High-Pressure 57Fe Mössbauer Spectroscopy of LaFeAsO

The electronic properties of an oxypnictide, LaFeAsO, pressurized in a diamond-anvil cell, were investigated by 57 Fe Mossbauer spectroscopy and electrical resistance measurements at pressures up to 24 and 35 GPa, respectively. The Neel temperature gradually decreased from ∼140 K at 0.1 MPa to ∼50 K at 20 GPa, and fell below 8 K or disappeared at 24 GPa. The hyperfine field at 8 K decreased from 5.3 T at 0.1 MPa to 2.2 T at 20 GPa. On the other hand, the onset of superconductivity occurred at ∼9 K at 2 GPa. The superconductivity peaked at ∼21 K at 12 GPa, and then began a perceptible decline, disappearing at ∼35 GPa. This suggests that suppression of antiferromagnetic order plays an important role in the emergence of pressure-induced superconductivity.