Pressure-Induced Phase Transitions and Correlation between Structure and Superconductivity in Iron-Based Superconductor Ce(O_0.84F_0.16)FeAs

High-pressure angle-dispersive X-ray diffraction experiments on iron-based superconductor Ce­(O_0.84F_0.16)­FeAs were performed up to 54.9 GPa at room temperature. A tetragonal to tetragonal isostructural phase transition starts at about 13.9 GPa, and a new high-pressure phase has been found above 33.8 GPa. At pressures above 19.9 GPa, Ce­(O_0.84F_0.16)­FeAs completely transforms to a high-pressure tetragonal phase, which remains in the same tetragonal structure with a larger a-axis and smaller c-axis than those of the low-pressure tetragonal phase. The structure analysis shows a discontinuity in the pressure dependences of the Fe–As and Ce–(O, F) bond distances, as well as the As–Fe–As and Ce–(O, F)–Ce bond angles in the transition region, which correlates with the change in T_c of this compound upon compression. The isostructural phase transition in Ce­(O_0.84F_0.16)­FeAs leads to a drastic drop in the superconducting transition temperature T_c and restricts the superconductivity at low temperature. For the 1111-type iron-based superconductors, the structure evolution and following superconductivity changes under compression are related to the radius of lanthanide cations in the charge reservoir layer.