High pressure neutron and synchrotron X-ray diffraction studies of tetragonal LaFeAsO0.9F0.1

We studied polycrystalline tetragonal LaFeAsO0.9F0.1 up to 9 GPa using time-of-flight neutron and synchrotron X-ray diffraction. Consistent with previous studies, the layer-structured LaFeAsO0.9F0.1 has a bulk modulus of 74–79 GPa, with the c−axis being twice as compressible as the a−axis. The refined structural parameters under pressure show non-monotonic variation with kink points at 4 GPa, well correlating with a maximum superconducting temperature (Tc) of 43 K previously reported for LaFeAsO0.9F0.1. At this pressure, however, both the As–Fe–As bond angle and anion height from the Fe layers deviate substantially from the respective ideal values of 109.47° and 1.38 A for a regular FeAs4 tetrahedron. These findings indicate that the correlation between the maximum Tc and the geometry of conductive Fe–As layers is material dependent and may also be sensitive to atomic doping in the parent Fe-based superconductors.