Anomalous lattice thermal conductivity in SrLiX (X = P, As) Zintl phases via anion engineering

Thermoelectric materials show promise for converting waste heat into electricity, addressing energy sustainability challenges. The ZrBeSi-type Zintl phase compounds, featuring a layered structure, intrinsically exhibit low lattice thermal conductivity (\kappa_L). Motivated by this feature, we employed first-principles calculations combined with Boltzmann transport theory to systematically investigate the thermoelectric transport properties of SrLiX (X = \rm P, As). SrLiP exhibits a \kappa_L of 0.98 W\cdotm^-1\cdotK^-1 at 900 K, which is 34% lower than that of SrLiAs (1.48 W\cdotm^-1\cdotK^-1), despite its smaller average atomic mass. This anomalous reduction originates from the broader phonon bandwidth in SrLiP, which is a direct consequence of anion regulation, and this broader bandwidth substantially expands the three-phonon scattering phase space and enhances phonon scattering rates. Meanwhile, n-type SrLiAs and SrLiP reach power factors of \(1.14 \rm mW\cdot\rm m^-1\cdot\rm K^-2\) and \(1.33 \rm mW\cdot\rm m^-1\cdot\rm K^-2\) at 900 K, respectively, benefiting from multiple band convergence near the Fermi level that yields a valley degeneracy. Ultimately, our calculations yield ZT values of 0.73 for n-type SrLiP and 0.48 for n-type SrLiAs at 900 K. Our results demonstrate that anion engineering successfully decouples thermal and electrical transport, simultaneously suppressing lattice thermal conductivity while maintaining favorable electrical transport properties. This offers a viable strategy for designing high-performance thermoelectrics.