Electrical and Thermal Transport Properties of n ‐type Bi 6 Cu 2 Se 4 O 6 (2BiCuSeO + 2Bi 2 O 2 Se)

New thermoelectric materials, n‐ type Bi 6 Cu 2 Se 4 O 6 oxyselenides, composed of well‐known BiCuSeO and Bi 2 O 2 Se 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 Bi 2 O 2 Se, it is found that the halogen‐doped Bi 6 Cu 2 Se 4 O 6 possesses n ‐type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi 2 O 2 Se, an extremely low thermal conductivity can be observed in Bi 6 Cu 2 Se 4 O 6 . 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 Bi 6 Cu 2 Se 4 O 6 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 Bi 6 Cu 2 Se 3.2 Br 0.8 O 6 and Bi 6 Cu 2 Se 3.6 Cl 0.4 O 6 , respectively, indicating the promising thermoelectric performance in n ‐type Bi 6 Cu 2 Se 4 O 6 oxyselenides.