated face-sharing polyhedron with 15 faces, can be classified as a transition metal (TM) superoxide. The long sought-after bulk yttrium monoxide (YO) is shown to become stable at high pressure. NaCl-type YO is superconducting with a critical temperature (Tc) of 13.0 K at 25 GPa, becoming the TM monoxide with the highest known Tc. Our work will inspire future studies exploring the chemistry and properties of O-rich TM oxides at high pressure.