Abstract
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 T-c. The above superconductors show a negative value of the Seebeck coefficient which indicates that electrons are the dominant charge carriers. (C) 2009 Elsevier B.V. All rights reserved.