Tetragonal-orthorhombic phase transition and F-doping effects on the crystal structure in the iron-based high-T_c superconductor LaFeAsO

Crystal structure refinements of undoped and 14 atomic% F-doped LaFeAsO (T_c = 20 K) were performed over the temperature region of 25 K to 300 K by synchrotron X-ray diffraction. The electronic structures of the crystals were also examined using density functional theory (DFT). Undoped LaFeAsO was observed to undergo a crystallographic transition from the tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ∼160 K. This crystallographic transition was accompanied with an antiferromagnetic spin ordering at ∼140 K. The DFT calculation results suggest that both transitions were caused by the "Cooperative Jahn-Teller effect". In contrast, an F-doped sample maintains the non-magnetic tetragonal phase down to 25 K. F-ion substitutions for the O sites changed the bond length and angle in the distorted LaO tetrahedron, whereas weaker geometric effects were observed in the FeAs layer. Further, the F-doping induced large decreases in the c-axis length and the La-As distance, suggesting an enhancement of the charge polarizations in the (LaO)^δ+ and (FeAs)^δ- layers as a result of electron transfer between two layers. The enhanced polarization and Fe-Fe interaction in the FeAs layer are likely to suppress the transitions leading to the emergence of a superconducting phase.