Enhanced Thermoelectric Performance by Inverted Thermal Conductivity Anisotropy in Layered <i>AE</i> ₂ ZnN ₂ ( <i>AE</i> = Ca, Sr, Ba) with Rigid Interlayer Bonds

Nitrides are thought to be candidates for eco-friendly thermoelectric (TE) materials; however, the TE performances of conventional nitrides are often limited by high lattice thermal conductivity (κlat). In this study, we focus on layered ternary nitride semiconductors, AE2ZnN2 (AE = Ca, Sr, Ba), as a novel class of TE nitrides, and we performed all first-principles calculations for electron and phonon transport to predict their ultimate TE performances. It is found that p-type AE2ZnN2 exhibits strong electronic anisotropy, i.e., the electronic conductivity (σ) is higher (corresponding to smaller carrier effective mass m*) in the in-plane direction and lower in the out-of-plane direction, as commonly expected for layered crystal structures due to their two-dimensional electronic structures. While layered crystals typically exhibit larger in-plane κlat than out-of-plane κlat, AE2ZnN2 shows unexpected, inverted κlat anisotropy. This inverted anisotropy comes from the coordination structures in AE2ZnN2, where AE–N layers are interconnected by strong −(AEN)-Zn-(AEN)– linear bonds along the out-of-plane direction, whereas in-plane chemical bonds are governed by weak interactions between pyramidal AE–N5 polyhedra. Among the AE2ZnN2 compounds, Ba2ZnN2 shows the lowest in-plane κlat, reaching 3.9 W/(m·K) at 300 K, owing to the heaviest Ba ion with the weakest chemical bonds. The in-plane Seebeck coefficients (S) of p-type Ba2ZnN2 are enhanced by the large in-plane density-of-state effective masses (mDOS*) arising from multiple hole pockets below the valence band maximum (VBM), yielding an in-plane ZT of 1.0 at 900 K. This high ZT is attained by the combination of a two-dimensional electronic structure, multivalley structures near the VBM, and inverted κlat anisotropy, which enables high σ and S together with low κlat in Ba2ZnN2. These results point to AE2ZnN2 compounds, particularly Ba2ZnN2, as potential environmentally friendly TE materials.