High-entropy strategy in designing La2Bi4Cu2O6Se4 superlattice thermoelectric material with band convergence and low thermal conductivity

Designing novel van der Waals layered materials with low thermal conductivity and large power factor is important for the development of layered thermoelectric materials. Therefore, the novel van der Waals intercalated compound La₂Bi₄Cu₂O₆Se₄, which is constructed by alternately stacking LaCuSeO and Bi₂O₂Se units along the c-axis in a 1:2 ratio, has designed for thermoelectric materials. The unique intercalated strategy leads to the four-band convergence at the valence band maximum, and the combination of multiple heavy band and light band, which significantly enhances the p-type doping power factor. The lattice thermal conductivities in La₂Bi₄Cu₂O₆Se₄ and LaCuSeO compounds are accurately calculated by considering the coherence contributions of the anharmonic phonon reformulations and the off-diagonal term of the heat flux operator. The weak bond property of the Cu d-Se p bonding causes phonon softening, reducing the lattice thermal conductivity. The intercalated Bi atom has stereochemically active lone-pair electrons, which causes acoustic-optical coupling and produces strong fourth acoustic-optical phonon scattering, suppressing low-frequency phonon transport. The carrier relaxation time is rationalized by considering multiple carrier scattering mechanisms. The p-type doping La₂Bi₄Cu₂O₆Se₄ achieves an average ZT of 2.3 at 700 K, and an optimal ZT of 2.7 along the in-plane direction. Our current work not only reveals the origin of the strong phonon scattering and large power factor of La₂Bi₄Cu₂O₆Se₄ compound, but also provides theoretical guidance for the design of La-based layered oxides for thermoelectric applications.