Undoped LaFePnO (Pn P, As) shows metallic resistivity and a tetragonal phase at room temperature. The electrical resistivity (ρ) of undoped LaFeAsO exhibits a sudden decrease around Tanom ∼160 K with decreasing the temperature and increases with decreasing the temperatures (T) below another threshold temperature (Tmin). It makes a contrast to undoped LaFePO, which shows neither of Tanom nor Tmin. The T anom corresponds to a crystallographic phase transition: undoped LaFeAsO shows an orthorhombic phase at temperatures < 160 K. A positive magnetoresistance (MR (ρ-ρ0) / ρ0) also appears below T anom. DFT calculations indicate that spin moment of LaFeAsO is 1.56 μB / Fe in an antiferromagnetic spin configuration in a tetragonal phase, while spin moment of LaFePO is almost quenched.
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