Gallium composition dependence of crystallographic and thermoelectric properties in polycrystalline type-I Ba8GaxSi46-x (nominal x=14-18) clathrates prepared by combining arc melting and spark plasma sintering methods

The gallium composition dependence of crystallographic and thermoelectric properties in polycrystalline n-type Ba8GaxSi46-x (nominal x=14-18) compounds with the type-I clathrate structure is presented. Samples were prepared by combining arc melting and spark plasma sintering methods. Powder x-ray diffraction, Rietveld analysis, scanning electron microscopy, and energy-dispersive x-ray spectroscopy show that the solubility limit of gallium in the type-I clathrate phase is close to x=15, which is slightly higher than that for a single crystal. The carrier concentration at room temperature decreases from 2 x 10(21) cm(-3) to 4 x 10(20) cm(-3) as the Ga content x increases. The Seebeck coefficient, the electrical conductivity, and the thermal conductivity vary systematically with the carrier concentration when the Ga content x varies. The effective mass (2.0m(0)), the carrier mobility (10 cm(2) V-1 s(-1)), and the lattice thermal conductivity (1.1 W m(-1) K-1) are determined for the Ga content x=14.51. The dimensionless thermoelectric figure of merit ZT is about 0.55 at 900 K for the Ga content x=14.51. The calculation of ZT using the experimentally determined material parameters predicts ZT=0.8 (900 K) at the optimum carrier concentration of about 2 x 10(20) cm(-3). (C) 2012 Elsevier Inc. All rights reserved.