Publication date: 10th April 2024
Donor-substituted (W, Mo) LaNbO4 materials have been reported to be good oxide ion conductors exhibiting excellent chemical stability with acceptable ionic conductivity, while their protonic conductivity is negligible [1]-[6]. However, most of these works were conducted at ambient conditions, and the conductivity behaviours at reducing conditions can be significantly different from non-reducing conditions.
In this work, LaNb0.84W0.16O4+δ and LaNb0.84Mo0.16O4+δ have been successfully synthesized via a solid-state reaction route. An abnormal conductivity behaviour at reducing conditions was found in electrochemical impedance spectroscopy measurements. At first, this abnormality is attributed to the emergence of the electronic conductivity. However, more evidence from different characterisations such as DC measurement, XPS and XAS suggest the simultaneous emergence of protonic conductivity later on. Protons originating from the protonic incorporation are indeed participated in the conductivity behaviour and worth further investigating.
In conclusion, the conductivity behaviour can be significantly different depending on the dopants. For W-substituted LaNbO4, the materials are stable at reducing conditions without the change of oxidation states of cations as well as the oxygen stoichiometry. Therefore, the electronic conductivity in W-substituted LaNbO4 is surprisingly low, and the conductivity enhancement is mainly, if not all, from the emergence of the protonic conductivity. In contrast, Mo-substituted LaNbO4 materials suffer from severe reductions and their conductivity behaviour is more complex than W-substituted LaNbO4 as there is a trade-off between the emergence of the electronic and protonic conductivity and the degradation of oxide ionic conductivity.
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