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

Abstract

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* = 2.3m ₀, Hall mobility μ = 7.4 cm² V⁻¹ s⁻¹, and the lattice thermal conductivity κ _L = 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, v _L = 6038 m/s and v _T = 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.