Effect of chemical pressure on spin density wave and superconductivity in undoped and 15% F-doped ${\text{La}}_{1-y}{\text{Y}}_y\text{FeAsO}$ compounds

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting ${\text{LaFeAsO}}_{0.85}{\text{F}}_{0.15}$ 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 $T_{\text{SDW}}$ 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.

Figure 1

(Color online) Rietveld refinement plot obtained for the samples with nominal composition $\left({\text{La}}_{0.70}{\text{Y}}_{0.30}\right)\text{FeAs}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$.

Figure 2

(Color online) Cell parameters of ${\text{La}}_{1-y}{\text{Y}}_y\text{FeAsO}$ (circle symbol) and ${\text{La}}_{1-y}{\text{Y}}_y{\text{FeAsO}}_{0.85}{\text{F}}_{0.15}$ (square symbol) as a function of yttrium nominal content.

Figure 3

(Color online) ${\text{La}}_{1-y}{\text{Y}}_y\text{FeAsO}$ normalized resistivity versus temperature behavior. The inset shows the shift toward low temperatures of the maximum of normalized $d\rho /dT$ with increasing the yttrium content.

Figure 4

(Color online) Temperature dependence of the resistivity for ${\text{La}}_{1-y}{\text{Y}}_y{\text{FeAsO}}_{0.85}{\text{F}}_{0.15}$ compounds.

Figure 5

Temperature evolution of $T_{\text{SDW}}$ and $T_c$ as a function of yttrium content for ${\text{La}}_{1-y}{\text{Y}}_y\text{FeAsO}$ and ${\text{La}}_{1-y}{\text{Y}}_y{\text{FeAsO}}_{0.85}{\text{F}}_{0.15}$ compounds.

Figure 6

(Color online) ZFC and FC magnetizations for ${\text{La}}_{1-y}{\text{Y}}_y{\text{FeAsO}}_{0.85}{\text{F}}_{0.15}$ compounds. The black arrows indicate the critical temperatures $T_c$ evaluated in the same way as the resistivity ones, that is, through the intersection of the lines corresponding to the normal state above $T_c$ and the steepest part of the magnetization in the superconducting one.

Figure 7

(Color online) (a) $T_c$ and (b) $T_{\text{SDW}}$ vs the $a$ axis, for the ${\text{La}}_{1-y}{\text{Y}}_y{\text{FeAsO}}_{0.75}{\text{F}}_{0.15}$ samples (full circle) in comparison with data collected from literature for 1111 compounds with different rare earths. (c) $T_c$ versus $T_{\text{SDW}}$ for La-Y series and other rare earths. The SC samples used for comparison are ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1-\delta }$ (Ref. 24), ${\text{CeFeAsO}}_{0.84}{\text{F}}_{0.16}$ (Ref. 25), ${\text{PrFeAsO}}_{0.84}{\text{F}}_{0.16}$ (Ref. 2), ${\text{NdFeAsO}}_{1-\delta }$ (Ref. 5), ${\text{SmFeAsO}}_{0.9}{\text{F}}_{0.1}$ (Ref. 3), ${\text{Gd}}_{0.8}{\text{Th}}_{0.2}\text{FeAsO}$ (Ref. 6), ${\text{TbFeAsO}}_{1-\delta }$ (Ref. 26), and ${\text{DyFeAsO}}_{1-\delta }$ (Ref. 26). $T_{\text{SDW}}$ data for undoped REFeAsO are, when different data are present for the same compound, the average between the maximum and minimum values reported: La [$T_{\text{SDW}}=138\text{K}$ (Ref. 19) and $T_{\text{SDW}}=143\text{K}$ (Ref. 27)]; Ce [$T_{\text{SDW}}=136\text{K}$ (Ref. 27) and $T_{\text{SDW}}=140\text{K}$ (Ref. 28)]; Pr [$T_{\text{SDW}}=127\text{K}$ (Ref. 21) and $T_{\text{SDW}}=139\text{K}$ (Ref. 27)]; Nd [$T_{\text{SDW}}=137\text{K}$ (Ref. 29) and $T_{\text{SDW}}=141\text{K}$ (Ref. 27)]; Sm [$T_{\text{SDW}}=131\text{K}$ (Ref. 26)]; Gd [$T_{\text{SDW}}=125\text{K}$ (Ref. 6)]; and Tb [$T_{\text{SDW}}=124\text{K}$ (Ref. 26)].