Compositionally controlled semimetal to superconducting transition in NaF doped LaOFeAs: Enhancement in Tc due to Na-doping

We report the synthesis of LaOFeAs based oxypnictide superconductors (La1−xNaxO1−xFxFeAs) using sodium fluoride as a fluorinating agent. NaF doping leads to a systematic decrease in both the a and c lattice parameters. Resistivity measurements show the onset resistivity transition temperature at 30.9 (±0.05) K and corresponding Meissner transition at 28 (±0.05) K in La0.85Na0.15O0.85F0.15FeAs which is highest in LaOFeAs type superconductors synthesized at ambient pressure. Further increase of NaF content in LaOFeAs leads to suppression of Tc. The above superconductors show a negative value of the Seebeck coefficient which indicates that electrons are the dominant charge carriers.