Superconductivity of Metal Deficient Silicon Clathrate Compounds, Ba8-Si46 (0 < x < 1.4)

Single crystals of the Ba-containing silicon clathrate superconductor Bay.ysSke were prepared using a high-pressure and high-temperature condition (3 GPa, 1300 °C). It crystallized in the cubic space group Pm~3n with a = 10.3141(7) A and Z = 1. There are two crystallographically different types of Ba sites, at the centers of Si dodecahederal (Ba@Si2o) and Si tetrakaidecahedral (Ba@Si) cages. On evacuation at 527 °C, a part of Ba atoms were removed from the Ba@Si2o sites. The superconducting transition temperature (Tc) decreased from 9.0 to 6.0 K with the decrease of the Ba content from 7.76 to 6.63 Ba/Sie. The Ba deficient sites and the deficiency were determined by the structural refinement in the single-crystal X-ray analyses.