Intrinsic properties of stoichiometric LaFePO

DC and ac magnetization, resistivity, specific-heat, and neutron-diffraction data reveal that stoichiometric LaFePO is metallic and non-superconducting above T=0.35 K, with γ=12.5mJmol K². Neutron-diffraction data at room temperature and T=10 K are well described by the stoichiometric, tetragonal ZrCuSiAs structure, and show no signs of structural distortions or long-range magnetic ordering to an estimated detectability limit of 0.07μB/Fe. We propose a model based on the shape of the iron-pnictide tetrahedron that explains the differences between LaFePO and LaFeAsO, the parent compound of the recently discovered high-Tc oxyarsenides, which, in contrast, shows both structural and spin-density wave transitions.