Crystallographic and superconducting properties of filled skutterudite SrOs4 P12

The crystallographic and physical properties of the recently discovered filled skutterudite superconductor ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$, 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 ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$ is 8.093(2) \\AA{} 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 ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$, 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 $\\ensuremath{\\gamma}$ and Einstein temperature $\\ensuremath{\\theta}{}_{\\mathrm{E}}$ of ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$ are deduced as $\\ensuremath{\\gamma}\\ensuremath{\\sim}26\\phantom{\\rule{4pt}{0ex}}\\mathrm{mJ}/{\\mathrm{mol}\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}}^{2}$ and $\\ensuremath{\\theta}{}_{\\mathrm{E}}\\ensuremath{\\sim}150$ K, respectively. A larger isotropic atomic displacement parameter ${U}_{\\mathrm{eq}}$ 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 ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$. ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$ exhibits two superconducting transitions at ${T}_{\\mathrm{c}1}=1.6$ K and ${T}_{\\mathrm{c}2}=1.0$ K. Specific-heat data indicate that the observed superconductivity is of bulk nature with approximate volume fractions of $27%$ and $38%$ for superconductivity at ${T}_{\\mathrm{c}1}$ and ${T}_{\\mathrm{c}2}$, respectively. The electrical resistivity under field and pressure as well as the specific heat under field indicate that ${T}_{\\mathrm{c}1}$ is sensitive to the magnetic field and ${T}_{\\mathrm{c}2}$ is sensitive to pressure. The results show that ${\\mathrm{SrOs}}_{4}{\\mathrm{P}}_{12}$ is an $s$-wave weakly coupled superconductor with an electron-phonon mass enhancement ${\\ensuremath{\\lambda}}_{\\text{ep}}\\ensuremath{\\sim}0.47$.