Superconducting VF with unusual delocalized electrons under high pressure

Superconductivity has been commonly found in vanadium (V) compounds, such as V-H, V-O, and V-N compounds, but with the exception of fluorides to date. Therefore, the study of V-F systems for superconductivity has drawn much attention. Here, we have systematically investigated the V-F compounds under high pressures by using first-principles swarm-intelligence structural search calculations. We found two new stoichiometries of VF and VF₂ to become stable under high pressure. Unlike the known V-F compounds, VF and VF₂ exhibit additional V-V interactions. In particular, there are delocalized electrons in the predicted VF structure, which gives rise to its conductivity. We further discussed the superconductive characteristics of VF by Bardeen-Cooper-Schrieffer (BCS) theory. The T_c value of the predicted C2/m VF is 0.85 K. The low-frequency phonon modes of VF play a predominant role in superconductivity. In addition, we proposed the high-pressure phases of VF₃, VF₄ and VF₅ and built the V-F high-pressure phase diagram. Our work provides useful information for deeply understanding the physical and chemical properties of V-F compounds under high pressure.