Predicted lithium oxide compounds and superconducting low-pressure ${\mathrm{LiO}}_4$

We study the stability of Li-O compounds as a function of pressure, with rich phase diagram, diverse properties, and fundamental chemical interest in mind. Using the ab initio evolutionary algorithm USPEX, we predict the stability of compounds ${\mathrm{LiO}}_4, {\mathrm{Li}}_5{\mathrm{O}}_3$, and ${\mathrm{Li}}_6\mathrm{O}$ under pressure. Unexpectedly, ${\mathrm{LiO}}_2$ will decompose to ${\mathrm{Li}}_2{\mathrm{O}}_2+{\mathrm{LiO}}_4$ in the pressure range 6–18 GPa. ${\mathrm{LiO}}_4$, formed at the pressure of just 6 GPa, can be seen as $\varepsilon -{\mathrm{O}}_8$ accepting two electrons from two Li atoms. This phase is superconducting, with $T_c$ up to 12.2 K at 10 GPa. This is remarkable, because elemental oxygen becomes superconducting at much higher pressure (96 GPa) and has much lower $T_c$ ($<0.6$ K), and suggests that chemical alloying with other elements has the potential of not only decreasing metallization pressure, but also of increasing $T_c$. Since $\varepsilon -{\mathrm{O}}_8$ is called red oxygen, ${\mathrm{LiO}}_4$ can be identified as “lithium red-oxide”, and is distinct from superoxide. Additionally, ${\mathrm{Li}}_5{\mathrm{O}}_3$ is stable at pressures above 70 GPa and can be represented as a hybrid structure $4{\mathrm{Li}}_2\mathrm{O}·{\mathrm{Li}}_2{\mathrm{O}}_2$, and electride suboxide ${\mathrm{Li}}_6\mathrm{O}$ is stable above 62 GPa.

Figure 1

Structures of ${\mathrm{LiO}}_2$. (a) Pnnm, (b) $P4/mbm$. (c) Enthalpies relative to $P4/mbm {\mathrm{LiO}}_2$ as a function of pressure. The discontinuous curve of the Pnnm phase means a spontaneous phase transition from high spin to low spin.

Figure 2

Thermodynamics of the Li-O system: (a), (b) Predicted convex hulls of the Li-O system at 0, 10, 25, 50, and 100 GPa. For clarity, we show separately (a) Li-rich and (b) O-rich parts of the system. In (c), we show the pressure-composition phase diagram of the Li-O system.

Figure 3

Conventional cell of ${\mathrm{LiO}}_4$ in (a) [010] and (b) [001] directions. (c) Band structure of ${\mathrm{LiO}}_4$ at 10 GPa. The red solid lines are calculated with HSE hybrid functional and the blue dotted lines are from the PBE functional. Bands formed by $2p$ electrons are indicated. Fermi level is set to zero. (d) Evolution of the band structure from 0 to 10 GPa (computed using PBE functional). (e) Phonon spectrum of ${\mathrm{LiO}}_4$ at 0 and 10 GPa.

Figure 4

Critical temperature of superconductivity ($T_c$) and DOS at the Fermi level as a function of pressure.

Figure 5

Crystal structures of (a) ${\mathrm{Li}}_6\mathrm{O}$ and (b) ${\mathrm{Li}}_5{\mathrm{O}}_3$ and their phonon spectra at 80 GPa [(c) and (d), respectively]. In (a) we also show a green isosurface with $\mathrm{ELF}=0.9$. In (b), the structure is shown in the [010] and [001] directions with peroxide ions colored green and the oxide ion colored red.