Here we show that a pressure of about 8 GPa suppresses both the vacancy order
and the insulating phase, and a further increase of the pressure to about 18
GPa induces a second transition or crossover. No superconductivity has been
found in compressed insulating 245 phase. The metallic phase in the
intermediate pressure range has a distinct behavior in the transport property,
which is also observed in the superconducting sample. We interpret this
intermediate metal as an orbital selective Mott phase (OSMP). Our results
suggest that the OSMP provides the physical pathway connecting the insulating
and superconducting phases of these iron selenide materials.Comment: 32 pages, 4 figure
