Unravelling the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ mixed ionic electronic conductors. Issue 6 (25th October 2020)

• Record Type: Journal Article
• Authors: Scherb, Tobias; Fantin, Andrea; Checchia, Stefano; Stephan-Scherb, Christiane; Escolástico, Sonia; Franz, Alexandra; Seeger, Janka; Meulenberg, Wilhelm A.; d'Acapito, Francesco; Serra, José M.
• Is Part Of: Journal of applied crystallography. Volume 53:Issue 6(2020)
• Page Start: 1471
• Page End: 1483
• Publication Date: 2020-10-25
• Subjects: powder diffraction -- mixed conductors -- X‐ray absorption spectroscopy (XAS) -- Nd6−yWO12−δ; Crystallography -- Periodicals; 548.05
• DOI: 10.1107/S1600576720012698
• Languages: English
• ISSNs: 0021-8898

Abstract

Abstract : The crystal structures of non‐substituted and Mo‐substituted neodymium tungstates are described in detail through neutron diffraction and high‐resolution X‐ray diffraction. Combined X‐ray and neutron diffraction refinements and electron probe micro‐analysis were employed to locate Mo atoms in the crystal structure of Nd6− y W1− z Mo z O12−δ ( z = 0, 0.25), while X‐ray absorption spectroscopy in the near‐edge regions confirmed no changes in the oxidation states of Nd and W. Abstract : Mixed ionic electronic conducting ceramics Nd6− y WO12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2 O or H2 S. 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 La6− y WO12−δ, the ionic/electronic transport mechanism in Nd6− y WO12−δ 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 Nd5.8 WO12−δ and molybdenum‐substituted Nd5.7 W0.75 Mo0.25 O12−δ 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 Nd5.8 WO12−δ and Nd5.7 W0.75 Mo0.25 O12−δ . Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm 3 m and doubled unit‐cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4 a and 48 h and is evenly distributed, in contrast with La6− y WO12−δ . X‐ray absorption spectroscopy as a function of partial pressure p O2 in the near‐edge regions excludes oxidation state changes of Nd (Nd 3+ ) and W (W 6+ ) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo‐substituted Nd6− y WO12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.