Simulation and conductivity studies of defects and ion transport in Sc2(WO4)3

The scandium tungstate [Sc2(WO4)3] material has been investigated using a combination of atomistic simulation and experimental techniques to probe the defect, dopant and ion conduction properties. The simulations reproduce the complex crystal structure with the calculated unit cell parameters within 0.8% of experimental data from diffraction studies. Frenkel and Schottky defect energies have been calculated, suggesting that such intrinsic defects are not significant within the structure. Vacancy migration (O2− or Sc3+) has been calculated to be unfavourable. Modelling the pathways of interstitial O2− and Sc3+ migration suggests that either mechanism is possible, although the process to create these defects is still not clear. Isovalent doping onto the Sc3+ site is shown to be energetically favourable for a range of ions (e.g., Ga3+, In3+, Yb3+). In terms of experimental doping studies, a range of strategies have been tried to introduce either vacancies or interstitial ions. These attempts were unsuccessful, showing that aliovalent doping on either cation site is extremely difficult, and so deviations from the ideal stoichiometry appear unfavourable. Isovalent doping was favourable for a range of ions (e.g., Ga3+, Al3+, In3+) which support the modelling results. Impedance data suggest that the main conduction mechanism is ionic rather than electronic in agreement with previous studies.