Multiple Valleys and Band Nesting Driven by Weak p‐d Orbital Hybridization for Excellent Thermoelectric Performance in Low‐Symmetry Cu 2 HfTe 3

Abstract Exploring new materials with intrinsically low lattice thermal conductivity is significant in advanced thermoelectric materials. The complex crystal structures of low‐symmetry materials induce strong anharmonic vibrations of atoms and local crystallographic distortion, consequently leading to potentially ultralow lattice thermal conductivity. However, the low energy valley number and degeneracy near the VBM originating from low symmetry make it challenging to obtain high thermoelectric performance. In this work, the thermoelectric properties of monoclinic Cu 2 HfTe 3 for the first time is have prepared and characterized. To address this controversy in low‐symmetry Cu 2 HfTe 3 , the weak Ti─Te bonding is introduced, which optimizes the band structure and softens the lattice simultaneously. Density functional theory (DFT) calculations indicates Ti‐alloying triggers multiple electronic valleys and band nesting, effectively increasing valley degeneracy and density‐of‐states effective mass. Consequently, the Seebeck coefficient ( S ) and power factor (PF) are significantly enhanced, elevating the average PF from 4.81 to 6.43 µW cm −1 K −2 . Furthermore, the formation of hierarchical bonding reduces phonon group velocities. Collectively, these effects yield a remarkable ZT peak of 1.11 at 873 K, highlighting the promise of Cu 2 HfTe 3 as a thermoelectric material. This work presents an effective strategy for band engineering and decoupling of electron‐phonon transport in low‐symmetry thermoelectric materials.