Direct observation of the influence of the As-Fe-As angle on theTcof superconducting SmFeAsO1−xFx

The electrical resistivity, crystalline structure, and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO${}_{0.81}$F${}_{0.19}$ have been studied up to pressures \ensuremath{\sim}20 GPa. The correlation between the pressure dependence of the superconducting transition temperature (${T}_{c}$) and crystallographic parameters on the same sample shows clearly that a regular FeAs${}_{4}$ tetrahedron maximizes ${T}_{c}$ through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.