Janus Effect of Phonons in Inducing Diffusons in Nanolayered XMg₂Bi₂ (X = Sr, Ba): Static–Dynamic Transition for Cations

The thermoelectric performance of XMg₂Bi₂ (X = Sr, Ba) materials is systematically investigated through integrated first-principles calculations, Boltzmann transport theory, and a two-channel model in this work. The temperature-activated static-dynamic transition in X (X = Sr, Ba) atoms vibrations induces a Janus effect of phonons, facilitating dual phonon transport regimes characterized by normal phonon and diffusons. The comparable ionicity between X²⁺ (X = Sr, Ba) and [Mg₂Bi₂]^2- layers disrupts the conventional Zintl-phase characteristics, leading to an atypically isotropic lattice thermal conductivity within their materials. Concurrently, charge-insulating X²⁺ (X = Sr, Ba) layers restrict out-of-plane carrier mobility, creating a distinctive two-dimensional (2D) electronic transport framework superimposed on three-dimensional (3D) phonon dynamics. By disentanglement of the cross-dimensional transport phenomena through a two-channel model and multicarrier scattering analysis, the XMg₂Bi₂ (X = Sr, Ba) materials achieve remarkable thermoelectric performance with the optimal figure of merits (<i>ZT</i>_max) of 1.5 (<i>p</i>-type) and 1.9 (<i>n</i>-type) at 600 K, respectively.