Enhanced conductivity in Mo- and W-doped Bi ₄ Nd ₆ O ₁₅ -type structure compounds

This work presents the first investigation of the total conductivity in Nd-modified bismuth oxides with the Bi₄Nd₆O₁₅-type structure. Polycrystalline samples of two binary (Bi₂Nd₄O₉, BNO-I and Bi₄Nd₆O₁₅, and BNO-II) and two ternary Mo- and W-doped ((Bi₂O₃)0.4(Nd₂O₃)0.5(MoO₃)0.1, BNMO; (Bi₂O₃)0.4(Nd₂O₃)0.52(WO₃)0.08, BNWO) compounds were synthesized via solid-state reactions. The phase purity, crystal structure, and thermal stability were confirmed by X-ray diffraction, scanning electron microscopy, and differential scanning calorimetry. The total electrical conductivity was studied by impedance spectroscopy under dry and wet air atmospheres (300-900 °C). The incorporation of Mo and W significantly enhances the total conductivity by over one order of magnitude, reaching 0.04 S cm⁻¹ at 900 °C (BNWO). The absence of humidity dependence, negligible mass loss in the dehydroxylation regime (400-600 °C), and a near-zero oxygen partial pressure dependence collectively confirm predominantly oxygen-ionic conduction. We propose that the conductivity enhancement arises not from vacancy creation, but from dopant-induced disordering of the intrinsic oxygen vacancy system, which lowers the migration barrier - a mechanism distinct from that in Bi-rich δ-Bi₂O₃-based conductors. These findings establish Nd-rich bismuth oxides as a promising family of materials for defect-engineering strategies targeting high oxide-ion mobility.