Synthesis, Electric Transport, M ssbauer Spectroscopy, Specific Heat and Induction of Superconductivity in ReFeAsO (Re=La, Pr, Gd and Sm)

We report an easy and versatile route of synthesis for ReFeAsO (Re = La, Gd, Pr and Sm) by both single step and precursor based double step solid-state reaction routes. The studied ground state compounds of the new FeAs based superconductor family crystallize in tetragonal P4/nmm structure, with lattice parameters variation following their ionic radii. The resistivity measurements exhibit clearly the appearance of a metallic step below say 150-130 K, which is reminiscent of the SDW (Spin Density Wave) magnetic character. Also, It is observed that oxygen stoichiometric ReFeAsO is not formed and rather the same is slightly oxygen deficient. The M ssbauer spectroscopy (MS) carried out on some of them at 290 K and 90 K i.e. both above and below the SDW temperature exhibits the Fe ordered state at 90 K. Further the MS is proved to be the most efficient technique in picking up the FeAs based impurities in these materials. The heat capacity measurements on SmFeAsO, clearly demonstrates the SDW temperature with a hump in Cp(T) at around 130 K. Further Sm spins are ordered anti-ferro-magnetically (AFM) at 4.5 K. Superconductivity is also induced in SmFeAsO with Co substitution at Fe site with Tc = 14 K. The general behavior of the ReFeAsO is discussed and it is concluded that (a) they can be synthesized by an easy and versatile methods, (b) their ground state is not oxygen stoichiometric, but deficient and (c) MS is the most efficient technique to pick up the Fe based impurities in these materials, that is yet rampant in this new class of superconductors.