Crystallographic and superconducting properties of filled skutterudite SrOs₄P₁₂

Abstract

The crystallographic and physical properties of the recently discovered filled skutterudite superconductor SrOs4P12, synthesized by a high-pressure and -temperature technique, are studied by measuring electrical resistivity, specific heat, and magnetization, and by performing electronic band calculations. X-ray powder diffraction with Rietveld refinement indicates that the lattice parameter of SrOs4P12 is 8.093(2) Å and that fractional coordinates of the P site are [0, 0.3607(9), 0.1450(9)], which is also confirmed by calculations based on density functional theory. The electrical resistivity indicates a metallic nature of SrOs4P12, which is consistent with the density of states at Fermi energy with 7.5 (states/eV)/f.u. deduced from the electronic band calculations. The Sommerfeld coefficient γ and Einstein temperature ΘE of SrOs4P12 are deduced as γ∼26mJ/molK2 and ΘE∼150 K, respectively. A larger isotropic atomic displacement parameter Ueq of Sr compared to other atomic species as obtained from a Rietveld analysis and a specific heat anomaly around 30 K refers to anharmonic lattice vibrations of Sr in SrOs4P12. SrOs4P12 exhibits two superconducting transitions at Tc1=1.6 K and Tc2=1.0 K. Specific-heat data indicate that the observed superconductivity is of bulk nature with approximate volume fractions of 27% and 38% for superconductivity at Tc1 and Tc2, respectively. The electrical resistivity under field and pressure as well as the specific heat under field indicate that Tc1 is sensitive to the magnetic field and Tc2 is sensitive to pressure. The results show that Tc2 is an s-wave weakly coupled superconductor with an electron-phonon mass enhancement λep∼0.47.