Carrier control in Ba8Ga16Ge30 single crystals

Carrier control of p and n in type-I crystal clathrates Ba8Ga16Ge30's (BGG's) is studied for their single crystals. The polycrystalline precursors with regulated various stoichiometric compositions are employed for single crystal growth under excess amount Ga as a flux. Hall coefficient measurements prove that both p and n type BGG single crystals are produced by modifying the relative ratios between the endohedral alkaline-earth atoms and the Ga atoms residing in the framework. ICP analyses together with Rietveld refinement based on the high resolution synchrotron X-ray diffraction suggests that the occupancy of Ba is less than one, and this explains the reason why p type can be possible in BGGs in contrast to the fact only n type is obtained in Sr8Ga16Ge30's. Superconductivity is not observed when the high doping levels are searched. (C) 2010 Elsevier B.V. All rights reserved.