Computer simulation of the oxygen mobility in CaMnO_3-x

Abstract

The perovskite-type metal oxide CaMnO_3-x is known to accommodate substantial amounts of oxygen vacancies. High-resolution electron microscope investigations give evidence for ordering of the vacancies, i.e. well-defined structures in the compositional range of CaMnO_2.5 < CaMnO_3-x < CaMnO₃. Within this range the metal cation positions do not change, i.e. perovskitic framework is conserved while a remarkably high oxygen anion mobility is recorded. In addition, the electronic and magnetic structure, and thus the physical properties, depend directly on the oxygen stoichiometry.

This contribution focusses on the oxygen mobility in different CaMnO_3-x phases exhibiting oxygen vacancy ordered structures, i.e. CaMnO_3.0, CaMnO_2.80, CaMnO_2.75, CaMnO_2.66, CaMnO_2.55 and CaMnO_2.50. In these compounds the formal oxidation state of manganese changes from Mn⁴⁺ (x=0) to Mn³⁺ (x=0.5).

For the computer simulation of the defect structure and for the mobility of the oxygen anions within these defect structures we applied the method of interatomic potentials in a simple rigid-ion approximation. The parameters of interaction were calibrated on the basis of empirical data. i.e. equilibrium geometry and cohesive energies of the binary oxides CaO, MnO₂ and Mn₂O₃ were taken into account for the present calculations.

Stabilities, oxygen migration barriers and dielectric constants of selected representants of CaMnO_3-x are presented.

Key Words:

• Perovskitic calcium manganese oxides
• anion deficient structures
• oxygen mobility
• computer simulation
• oxygen migration barriers
• dielectric constants