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Unravelling the crystal structure of Nd_5.8WO_12-? and Nd_5.7W_0.75Mo_0.25O_12-? mixed ionic electronic conductors.

ABSTRACT: Mixed ionic electronic conducting ceramics Nd_6-y WO_12-? (? 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 La_6-y WO_12-?, the ionic/electronic transport mechanism in Nd_6-y WO_12-? 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_5.8WO_12-? and molybdenum-substituted Nd_5.7W_0.75Mo_0.25O_12-? 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_5.8WO_12-? and Nd_5.7W_0.75Mo_0.25O_12-?. 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 4a and 48h and is evenly distributed, in contrast with La_6-y WO_12-?. X-ray absorption spectroscopy as a function of partial pressure pO₂ in the near-edge regions excludes oxidation state changes of Nd (Nd³⁺) and W (W⁶⁺) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd_6-y WO_12-? is therefore explained by the higher Mo reducibility and the creation of additional - disordered - oxygen vacancies.

SUBMITTER: Scherb T

PROVIDER: S-EPMC7710492 | biostudies-literature | 2020 Dec

REPOSITORIES: biostudies-literature

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Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ mixed ionic electronic conductors.

Scherb Tobias T Fantin Andrea A Checchia Stefano S Stephan-Scherb Christiane C Escolástico Sonia S Franz Alexandra A Seeger Janka J Meulenberg Wilhelm A WA d'Acapito Francesco F Serra José M JM

Journal of applied crystallography 20201026 Pt 6

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, 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 La6-y WO12-δ ...[more]

PMID: 33304224

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Project description:The series BaIn1-x Fe x O2.5+δ, x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba2In2O5 (BaInO2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba2Fe2O5 (BaFeO2.5) crystallizes in a complex monoclinic structure, P21/c, showing seven Fe3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm3̅m, with long-range disorder on the small cation and oxygen sites. 57Fe Mössbauer studies indicate a mixed valency, Fe4+/Fe3+, for both the air-fired and argon-fired samples. The increased Fe3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm3̅m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ'(max) and spin glass behavior, while for x = 0.75, χ'(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe3+-Fe3+ antiferromagnetic exchange and ferromagnetic Fe3+-Fe4+ exchange. The PDF and 57Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm3̅m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe3+ sites are largely fourfold-coordinated and Fe4+ sites are fivefold- or sixfold-coordinated.

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Unravelling the crystal structure of Nd_5.8WO_12-? and Nd_5.7W_0.75Mo_0.25O_12-? mixed ionic electronic conductors.

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Unravelling the crystal structure of Nd<sub>5.8</sub>WO<sub>12-δ</sub> and Nd<sub>5.7</sub>W<sub>0.75</sub>Mo<sub>0.25</sub>O<sub>12-δ</sub> mixed ionic electronic conductors.

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Unravelling the crystal structure of Nd5.8WO12-? and Nd5.7W0.75Mo0.25O12-? mixed ionic electronic conductors.

Publications

Unravelling the crystal structure of Nd<sub>5.8</sub>WO<sub>12-δ</sub> and Nd<sub>5.7</sub>W<sub>0.75</sub>Mo<sub>0.25</sub>O<sub>12-δ</sub> mixed ionic electronic conductors.

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OmicsDI is part of the ELIXIR infrastructure

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Unravelling the crystal structure of Nd_5.8WO_12-? and Nd_5.7W_0.75Mo_0.25O_12-? mixed ionic electronic conductors.