Superconductivity and crystal structure of the solid solutions of Ba8-δSi46-xGex (0≤x≤23) with Type I clathrate structure

The solid solutions of barium containing Type I clathrate, Ba8-deltaSi46-xGex (0 less than or equal to x less than or equal to 23) were prepared under high-pressure and high-temperature conditions of 3 GPa at 800degreesC. All the solid solutions showed superconductivity, and the transition temperature (T-c) decreased from 8.0 to 2.0 K as the germanium content increased from x = 0 to 23 in Ba8-deltaSi46-xGex. The single crystals with five different compositions were obtained and the structures, compositions, and site occupancies were determined from X-ray single-crystal analysis. A slight barium deficiency was observed at Ba1 (2a) sites for all the clathrates. The Ge atoms replaced the Si atoms at the Si3 (24k) site in the composition range of x < 8, and then at the Si2 (16i) site. The crystals had a slight deficiency in the covalent (Si, Ge) network and the deficiency increased with the increase of the Ge content. (C) 2003 Elsevier Inc. All rights reserved.