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

► Crystallographic properties of Ba8GaxSi46−x clathrates are characterized. ► Arc melting and spark plasma sintering process enables increase of Ga content. ► We elucidate the Ga composition dependence of thermoelectric properties. ► Thermoelectric figure of merit ZT is improved due to the increased Ga content. ► Calculation predicts a potential ZT=0.8 at 900 K at optimized carrier concentration.