Charge Transport by Polyatomic Anion Diffusion in Sc₂(WO₄)₃

Discussions about the nature of the charge carriers in the scandium tungstate and other isostructural tungstates and molybdates have persisted in the literature since a variety of experimental indications pointed toward trivalent cations as the mobile species. Here variations of the structure over a wide temperature range are analyzed by XRD and computational methods, demonstrating that the negative thermal expansion persists throughout the range of 11−1300 K. Over a limited temperature range (<500 K) molecular dynamics simulations with an optimized forcefield reproduce this negative thermal expansion. Likewise, charge transport is monitored both experimentally by impedance spectroscopy and Tubandt experiments and computationally based on the molecular dynamics simulation trajectories. Extended isothermal−isobaric simulations suggest a complex migration of polyatomic tungstate anions as the energetically most favorable transport mechanism in Sc 2 (WO 4 ) 3 . A bond valence analysis depicts possible diffusion pathways for WO 4 2−, although there is no indication of a pathway for Sc 3+ . The hopping mechanism of tungstate ions from one equilibrium site to another one follows the instantaneous diffusion pathways. A long-range transport still requires the rare formation of WO 4 2− Frenkel defects limiting the accuracy of the simulated absolute conductivity. Both MD simulations and bond valence analysis suggest WO 4 2− be the mobile species, which follow the interstitialcy diffusion mechanism. Our 3-section Tubandt-type experiments qualitatively show that the transfer of W occurs in the form of a negatively charged complex. This should be the first example of polyatomic diffusion species and opens a new field in the search for new ionic conductors.