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DF: Fachverband Dielektrische Festkörper

DF 11: Poster Session

DF 11.21: Poster

Donnerstag, 29. März 2007, 14:30–18:00, Poster C

First-principles study of point defects in barium titanate: thermodynamics and electrical conductivity — Paul Erhart and •Karsten Albe — Technische Universität Darmstadt, Institut für Materialwissenschaft, Petersenstraße 23, 64287 Darmstadt

The thermodynamic and kinetic properties of mono and di-vacancy defects in cubic (para-electric) barium titanate are studied by means of density-functional theory calculations. Depending on the thermodynamic boundary conditions either metal or oxygen vacancies prevail. The assumption that the vacancies occur in their nominal charge states throughout the band gap, which underlies the most widely employed defect models, is confirmed. Only within about 0.1 eV of the band edges transition levels are found. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which under metal-rich (oxygen-rich) conditions metal (oxygen) vacancies are bound in di-vacancy clusters. Since oxygen vacancies readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The formation and migration energies are employed to derive the dependence of the equilibrium Fermi level and the charge carrier concentrations on the chemical conditions and the temperature. Thereby, it is also possible to deduce the relation between the conductivity and the oxygen partial pressure which compares very well with experiments. Furthermore, we are able to demonstrate the correspondence between the Kröger-Vink analysis, widely applied in defect chemistry, and the equations of semiconductor physics.