New oxypnictide superconductors: PrOFe<SUB>1−x</SUB>Co<SUB>x</SUB>As

Oxypnictides of the type PrOFe<SUB>1−x</SUB>Co<SUB>x</SUB>As (x≤0.3) were synthesized for the first time by the sealed tube method. All the compounds were found to be monophasic and crystallize in the tetragonal ZrCuSiAs type structure (space group=P4/nmm) and the lattice parameters (a and c) decrease with increase in cobalt content. Mössbauer measurements of the compounds indicate low spin Fe<SUP>2+</SUP> in tetrahedral coordination. Resistivity and magnetization studies reveal superconducting transitions in compounds with 'x'=0.05, 0.10 and 0.15, with maximum transition temperature (T<SUB>c</SUB>) at ~14 K in the compound with 'x'=0.1. The variation of resistivity with temperature under different magnetic field has been studied to estimate the upper critical field (H<SUB>c2</SUB>) (~50.2 T for the 'x'=0.1 composition). The Seebeck and Hall coefficient (R<SUB>H</SUB>) suggests electron type charge carriers in these compound and the charge carrier density increases with increase in Co-doping