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 Ba₈Ga_16.6Ge_29.4, while those grown from 8:38:34 initial ratio are n-type with average composition Ba₈Ga_15.9Ge_30.1. More importantly, the thermopower S(T), resistivity ρ(T) and thermal conductivity κ(T) show radically different behaviors. The p-type crystals show: (i) monotonically increasing S(T) up to S(300 K)≈+200 μV/K, (ii) semiconducting ρ(T) (which decreases two orders of magnitude between 10 and 300 K), and (iii) a glass-like low-temperature plateau in κ(T). In contrast, the n-type crystals show: (i) monotonically decreasing S(T) down to S(300 K)≈-200 μV/K, (ii) metallic ρ(T) in the 1-3 mΩ cm range, and (iii) a low-temperature crystalline peak in κ(T).