Polyanion Conduction Mechanism in Solid Scandium Tungstate
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1177-Z09-20
Polyanion Conduction Mechanism in Solid Scandium Tungstate Zhou Yongkai1, Stefan N. Adams1, Arkady Neiman2 1 Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Singapore 2 Chemical Department, Ural State University, Ekaterinburg, Russian Federation ABSTRACT Scandium tungstate is investigated as a model material for solid electrolytes in which polyatomic anions, here WO42–, are mobile in the solid state. Simulations using structures with artificially induced WO42– vacancy, Frenkel defect and Schottky defects produced lower activation energy compared to the initially defect-free model. Simulations with Frenkel defect structures show low activation energy but the interstitial WO42– has initially a strong preference to return to the vacant tungstate site. The vacancy defect model reproduces the activation energy to the experimental conductivity studies more closely. Qualitative considerations support the idea that vacancies formed during the sample preparation are the most abundant mobile defect among the investigated cases. Nonstoichiometric samples with varying initial Sc2O3:WO3 ratios Sc2O3 xWO3, where x = 2.9, 3.0 and 3.1, are synthesized and characterized by XRD and impedance measurements, but a significant influence on the conductivity could not be confirmed experimentally.
INTRODUCTION In our recent work [1, 2], we could by combined computational, electrochemical and diffraction studies identify WO42- anions (and not as previously reported in the literature Sc3+ cations [3-11]) as the mobile species in the solid electrolyte scandium tungstate, Sc2(WO4)3. Conduction by polyatomic anions is otherwise rare in solid state ionics (except for diatomic OH). In our Molecular Dynamics (MD) simulations for initially defect-free Sc2(WO4)3 structure models the diffusion of WO42- groups follows a unique correlated mechanism, which is triggered by a rare high energy step: the generation of a tungstate Frenkel defect. Consequently the simulated activation energy is significantly higher than the experimentally observed value. It may be presumed that the ion transport in scandium tungstate is dependent on the concentration of defects that are produced during the high temperature synthesis of the material. In this work, simulations of structure models with artificially induced defects, e.g. WO42- vacancy, Frenkel defect and Schottky defect were investigated in details. To complement the computational study, nonstoichiometric samples with varying initial Sc2O3:WO3 ratios Sc2O3 - xWO3, where x = 2.9, 3.0 and 3.1, were synthesized and characterized by XRD and impedance measurements. EXPERIMENTAL DETAILS The same modified Universal forcefield as described in detail in our previous studies [1, 2] is used for all MD simulations in this work. Therein the default Lennard Jones–type nonbonded Sc-O interaction of UFF is replaced by a Morse-type interaction potential (see eq. 1) with bond energy D0 = 2.0 kcal / mol, equilibrium interaction distance r0 = 2.
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