Tetragonal-orthorhombic phase transition and F-doping effects on the crystal structure in the iron-based high-Tc 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 similar to 160 K. This crystallographic transition was accompanied with an antiferromagnetic spin ordering at similar to 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)(delta+) and (FeAs)(delta-) 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.