Tetragonal–Orthorhombic Phase Transition and F-Doping Effects on the Crystal Structure in the Iron-Based High-TcSuperconductor LaFeAsO

Abstract

Crystal structure refinements of undoped and 14 at.% F-doped LaFeAsO ( T c =20 K) were performed over the temperature region of 25 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 ( P 4/ n m m ) to orthorhombic ( C m m a ) 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 ...