Improvement of Low-Temperature zT in a Mg₃Sb₂–Mg₃Bi₂ Solid Solution via Mg-Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n-Type Mg₃Sb₂-based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain-boundary effect has been limited due to loss of Mg, which hinders a sample's n-type dopability. A Mg-vapor anneal processing step that grows a sample's grain size and preserves its n-type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon-scattering-dominated T^−3/2 trend over a large temperature range, further supporting the conclusion that the temperature-activated mobility in Mg₃Sb₂-based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm² V⁻¹ s⁻¹ in annealed 800 °C sintered Mg_{3 + δ}Sb_1.49Bi_0.5Te_0.01, the highest ever reported for Mg₃Sb₂-based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n-type Bi₂Te₃) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.