High-pressure synthesis and superconductivity of the Yb-substituted Ba8−xYbxSi46 clathrates

Summary form only given. Clathrate materials are extended Si, Ge and Sn cage-like solids with sp³-hybridized networks. As a new generation of energy conversion materials, clathrates have received increasing attention over the past few years. Clathrates with an open network are tailorable materials containing polyhedral cages, which can capture alkali metal, alkaline-earth metal and europium atoms. These atoms on the center of cage arrange in such a nanoscale array. Superconductivity mediated by the Si framework was demonstrated in Ba₈Si₄₆ clathrate. In contrast to carbon in C₆₀, silicon atoms do not form sp²-like networks in clathrates. Therefore, superconductivity of Si clathrate with sp³ networkshould be unique. The strong hybridization of the Ba state with the Si framework conduction band state plays a key role in the superconductivity. The introduction of rare earth elements as guest atoms into the cages would create a strongly correlated electron system through Kondo interaction of the local 4f states with the conduction electrons. However, such influence on superconductivity still has remained elusive to date. Here we report the incorporation of Yb as a guest atom into the clathrate crystal structure, and relative magnetic measurements and Raman spectrum for these Yb-doped samples .

The new clathrates Ba_8-xYb_xSi₄₆ were synthesized at a pressure of 3 GPa and temperature of 1113 K. X-ray powder diffraction data indicate that the cubic lattice parameter a decreases with Yb doping. EDS energy spectrum shows that the Yb peak is really evident above the background, which means a sizable amount of Yb incorporated into the clathrate phase. According to the result of EDS, the averaged chemical composition is Ba_7.92Yb_0.08Si₄₆, assuming that the number of Si atoms is fixed to 46. The temperature dependence of magnetization revealed that Ba- sub>7.5Yb_0.5Si₄₆ is superconductive below T_C = 7 .3 K and shows Curie-paramagnetic behavior at high temperature. A Curie-Weiss fit yielded the negative Curie-Weiss temperature and the effective magnetic moment is 5.59 μ_B, which is approximately equal to the Yb³⁺ free ions value . As temperature decreases, electrical resistance decreases and drops to zero at 7.3 K . The high temperature resistance significantly deviates from the Bloch-Grüneisen law associated with simple metallic conductivity. In particular, the broad curvature around 175 K resembles a behavior frequently found in strongly correlated electron systems induced by rare earth incorporation. Room-temperature Raman-scattering spectrum is reported for Ba_7.5Yb_0.5Si₄₆ and compared to that of Ba₈Si₄₆. The peaks at 279 and 488 cm^-1 for Ba₈Si₄₆show clear shift to 295 and 510 cm^-1 for the Yb doped Ba_7.5Yb_0.5Si₄₆. These shifts are caused by the incorporation of Yb into the cages. In contrast, the peaks at 125 and 358 cm^-1 do not show any shift .

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