A 57Fe Mossbauer spectroscopy study of the 7 K superconductor LaFePO

A polycrystalline sample of superconducting LaFePO was prepared in a tin flux at 1123 K. The structure was determined from single-crystal data (ZrCuSiAs type, P4/nmm, a = 3.9610(1), c = 8.5158(2) angstrom, Z = 2), and the phase analysis was performed by the Rietveld method. LaFePO is Pauli-paramagnetic and becomes superconducting at 7 K after removing the ferromagnetic impurity phase Fe2P from the sample. Fe-57 Mossbauer spectroscopy measurements at 298, 77, 4.2. and 4 K show single signals at isomer shifts around 0.35 mm s(-1), subject to weak quadrupole splitting. At 4 K, a symmetric line broadening appears, resulting from a small transferred magnetic hyperfine field of 1.15(1) T and accompanied by an angle of 54.7(5)degrees between B-hf and V-zz, the main component of the electric field gradient tensor.