Unravelling the crystal structure of NdWO and NdWMoO mixed ionic electronic conductors.

Mixed ionic electronic conducting ceramics Nd WO (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO, CO, H, HO or HS. 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 La WO, the ionic/electronic transport mechanism in Nd WO 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 NdWO and molybdenum-substituted NdWMoO 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 NdWO and NdWMoO. Both investigated samples crystallize in a defect fluorite crystal structure with space group 3 and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4 and 48 and is evenly distributed, in contrast with La WO. X-ray absorption spectroscopy as a function of partial pressure O in the near-edge regions excludes oxidation state changes of Nd (Nd) and W (W) in reducing conditions: the enhanced hydrogen permeation, ambipolar conduction, observed in Mo-substituted Nd WO is therefore explained by the higher Mo reducibility and the creation of additional - disordered - oxygen vacancies.