Superconductivity in the (Ba_(1-x)Sr_x)_8Si_(46) clathrates (x<0.75): Experimental and ab initio investigation

摘要

Si clathrates (Ba_(1-x)Sr_x)_8 Si_(46) with x ≤ 0.75 have been successfully synthesized and their superconductivity studied both experimentally and by ab initio calculations. The linear decrease of the cubic lattice parameter with the Sr content shows the continuous shrinkage of the constituting (Ba/Sr) @ Si20 and (Ba/Sr) @ Si24 cages, while the critical superconducting transition temperature T_c decreases from 8 K to 4.1 K at 75% of substitution. The experimentally extrapolated value for the T_c of the hypothetical Sr_8Si_(46) is in good agreement with the calculated one of 3 K. The lower contribution of the Sr d-electrons to the density of states at the Fermi energy explains the observations.