Computer modelling of ion migration in zirconia

Abstract

Defect structure and migration pathways of cations in cubic zirconia (ZrO₂) have been calculated using two computer modelling techniques. The first is based on the Mott–Littleton method, which considers defects to be embedded in an otherwise perfect crystal, and the second is the supercell approach, which allows finite defect concentrations to be modelled. Using the first approach, migration pathways for both intrinsic and dopant cations have been calculated. Activation energies ranging from 3.1 to 5.8 eV have been calculated assuming a vacancy mechanism. For highly charged dopants a curved pathway was found to be favoured over a straight pathway. The effect of stabilizer concentration on the properties of the system investigated has been analysed using the supercell method; 3 × 3 × 3 and 4 × 4 × 4 supercells containing 3–40 mol% calcia (CaO) or yttria (Y₂O₃) have been constructed assuming a random distribution of both dopant cations and oxygen vacancies. After relaxation the oxygen vacancies were found to be located adjacent to the zirconium cations in the CaO-doped system, while remaining randomly ordered in the Y₂O₃-doped system. Also cation vacancies were created, and after relaxation they were surrounded in all systems (CaO-stabilized ZrO₂ and Y₂O₃-stabilized ZrO₂) on average by 2.7 oxygen vacancies.

Acknowledgements

The authors are grateful to the Deutsche Forschungsgemeinschaft, the Deutscher Akademischer Austauschdienst and the British Council (through the joint ARC programme, Grant No. 1076) for substantial financial support. Gerhard Heide, Clausthal TU and Steven R. Ivins, Keele University, are gratefully acknowledged for their help to optimize the calculations.