Crystallographic, thermoelectric, and mechanical properties of polycrystalline type-I Ba8Al16Si30-based clathrates

Crystallographic, thermoelectric, and mechanical properties of polycrystalline Ba8Al16Si30-based samples with type-I clathrate structure prepared by combining arc melting and spark plasma sintering methods were investigated. The major phase of the samples was a type-I clathrate with an actual Al/Si ratio of ~15/31, strongly suggesting that framework deficiency was absent or was present in very low concentration in the samples. The Hall carrier concentration n of the samples was approximately 1 × 1021 cm−3, which is lower than the values reported so far for the Ba8Al16Si30 system. Other important material parameters of the samples were as follows: the density-of-states effective mass m* = 2.3m0, Hall mobility μ = 7.4 cm2 V−1 s−1, and the lattice thermal conductivity κL = 1.2 W m−1 K−1. The thermoelectric figure of merit ZT reached approximately 0.4 (900 K) for a sample with n = 9.7 × 1020 cm−3. Simulation using the experimentally determined values of material parameters showed that ZT reached values >0.5 if the carrier concentration is optimized at about 3 × 1020 cm−3. Young’s, shear, and bulk moduli were estimated to be approximately 98, 39, and 117 GPa, respectively, and Poisson’s ratio was found to be 0.25 from the longitudinal and transverse velocities of sound, vL = 6038 m/s and vT = 3503 m/s, respectively, for a sample with ZT = 0.4. The coefficient of thermal expansion (CTE) ranged from approximately 8 × 10−6 K−1 to 10 × 10−6 K−1 (330–690 K), which is smaller than the values reported for Ba8Ga16Ge30 and Sr8Ga16Ge30 clathrates.