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

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(R_F = 1.33 Ǻ) at O^2- (R_O = 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 SDWtransition. The superconductivity in the F-doped sample is confirmed by resistivity measurements as well as magnetic measurements. Superconducting transition temperature (T_c) 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 H_c2(0) is calculated from detailed up to 14 Tesla R(T)H measurements using the extrapolation method employing Ginzburg- Landau (GL) theory. Thus calculated H_c2(0) is found to be above 200 Tesla, which is second best to the High T_c cuprates.