Pressure effect on iron-based superconductor LaFeAsO_1-xH_x: Peculiar response of 1111-type structure

A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO_1-xH_x as a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As-Fe-As angle of the FeAs₄ tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs₄ coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (T_c) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-T_c under pressure.