Superconductivity in (La,Y)FeAs(O,F) pncitides

The structural, magnetic and resistive properties of (La$_{1-x}$Y$_{x})$FeAs(O$_{0.85}$F$_{0.15})$ (x $\le $ 0.7) compounds prepared at normal pressure were investigated. Substituting La with Y decreases the ionic size at the rare earth site, determining a progressive decrease of both cell edges; as a result a notable increase of the superconductive transition temperature is observed. In particular T$_{C}$ increases with Y content, up to a maximum value of 39.8 K for x = 0.5, followed by the a slight decrease for x = 0.7 (T$_{C}$ = 35.9 K). The resistivity curve of the corresponding undoped compound, (La$_{0.5}$Y$_{0.5})$FeAsO, exhibits the typical bump related to the SDW onset; the first derivative curves for (La$_{0.5}$Y$_{0.5})$FeAsO and LaFeAsO are almost superposed around T$_{SDW}$.