Role of p–p and d–p orbital interactions in the electronic structure and phonon localization driving high thermoelectric performance
Abstract
In materials engineering, orbital interaction is essential for modulating carrier transport properties in order to tune the intrinsic coupling of transport coefficients and improve thermoelectric performance. In this regard, we demonstrated an in-depth theoretical understanding of orbital interactions (d–p and p–p coupling) present in transition metal dichalcogenides (TMDs) having an octahedral symmetry through focusing on two prototypical superlattices (SLs), HfSe2–ZrSe2 and HfSe2–SnSe2. Employing first-principles density functional theory (DFT) with the Boltzmann transport formalism, we elucidated the decisive role of specific orbital interaction, particularly between metallic (Hf/Zr) d-orbitals and chalcogen (Se) p-orbitals, as well as p–p coupling between metallic Sn and chalcogen Se, governing the electronic structure and lattice dynamics. In the HfSe2–ZrSe2 SL, strong d–p orbital interactions give rise to localized optical phonons originating from ZrSe2 sublayers, which facilitate enhancement of three-phonon scattering and lead to a pronounced suppression of lattice thermal conductivity (κlatt). Simultaneously, these d–p interactions between Zr and Se atoms, inducing resonant electronic states near the conduction band edge, increase the density of states and thereby enhance the Seebeck coefficient. Conversely, in the HfSe2–SnSe2 SL, the p–p orbital overlap instigates linear band dispersion near the Fermi level, resulting in high carrier mobility and a significantly improved electronic transport response. Consequently, both SLs demonstrate exceptional TE performance, with the HfSe2–ZrSe2 system exhibiting peak ZT values of ∼3.6 (n-type) and ∼1.7 (p-type) at 800 K and the HfSe2–SnSe2 system achieving ZT values of ∼3.5 (n-type) and ∼1.1 (p-type). Our work not only deepens the fundamental understanding of orbital interaction into the intricate electrical and thermal transport mechanisms governing superlattices across different orbital coupling, but also provides an exceptional example for the targeted design and discovery of energy materials with tailored thermal and thermoelectric properties.