LAUSR

New thermoelectric materials, n‐type Bi6Cu2Se4O6 oxyselenides, composed of well‐known BiCuSeO and Bi2O2Se oxyselenides, are synthesized with a simple solid‐state reaction. Electrical transport properties, microstructures, and elastic properties are investigated with an emphasis on thermal transport properties. Similar to Bi2O2Se, it is found that the halogen‐doped Bi6Cu2Se4O6 possesses n‐type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi2O2Se, an extremely low thermal conductivity can be observed in Bi6Cu2Se4O6. To reveal the origin of low thermal conductivity, elastic properties, sound velocity, Grüneisen parameter, and Debye temperature are evaluated. Importantly, the calculated phonon mean free path of Bi6Cu2Se4O6 is comparable to the interlayer distance for BiO─CuSe and BiO─Se layers, which is ascribed to the strong interlayer phonon scattering. Contributing from the outstanding low thermal conductivity and improved electrical transport properties, the maximum ZT ≈0.15 at 823 K and ≈0.11 at 873K are realized in n‐type Bi6Cu2Se3.2Br0.8O6 and Bi6Cu2Se3.6Cl0.4O6, respectively, indicating the promising thermoelectric performance in n‐type Bi6Cu2Se4O6 oxyselenides.