Interplay of electronic and lattice degrees of freedom in A_{1−x}Fe_{2−y}Se_{2} superconductors under pressure

The local structure and electronic properties of Rb1-xFe2-ySe2 are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium K edges as a function of pressure. A combination of dispersive geometry and a nanodiamond anvil pressure cell has permitted us to reveal a steplike decrease in the Fe-Se bond distance at p similar or equal to 11 GPa and, at the same pressure, the local atomic structure of Rb1-xFe2-ySe2 becomes more ordered. Thus, at this pressure the phase separation might be suppressed, which has a large influence on the electronic structure, as evidenced by changes in the Fe K-edge prepeak. The present results provide compelling evidence that the reemerging superconductivity in A(1-x)Fe(2-y)Se(2) is closely related to the transition to a locally more ordered state, which has a different electronic structure.