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Recent experiments indicated that both layered Bi _2 O _2 Se and Bi _2 O _2 Te are promising thermoelectric materials with low thermal conductivities. However, theoretical study on the thermoelectric properties, especially the phonon transport properties, is rare. In order to understand the thermoelectric transport mechanism, we here investigate the electron and phonon transport properties by using the first-principles calculations combined with the Boltzmann transport theory. Our results indicate that both Bi _2 O _2 Se and Bi _2 O _2 Te are semiconductors with indirect energy gaps of 0.87 eV and 0.21 eV within spin–orbit coupling, respectively. Large Seebeck coefficient and power factor are found in the p -type than the n -type for both compounds. Low lattice thermal conductivities at room temperature are obtained, 1.14 W m ^−1 K ^−1 for Bi _2 O _2 Se and 0.58 W m ^−1 K ^−1 for Bi _2 O _2 Te, which are close to the experimental values. It is found that the low-frequency optical phonon branches with higher group velocity and longer lifetime also make a main contribution to the lattice thermal conductivity. Interestingly, the lattice thermal conductivity exhibits obvious anisotropy especially for Bi _2 O _2 Te. These results are helpful for the understanding and optimization of thermoelectric performance of layered Bi _2 O _2 Se and Bi _2 O _2 Te.

Minimizing the band splitting energy to approach orbital degeneracy has been shown as a route to improved thermoelectric performance. This represents an open opportunity in some promising layered materials where there is a separation of p orbitals at the valence band edge due to the crystal field splitting. In this work, using ab initio calculations and semiclassical Boltzmann transport theory, we try to figure out how orbital degeneracy influences the thermoelectric properties of layered transition-metal dichalcogenide ZrS _2 . We tune the splitting energy by applying compressive biaxial strain, and find out that near-degeneration at the ${\rm{\Gamma }}$ point can be achieved for around 3% strain. As expected, the enhanced density-of-states effective mass results in an increased power factor. Interestingly, we also find a marked decline in the lattice thermal conductivity due to the effect of strain on phonon velocities and scattering. The two effects synergetically enhance the figure of merit. Our results highlight the convenience of exploring this optimization route in layered thermoelectric materials with band structures similar to that of ZrS _2 .