(color online) Pressure–superconducting transition temperature (T_C) phase diagram for Tl_0.6Rb_0.4Fe_1.67Se₂, K_0.8Fe_1.7Se₂ and K_0.8Fe_1.78Se₂. The symbols represent the pressure–temperature conditions at which T_C values were observed from the resistive and alternating current susceptibility measurements; symbols with downward arrows represent the absence of superconductivity to the lowest temperature (4 K). All Tl_0.6Rb_0.4Fe_1.67Se₂, K_0.8Fe_1.7Se₂, and K_0.8Fe_1.78Se₂ samples show two superconducting regions (SC-I and SC-II) separated by a critical pressure at ∼ 10 GPa. NSC is the nonsuperconducting region at pressure > 13.2 GPa. The maximum T_C is found to be 48.7 K in K_0.8Fe_1.7Se₂ at a pressure of 12.5 GPa. At higher pressures > 13.2 GPa, the samples are nonsuperconducting. Error bars are one standard deviation. Reproduced with permission from Ref. [36].