High-pressure phases of Weyl semimetals NbP, NbAs, TaP and TaAs

The high pressure phase diagrams of TaAs family (NbP, NbAs, TaP and TaAs) are explored systematically using crystal structure search together with first-principles calculations. Our calculations show that NbAs and TaAs have similar phase diagrams and possess the same structural phase transition sequence of I41md $\rightarrow$ P-6m2 $\rightarrow$ P21/c $\rightarrow$ Pm-3m, with a bit different transition pressures. The phase transition sequence of NbP and TaP are somewhat different from that of NbAs and TaAs. In which new structures emerge, for instance, the Cmcm structure in NbP and the Pmmn structure in TaP. It is found interestingly that in the electronic structure of the high-pressure phase P-6m2-NbAs, there are Weyl points and triple degenerate points coexisted, similar to the high-pressure P-6m2-TaAs.