Synthesis and superconductivity of barium deficient type I silicon clathrate compounds, Ba8−xSi46 Original Research Article

Barium deficient Type I silicon clathrate compounds, Ba8−xSi46, were prepared by high-pressure and high-temperature conditions. The single phases were obtained by the reactions under pressures higher than 3 GPa and temperatures higher than 800 K. They showed superconductivity with critical temperatures (Tcs) ranging from 8.2 to 9.0 K. The composition of the compound having the highest Tc of 9.0 K was determined to be Ba7.76Si46 from the single crystal X-ray analysis. On annealing at 800 K in vacuum, a part of Ba atoms were deintercalated from the silicon cages of the clathrate structure. The Tc decreased with the increase of the Ba deficiency down to 5.0 K for Ba6.62Si46. The density of states of Ba6Si46 was calculated, and compared with those of the defect-free Ba8Si46.