Prediction of high-pressure phases of Weyl semimetal NbAs and NbP.

As the first known Weyl semimetals, binary compounds including TaP, TaAs, NbAs, and NbP have received worldwide interest. This work explored the phase behaviours of NbAs and NbP under high pressure up to 200 GPa via first-principles calculations combined with intelligent particle swarm optimization. Upon compression, a new phase of NbAs with P-6m2 symmetry appeared above 23 GPa and remained stable until 38 GPa, whereupon a monoclinic structure with space group P2 /c became more energetically favourable. This lasted until 73 GPa, when a Pm-3m phase followed. Surprisingly, NbP underwent a single phase transition around 63.5 GPa to a new phase with Cmcm symmetry that was completely distinct from the structures shown by TaAs-class compounds such as NbAs and TaAs. All these newly pressure-stabilized structures were dynamically stable at both high and ambient pressure. Electronic band structure calculations revealed a transition from semimetal to metal under high pressure. This work is meaningful and fundamental for future studies and applications of TaAs-class Weyl semimetals under compression or extreme conditions.