Crystallographic and superconducting properties of filled skutterudite SrOs4P12

Abstract

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 ${\ensuremath{\lambda}}_{\text{ep}}\ensuremath{\sim}0.47$.