A computational study of doped olivine structured Cd₂GeO₄: local defect trapping of interstitial oxide ions

Computational modelling techniques have been employed to investigate defects and ionic conductivity in Cd₂GeO₄. We show due to highly unfavourable intrinsic defect formation energies the ionic conducting ability of pristine Cd₂GeO₄ is extremely limited. The modelling results suggest trivalent doping on the Cd site as a viable means of promoting the formation of the oxygen interstitial defects. However, the defect cluster calculations for the first time explicitly suggest a strong association of the oxide defects to the dopant cations and tetrahedral units. Defect clustering is a complicated phenomenon and therefore not trivial to assess. In this study the trapping energies are explicitly quantified. The trends are further confirmed by molecular dynamic simulations. Despite this, the calculated diffusion coefficients do suggest an enhanced oxide ion mobility in the doped system compared to the pristine Cd₂GeO₄.