Thermoelectric properties of pristine and pnictogen P- and As-doped LiMgBi crystal: an ab initio study

Abstract

Thermoelectric materials capable of efficiently converting heat to electrical energy are important for sustainable energy technologies. In this study, we investigate the thermoelectric properties of pristine LiMgBi and its pnictogen-substituted derivatives, of LiMgBi 0 . 75 P 0 . 25 and LiMgBi 0 . 75 As 0 . 25 , using the first-principles density-functional theory combined with Boltzmann transport equations and the Slack model. The calculated elastic constants satisfy the Born stability criteria, indicating mechanical stability of all the compounds. Pristine LiMgBi is found to be a narrow-band-gap semiconductor with a band gap of 0.49 eV, leading to a high Seebeck coefficient ( S ) and favourable balance between electrical and thermal transport. This results in a maximum figure of merit, ZT = 1.01 at 1000 K. In contrast, substitution of Bi with P or As induces a semiconductor-to-metal transition. This reduces the S and increases the electronic thermal conductivity due to enhanced carrier concentration. Although electrical conductivity improves in the doped systems, the simultaneous increase in total thermal conductivity leads to a reduced thermoelectric performance, with ZT values of 0.52 and 0.60 for LiMgBi 0 . 75 P 0 . 25 and LiMgBi 0 . 75 As 0 . 25 , respectively. These results indicate that pnictogen substitution disrupts the transport balance in LiMgBi, and alternative strategies that preserve the semiconducting nature may be more effective for improving thermoelectric performance. • Pristine LiMgBi shows the highest figure-of-merit (ZT) • P and As doping collapses the band gap, driving LiMgBi into a metallic regime • Doped systems exhibit reduced Seebeck coefficient and increased thermal conductivity • Lattice stiffening from pnictogens raises lattice thermal conductivity (K L ) • Pnictogen doping at 25% concentration degrades ZT