Pressure-induced yttrium oxides with unconventional stoichiometries and novel properties

The oxygenic motifs (e.g., ${\mathrm{O}}^{2–}$, ${{\mathrm{O}}_2}^{2\text{–}}$ ${{\mathrm{O}}_2}^{2\text{–}}$, and ${{\mathrm{O}}_2}^{\text{–}}$) 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., ${\mathrm{YO}}_2$, ${\mathrm{Y}}_2{\mathrm{O}}_5$, and ${\mathrm{YO}}_3$) 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., ${\mathrm{O}}^{2\text{–}}$ in YO and ${\mathrm{Y}}_2{\mathrm{O}}_3$, the coexistence of ${\mathrm{O}}^{2\text{–}}$ and ${{\mathrm{O}}_2}^{2\text{–}}$ in ${\mathrm{YO}}_2$ and ${\mathrm{Y}}_2{\mathrm{O}}_5$, ${{\mathrm{O}}_2}^{2\text{–}}$ in $Pm\text{−}3$ ${\mathrm{YO}}_3$, and ${\mathrm{O}}^{2\text{–}}$ in Cmcm ${\mathrm{YO}}_3$), and are accompanied by a transition in the electronic structure from superconducting to metallic to semiconducting. Notably, the Cmcm symmetry ${\mathrm{YO}}_3$ 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 $\left(T_c\right)$ of 13.0 K at 25 GPa, becoming the TM monoxide with the highest known $T_c$. Our work will inspire future studies exploring the chemistry and properties of O-rich TM oxides at high pressure.

Figure 1

(a) Phase stabilities of the Y-O compounds with respect to elemental Y and ${\mathrm{O}}_2$ solids. The compounds sitting on the solid line (filled symbols) are thermodynamically stable, whereas the ones on the dotted lines (unfilled symbols) are metastable. The $P{6}_3/mmc$, $C2/m$, and Fddd phases for elemental Y solid [34, 70, 71], α and ζ phases of ${\mathrm{O}}_2$ with $C2/m$ symmetry [72, 73] are used to calculate the formation enthalpy per atom for each composition. (b) Pressure-composition phase diagram of the Y-O system in the range of 0–300 GPa.

Figure 2

Crystal structures of the predicted Y-O compounds and electron localization functions of two ${\mathrm{YO}}_3$ phases. (a) $Pm\text{−}3$ ${\mathrm{YO}}_3$ at 100 GPa, (b) Cmcm ${\mathrm{YO}}_3$ at 300 GPa, (c) $C2/m$ ${\mathrm{Y}}_2{\mathrm{O}}_5$ at 100 GPa, (d) ${\mathrm{YO}}_2$ with Cc symmetry at 50 GPa, (e) Pmmn ${\mathrm{YO}}_2$ at 170 GPa, (f) NaCl-type YO at 50 GPa, (g) CsCl-type YO at 200 GPa. In these structures, O atoms are represented by red and black spheres, and the blue spheres denote Y atoms. The calculated electron localization functions for (h) $Pm\text{−}3$ ${\mathrm{YO}}_3$ and (i) Cmcm ${\mathrm{YO}}_3$ with an isosurface of 0.6.

Figure 3

(a) Projected density of states (PDOS) of $Pm\text{−}3$ ${\mathrm{YO}}_3$ at 100 GPa. (b) Spin-dependent PDOS of ${\mathrm{YO}}_3$ with Cmcm symmetry at 300 GPa. The complete symmetry of spin-up and spin-down PDOS indicates that ${\mathrm{YO}}_3$ is nonmagnetic. (c) The band gap of $Pm\text{−}3$ ${\mathrm{YO}}_3$ and ${\mathrm{Y}}_2{\mathrm{O}}_5$ as a function of pressure from 120 to 220 GPa at the PBE level. (d) PDOS of Cmcm ${\mathrm{YO}}_3$ at 300 GPa. O1 corresponds to ${\mathrm{O}}^{2\text{–}}$, and O2 atoms comprise the ${{\mathrm{O}}_2}^{2\text{–}}$ motifs. (e) PDOS of Fm-$3m$ YO at 50 GPa. (f) The Eliashberg spectral function, ${\alpha }^{2}F\left(\omega \right)$, and integrated electron-phonon coupling parameter, λ(ω), of Fm-$3m$ YO at 25 GPa. (g) The electron-phonon coupling coefficient λ (red dashed line) and the logarithmic average phonon frequency ${\omega }_{\log }$ (blue dashed line) as a function of pressure for YO. The critical temperature, $T_c$, (orange line) as a function of pressure is shown in the inset. (h) The $T_c$ values of simple TM, TiO [45], and NbO [44], are obtained from the literature at ambient conditions. Other λ and $T_c$ are calculated by using the same accuracy as used for Fm-3m YO (B1) at 25 GPa.