The oxidation states of elements in pure and Ca-doped BiCuSeO thermoelectric oxides

Bi_1-xCa_xCuSeO (x = 0-0.3) was synthesized at 650°C in an air-tight system flowing with pure argon. The Ca doping resulted in an increase in the thermoelectric figure of merit (ZT) as the consequence of increased carrier concentration. X-ray photoelectron spectroscopy (XPS) was carried out to check the oxidation states in Bi_1-xCa_xCuSeO. The results indicated that in addition to the expected Bi³⁺ and Cu¹⁺, there existed Bi²⁺ and Cu²⁺ in the undoped BiCuSeO, whereas in the Ca-doped BiCuSeO, Bi⁴⁺, Cu³⁺ and Cu²⁺ were observed. The Ca dopant was confirmed to be in the 2+ oxidation state. Two broad peaks centered at 54.22 and 58.59 eV were recorded in the vicinity around the binding energy of Se 3d. The former is often observed in the Se-containing intermetallics while the latter is often found in the Se-containing oxides, indicating that along with the expected Se-Cu bonding, a bonding between Se and O may also exist. Based on the XPS results, the charge compensation mechanisms were proposed for Bi_1-xCa_xCuSeO, which may shed some light on the origins of charge carriers. BiCuSeO based oxides have recently be discovered to have a large ZT comparable to the best alloys currently in use, because of the large Seebeck coefficient and small thermal conductivity. However, their electrical conductivity is lower compared to the best thermoelectrics. This work may provide some hints for the further improvement of ZT in BiCuSeO based oxides.