Carrier-tuning of single-crystalline Ba8Ga16Ge30

Despite the fact that the only practical route to self-flux single crystal growth of Ba8Ga16Ge30 clathrates is through excess Ga flux, we have developed a method which results in p-type or n-type samples, depending on the relative Ge content in the starting mixture, and thus allows us to gain effective control over the charge carrier nature and density. An initial Ba:Ga:Ge ratio of 8:38:30 results in p-type crystals with average composition Ba8Ga16.6Ge29.4, while those grown from 8:38:34 initial ratio are n-type with average composition Ba8Ga15.9Ge30.1. More importantly, the thermopower S(T), resistivity p(T) and thermal conductivity K(T) show radically different behaviors. The p-type crystals show: (i) monotonically increasing S(T) up to S(300 K)approximate to + 200 mu V/K, (ii) semiconducting rho(T) (which decreases two orders of magnitude between 10 and 300K), and (iii) a glass-like low-temperature plateau in K(T). In contrast, the n-type crystals show: (i) monotonically decreasing S(T) down to S(300 K)approximate to -200 mu V/K, (ii) metallic p(T) in the 1-3 m Omega cm range, and (iii) a low-temperature crystalline peak in K(T). (c) 2006 Elsevier B.V. All rights reserved.