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 K 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 T_c values as high as 55 K 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.
@article{arxiv.0812.0774,
title = {Superconductivity in LnFePO (Ln = La, Pr, and Nd) single crystals},
author = {R. E. Baumbach and J. J. Hamlin and L. Shu and D. A. Zocco and N. M. Crisosto and M. B. Maple},
journal= {arXiv preprint arXiv:0812.0774},
year = {2009}
}