Decoupling thermoelectric parameters in novel ionic layered materials: a charged monolayer stabilization strategy for enhanced anisotropy

Abstract

The central challenge in advancing solid-state energy conversion is the discovery of materials capable of decoupling the intrinsically coupled thermoelectric parameters. While existing layered ionic materials, such as hexagonal Mg₃Sb₂, exhibit structural anisotropy, they often lack the requisite directional transport and suffer from imbalanced performance at different doping conditions. To address this limitation, we introduce a general design principle, namely the cation-stabilized monolayer network principle. This strategy conceptually segregates the crystal structure into tunable building blocks, which facilitates the rational engineering of new phases with customized anisotropy. Utilizing this principle, we computationally discovered three novel compound categories: tetragonal C^IC^IIA^V ternaries, tetragonal C^II₃A^V₂ binaries, and orthorhombic C^IC^II₂A^V₂ ternaries. A systematic high-throughput screening of 370 candidates identified 26 thermodynamically stable materials. We confirm the strong anisotropy in both electron and phonon transport achieved in these phases, exemplified by CsMgBi (a C^IC^IIA^V compound) which demonstrates an ultra-high anisotropy in the directional thermal conductivities of ρ_κ = 3.5. Moreover, the identification of tetragonal β-Mg₃Sb₂ (a C^II₃A^V₂ compound) as a superior p-type material addresses a significant performance deficit present in the prevailing hexagonal Mg₃Sb₂. This work validates the charged monolayer stabilization principle as a powerful approach for the rational discovery of novel, highly anisotropic, and high-performance ionic layered thermoelectrics.