Superconductivity in LnFePO (Ln = La, Pr and Nd) single crystals

Single crystals of the compounds LaFePO, PrFePO and NdFePO have been prepared by means of a flux growth technique and studied by electrical resistivity, magnetic susceptibility and specific heat measurements. We have found that PrFePO and NdFePO display superconductivity with values of the superconducting critical temperature T-c of 3.2 and 3.1 K, respectively. The effect of annealing on the properties of LaFePO, PrFePO and NdFePO is also reported. The LnFePO (Ln = lanthanide) compounds are isostructural with the LnFeAsO(1-x)F(x) compounds that become superconducting with Tc values as high as 55K for Ln = Sm. A systematic comparison of the occurrence of superconductivity in the series LnFePO and LnFeAsO(1-x)F(x) points to a possible difference in the origin of the superconductivity in these two series of compounds.