Superconducting VF with unusual delocalized electrons under high pressure

Abstract

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.