Na-induced n-to-p-type conversion and thermoelectric properties of the layered oxysulfide LaOBiPb_1-xNa_xS₃

The electrical transport and thermoelectric properties of Na-doped LaOBiPbS₃ are systematically investigated. Na substitution at the Pb site acts as an effective acceptor, converting the carrier type from n-type to p-type, as evidenced by a well-defined compensation point near x = 0.04 where the resistivity peaks and the Hall coefficient changes sign. The optimal p-type composition LaOBiPb_0.94Na_0.06S₃ delivers a power factor of 5.45 µW cm^-1 K^-2 and a maximum ZT of 0.33 at 773 K, reaching 3.2 and 2.5 times the PF and ZT values of the parent compound, respectively. For the heavily doped x = 0.08 sample, a simultaneous anomaly in resistivity and Seebeck co-efficient emerges above 600 K, ascribed to the interplay of bipolar conduction and local lattice strain induced by excessive Na incorporation. These findings establish Na doping as an effective strategy for realizing p-type conductivity and enhancing thermoelectric performance in this layered oxysulfide system.