Effect of chemical pressure on the local structure of La1−xSmxFeAsO system

We have studied the effect of chemical pressure on the local structure of La1-xSmxFeAsO oxypnictides. The combination of arsenic K-edge and rare-earth (RE = La, Sm) L-3-edge x-ray absorption spectra has permitted us to determine local bondlengths and unoccupied electronic states with varying chemical pressure controlled by the size difference of La and Sm. The Fe-As distance is hardly affected by the chemical pressure, and remains covalent as shown by As Kedge extended x-ray absorption fine structure (EXAFS) analysis. On the other hand, the As-RE and As-As distances show a gradual change. The changes indicate a larger effect of chemical pressure in the REO spacer layer and on the interlayer interactions, consistent with x-ray absorption near edge structure (XANES) data. The results suggest that interlayer atomic correlations are likely to have an important role in the physical properties of the La1-xSmxFeAsO parent phases of the superconducting oxypnictides.