Effect of chemical pressure on spin density wave and superconductivity in undoped and 15% F-doped La1-yYyFeAsO compounds

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO(0.85)F(0.15) compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase in the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both T(SDW) and T(c) in 1111 compounds. Our data suggest that the structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.