Abstract
We have successfully synthesized the fluoride-arsenide compounds Ca1−xRExFeAsF (RE=Nd, Pr; x=0,0.6). The X-ray powder diffraction confirmed that the main phases of our samples are Ca1−xRExFeAsF with the ZrCuSiAs structure. By measuring resistivity, superconductivity was observed at 56 K in Nd-doped and 52 K in Pr-doped samples with x=0.6. Bulk superconductivity was also proved by the DC magnetization measurements in both samples. Hall effect measurements revealed hole-like charge carriers in the parent compound CaFeAsF with a clear resistivity anomaly below 118 K, while the Hall coefficient RH in the normal state is negative for the superconducting samples Ca0.4Nd0.6FeAsF and Ca0.4Pr0.6FeAsF. This indicates that the rare-earth element doping introduces electrons into CaFeAsF, which induces high-temperature superconductivity.