Predicted lithium oxide compounds and superconducting low-pressure LiO4

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 LiO4, Li5O3, and Li6O under pressure. Unexpectedly, LiO2 will decompose to Li2O2 + LiO4 in the pressure range 6-18 GPa. LiO4, formed at the pressure of just 6 GPa, can be seen as epsilon-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 epsilon-O-8 is called red oxygen, LiO4 can be identified as lithium red-oxide, and is distinct from superoxide. Additionally, Li5O3 is stable at pressures above 70 GPa and can be represented as a hybrid structure 4Li(2)O center dot Li2O2, and electride suboxide Li6O is stable above 62 GPa.