Effect of Al substitution on the thermoelectric properties of the type VIII clathrate Ba ₈Ga ₁₆Sn ₃₀

Single-crystal samples of the type VIII clathrate Ba ₈Ga _16-x Al _x Sn ₃₀ (0 ≤ x ≤ 12) were prepared by the Sn-flux method, and the structural and electrical properties were studied from 300 K to 600 K. The lattice parameter increases linearly as x is increased from 0 to 10.5, which is the solubility limit of Al. For all samples, the electrical conductivity σ decreases monotonically as the temperature is increased. σ(T = 300 K) increases from 1.88 × 10 ⁴ S/m for x = 0 to 3.03 × 10 ⁴ S/m for x = 2, and then gradually decreases to 2.4 × 10 ⁴ S/m with further increase of x to 8. The increase of σ for Al-substituted samples is attributed to enhancement of carrier mobility. The Seebeck coefficients of samples with 0 ≤ x ≤ 8 are negative with large values, and the absolute values increase from 240 μV/K to 320 μV/K as the temperature increases from 300 K to 600 K. At 300 K, the effective mass m*/m ₀ is in the range from 0.53 to 0.67, and the samples with x = 6 and x = 8 have a rather low thermal conductivity of 0.72 W/mK and 0.78 W/mK, respectively. ZT reaches 1.2 at 500 K for x = 6.