PHYSICAL REVIEW B 103, 085139 (2021) Crystallographic and superconducting properties of filled skutterudite SrOs 4 P 12 Yukihiro Kawamura , 1 Shingo Deminami, 1 Keiki Takeda, 1 Toshihiro Kuzuya, 1 Leonid Salamakha, 2 Herwig Michor , 2 Ernst Bauer, 2 Jun Gouchi, 3 Yoshiya Uwatoko, 3 Tatsuya Kawae, 4 and Chihiro Sekine 1 1 Muroran Institute of Technology, Mizumoto 27-1, Muroran, Hokkaido 050-8585, Japan 2 Institute of Solid State Physics, TU Wien, Wiedner Hauptstraße 8-10, A-1040 Vienna, Austria 3 Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan 4 Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan (Received 20 November 2020; revised 13 February 2021; accepted 15 February 2021; published 25 February 2021) The crystallographic and physical properties of the recently discovered filled skutterudite superconductor SrOs 4 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 SrOs 4 P 12 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 SrOs 4 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 γ and Einstein temperature θ E of SrOs 4 P 12 are deduced as γ ∼ 26 mJ/mol K 2 and θ E ∼ 150 K, respectively. A larger isotropic atomic displacement parameter U 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 SrOs 4 P 12 . SrOs 4 P 12 exhibits two superconducting transitions at T c1 = 1.6 K and T c2 = 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 c1 and T c2 , respectively. The electrical resistivity under field and pressure as well as the specific heat under field indicate that T c1 is sensitive to the magnetic field and T c2 is sensitive to pressure. The results show that SrOs 4 P 12 is an s-wave weakly coupled superconductor with an electron-phonon mass enhancement λ ep ∼ 0.47. DOI: 10.1103/PhysRevB.103.085139 I. INTRODUCTION Filled skutterudite compounds have the general chemi- cal formula AT 4 X 12 (where A denotes alkaline-earth metal and rare-earth metal, T denotes transition metal, and X de- notes pnictogen) [1]. The A site, which is located at the center of a regular icosahedron oversized cage, often ex- hibits a characteristic motion in an anharmonic potential, called rattling. Because of this rattling, several skutterudites exhibit high thermoelectric performance [2] as well as spe- cific superconducting features. In filled skutterudites, some unique superconductors have been explored. Among them are the heavy fermion superconductor PrOs 4 Sb 12 [3], the multiple gap superconductor LaRu 4 As 12 [4–7], and RFe 4 P 12 (R = Y,La) with a positive pressure dependence of the superconducting transition temperature [8–10]. The ongo- ing discovery of unique superconducting properties among filled skutterudites motivates the search for new supercon- ductors among these compounds. Although many physical properties of rare-earth-based filled skutterudites RT 4 X 12 (R = rare-earth metal) have been investigated, much fewer of the alkaline-earth-based filled skutterudite AT 4 X 12 (A = Ca, Sr, and Ba) are known and reported. This is because it is difficult to synthesize these compounds at ambient pressure. Some alkaline-earth-based filled skutterudite su- perconductors have been reported, and they can only be synthesized at high pressure and high temperature (HPHT), such as CaOs 4 P 12 , BaOs 4 P 12 , SrOs 4 As 12 , and BaOs 4 As 12 [11–14]. However, they have not yet been investigated with respect to both physical properties and electronic band calculations. Recently, we succeeded in synthesizing SrOs 4 P 12 by using a HPHT procedure. In this article, the superconducting as well as normal state properties of the alkaline-earth filled skutterudite SrOs 4 P 12 are investigated by means of electrical resistivity, specific heat, magnetization measurements, and by electronic band-structure calculations. SrOs 4 P 12 shows metallic behavior with a moderate rattling effect. It exhibits superconductivity below the critical temperatures T c1 = 1.6K and T c2 = 1.0 K, where 27% and 38% of the volume fraction becomes superconducting. Superconductivity at T c1 and T c2 is easily suppressed by applying magnetic fields and pressure, respectively. The Ginzburg-Landau parameter κ , the penetra- tion depth λ, the coherence length ξ , and the electron phonon coupling constant λ ep are deduced as 29, 273 nm, 33 nm, and 0.47, respectively; refer to T c2 . II. METHODOLOGIES Polycrystalline SrOs 4 P 12 was synthesized at 4 GPa and 1320 K; this pressure and temperature were maintained for 90 min using a Kawai type-two stage multi-anvil high- pressure apparatus under an argon atmosphere with an O 2 2469-9950/2021/103(8)/085139(9) 085139-1 ©2021 American Physical Society