Abstract
The silicon clathrate superconductor is uncommon as its structure is dominated by strong Si-Si covalent bonds, rather than the metallic bond, that are more typical of traditional superconductors. To understand the influence of large magnetic moment of Eu on superconductivity for type-I clathrates, a series of samples with the chemical formula Ba8−xEuxAl6Si40 (x = 0, 0.5, 1, and 2) were synthesised in which Eu occupied Ba sites in cage center. With the increase of Eu content, the cubic lattice parameter decreases monotonically signifying continuous shrinkage of the constituting (Ba/Eu)@Si20 and (Ba/Eu)@Si24 cages. The temperature dependence of magnetization at low temperature revealed that Ba8Al6Si40 is superconductive with transition temperature at TC = 5.6 K. The substitution of Eu for Ba results in a strong superconductivity suppression; Eu-doping largely decreases the superconducting volume and transition temperature TC. Eu atoms enter the clathrate lattice and their magnetic moments break paired electrons. The Curie-Weiss temperatures were observed at 3.9, 6.6, and 10.9 K, respectively, for samples with x = 0.5, 1.0, and 2.0. Such ferromagnetic interaction of Eu can destroy superconductivity.
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