Weakening the polarity of chemical bonds to improve carrier mobility for realizing high thermoelectric performance in N-typed Mg3(Sb,Bi)2

Abstract

N-type Mg₃(Sb,Bi)₂ Zintl compounds have emerged as promising candidates for high-temperature energy applications due to their exceptional thermoelectric performance, making them pivotal in the development of sustainable energy technologies. Despite recent advancements, these materials suffer from low carrier mobility caused by polar covalent bonds, which degrade electrical conductivity and overall thermoelectric efficiency. In this study, we introduce beryllium, a bivalent homologous group element, as a cationic dopant to substitute for Mg in Mg_3.2Sb_1.5Bi_0.49Te_0.01. This substitution weakens the polarity of the chemical bonds, significantly enhancing carrier mobility from ∼62 to ∼138 cm² V⁻¹ s⁻¹. Theoretical analysis using the single parabolic band model confirms that the effective mass decreases with increasing Be doping content. First-principles calculations further reveal that Be doping leads to stronger charge localization due to their higher electronegativity and shifts the Fermi level into the conduction band and narrows the band gap, strengthening the n-type semiconducting properties. This optimization yields an impressive power factor of ∼2022 μW m⁻¹ K⁻² at 523 K in Mg_3.12Be_0.08Sb_1.5Bi_0.49Te_0.01, owing to the improved carrier mobility. Furthermore, the Be atoms as point defects induces significant lattice distortions and strains, effectively suppressing lattice thermal conductivity to ∼0.38 W m⁻¹ K⁻¹ at 573 K. Consequently, we achieve a remarkable ZT of 1.54 at 673 K and a high average ZT of 1.17 in n-type Mg_3.14Be_0.06Sb_1.5Bi_0.49Te_0.01. Our work offers new strategies to enhance the thermoelectric properties of n-type Mg₃(Sb,Bi)₂ materials, advancing high-temperature sustainable energy technologies.