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Unravelling the crystal structure of Nd 5.8 WO 12−δ and Nd 5.7 W 0.75 Mo 0.25 O 12−δ mixed ionic electronic conductors
Author(s) -
Scherb Tobias,
Fantin Andrea,
Checchia Stefano,
Stephan-Scherb Christiane,
Escolástico Sonia,
Franz Alexandra,
Seeger Janka,
Meulenberg Wilhelm A.,
d'Acapito Francesco,
Serra José M.
Publication year - 2020
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576720012698
Subject(s) - crystal structure , neutron diffraction , rietveld refinement , crystallography , extended x ray absorption fine structure , oxidation state , materials science , ionic bonding , crystal (programming language) , chemical stability , xanes , absorption spectroscopy , chemistry , analytical chemistry (journal) , spectroscopy , ion , metal , chromatography , metallurgy , programming language , physics , organic chemistry , quantum mechanics , computer science
Mixed ionic electronic conducting ceramics Nd 6− y WO 12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO 2 , CO, H 2 , H 2 O or H 2 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 La 6− y WO 12−δ , the ionic/electronic transport mechanism in Nd 6− y WO 12−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 Nd 5.8 WO 12−δ and molybdenum‐substituted Nd 5.7 W 0.75 Mo 0.25 O 12−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 Nd 5.8 WO 12−δ and Nd 5.7 W 0.75 Mo 0.25 O 12−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 La 6− y WO 12−δ . X‐ray absorption spectroscopy as a function of partial pressure p O 2 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 Nd 6− y WO 12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.