  We present a study concerning the partial substitution of yttrium at the
lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15
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 of 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
TSDW and Tc in 1111 compounds. Our data suggest that structural disorder
induced by the partial substitution in the La site or by doping could play an
important role as well.

Superconductivity