Unconventional stable stoichiometry of vanadium peroxide.

Peroxides have attracted considerable attention due to their intriguing electronic properties and diverse applications. However, only a few transition metal peroxides have been known thus far, limiting the variety of peroxide examples. Here, we demonstrate the stabilization of peroxides in the O-rich V-O system through first-principles calculations coupled with a swarm-intelligence structure search. As well as reproducing the known stoichiometries of VO, VO, VO, and VO, two hitherto unknown VO and VO stoichiometries are predicted to be thermodynamically stable at megabar pressures. VO has the highest oxygen content among the known peroxides to date. More interestingly, its electronic band gap increases with pressure, originating from the pressure-induced decrease of O-O bonding length in the peroxide group. V-rich VO exhibits superconductivity, becoming the first example in the V-O system. Our work not only unravels the unusual vanadium peroxide, but also provides further insight into the diverse electronic properties of vanadium oxides under high pressure.