Synthesis and superconductivity of barium deficient type I silicon clathrate compounds, Ba8-xSi46

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 (T(c)s) ranging from 8.2 to 9.0 K. The composition of the compound having the highest T-c 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 T-c 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. (C) 2003 Elsevier Ltd. All rights reserved.