Scherb, T.; Fantin, A.; Checchia, S.; Stephan-Scherb, C.; Escolastico, S.; Franz, A.; Seeger, J.; Meulenberg, W.A.; d'Acapito, F.; Serra, J.M.: Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-delta mixed ionic electronic conductors. Journal of Applied Crystallography 53 (2020), p. 1471-1483
10.1107/S1600576720012698
Open Accesn Version

Abstract:
Mixed ionic electronic conducting ceramics Nd6yWO12 ( is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or H2S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6yWO12, the ionic/electronic transport mechanism in Nd6yWO12 is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12 and molybdenum-substituted Nd5.7W0.75Mo0.25O12 prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12 and Nd5.7W0.75Mo0.25O12. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit-cell parameter due to cation ordering. Mo replacesWat bothWyckoff sites 4a and 48h and is evenly distributed, in contrast with La6yWO12. X-ray absorption spectroscopy as a function of partial pressure pO2 in the near-edge regions excludes oxidation state changes of Nd (Nd3+) andW(W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd6yWO12 is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.