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 42be the mobile species, which follow the interstitialcy diffusion mechanism. Our 3-section Tubandttype 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.

The conduction and transport numbers of charge carriers for composites in the systems MeWO 4 -WO 3 (Me = Ca, Sr, Ba) are studied as a function of the temperature and the activity of oxygen in a gas phase. The transport numbers are determined by an emf method and are estimated from dependences of conductance on the activity of oxygen in a gas phase. The deficiencies of the given procedure as applied to investigation of properties of composite phases are analyzed. The materials under study are classified with a class of metacomposites. The concentration intervals of the ionic, mixed, and electronic conductions are determined. The conduction of composites of compositions (100 -ı )å WO 4 · ı WO 3 is predominantly ionic at x ≤ 10 (Me = Ca), x ≤ 60 (Me = Sr), and x ≤ 45 (Me = Ba). The obtained data are explained in the framework of a model that represents a composite as a distributed matrix system where a film of surface phase MeW-s with a high mobility of oxygen ions plays the role of a connected matrix. It is presumed that the surface phase MeW-s possesses double-sided surface activity ( α å W -s ≤ , ) and a higher mobility with respect to å WO 4 and WO 3 . The discovered anomalies of dependences ( í ) and partial dependences , σ el ( T ) are explained by a change in the stoichiometry, morphology, and the degree of connectedness of surface phase MeW-s caused by with a change in the temperature and composition of composites.

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