Figure 5. Phase diagram.

Phase diagram of the excitonic insulator phase in Ta₂NiSe₅ given by the transition temperature T_C as a function of S doping (right) and pressure and Te doping (left), corresponding to the semiconducting and the semimetallic regions in the predicted phase diagram respectively. The pressure effect on Ta₂Ni(Se_0.8Te_0.2)₅ is also shown in the left region with an offset of chemical pressure of 0.5 GPa (see text). The shaded area in the left region indicates the region for phase separation behaviour at high pressures (see text), likely originating from a first-order transition. The inset shows the S doping x dependence of the transport activation energy of resistivity at high temperatures (500–550 K), E_550 K. E_550 K gives a measure of the one electron gap E_G and indicates that E_G increases monotonically as a function of x from almost zero for x=0 to at least a few tenths of an eV for x=1.0. The sulfur substitution level x and pressure P may therefore be mapped onto the single band gap E_G in the high-temperature phase.