Synthesis and physical properties of pure and f-doped REFeAsO (Re=Pr, Nd & Sm)

Abstract

We report a comparative study of crystal structure, electrical, and magnetic properties of the pure and F-doped superconducting REFeAsO (Re = Pr, Nd, and Sm) samples. The presence of superconductivity in these iso-structural compounds provided an opportunity to understand the doping mechanism in oxy-pnictide superconductor. Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. The Rietveld analysis of room temperature X-ray diffraction (XRD) data show that all the studied samples crystallize in single phase in a tetragonal structure with space group P4/nmm. The lattice parameters of the studied samples follow the well-known rare earth contraction. The decrease in c-parameter and the volume is indicative of successful substitution of F−1(RF = 1.33 Ǻ) at O2-(RO = 1.40 Ǻ) site. The ground state REFeAsO compounds shows a metallic step in resistivity measurements below say 150–130 K. This metallic step is attributed to structural and SDW transition. The superconductivity in the F-doped sample is confirmed by resistivity measurements as well as magnetic measurements. Superconducting transition temperature (Tc) is found to be at 51K, 48K and 38 K respectively for RE = Sm, Nd and Pr. The superconducting transition temperature of the F-doped samples increase with deceasing the ionic radii of the rare earth. The temperature dependent upper critical field Hc2(0) is calculated from detailed up to 14 Tesla R(T)H measurements using the extrapolation method employing Ginzburg- Landau (GL) theory. Thus calculated Hc2(0) is found to be above 200 Tesla, which is second best to the High Tc cuprates.