Polyatomic Anion Conduction in Sc2(WO4)3 Type Structures

Abstract

Various studies recently reported that the ion transport in scandium tungstate type structures A2(MO4)3, where A represents trivalent Sc3+, In3+, Al3+ or rare earth cations RE3+, and M is usually W6+ or Mo6+, is due to the mobility of the trivalent cations in these compounds. In this study, Sc2(WO4)3 is reinvestigated by a combination of computational, electrochemical and X-ray diffraction approaches to clarify the mobile species and the ion transport mechanism in this structure. The motion of the effective charge carriers in solid state ionics can be visualized by Molecular Dynamics (MD) simulations if proper initial structure and forcefield are known. Rietveld refinements of XRD data for the temperature range of 11K to 1300K provide not only initial structures for MD simulations but also yardsticks for fine-tuning the forcefield. By successfully reproducing the negative thermal expansion for the temperature range T<500K and an orthorhombic to monoclinic structural phase transition under external pressure in MD simulations, we have designed and verified a valid forcefield to predict the mobile species in Sc2(WO4)3. Using the same forcefield, a series of correlated WO42- migrations are observed in extended isothermal-isobaric MD simulations with both defect-free starting models and starting models with artificially induced defects. Simulations with pre-existing defects produce lower activation energy compared to the defect-free model. Models with WO42- vacancy and Schottky defects both reproduce the activation energy to the experimental conductivity studies closely. Even though the Schottky defect model includes the creation of Sc3+ vacancies, no Sc3+ diffusion is observed in our simulations. Dynamic BV analysis demonstrates that the hopping of WO42- between equilibrium sites in the MD trajectory follows the instantaneous diffusion pathways (pathways of low BV mismatch in the instantaneous structure model), except for the initial defect formation step in defect-free structure models. The anion conduction mechanism is also qualitatively substantiated by Tubandt-type electrolysis experiments, which find mass transfer from the anode pellet to the cathode pellet. Conduction by polyatomic anions is rarely observed in solid state ionics. Scandium tungstate is thus the prototype of a WO42- polyanion conductor.