Stabilization of charged substitutional ions in tetragonal zirconia

Abstract

The formation of substitutional and interstitial diluted defects of tantalum, niobium and molybdenum in a tetragonal zirconia (T-ZrO2) crystal was studied with Density Functional Theory calculations. Bader charge analysis was used to select the oxidation states (OS) which reached electronic localization around the substitutional ion. Oxidation states from +5 to +3 for Nb, +6 to +2 for Mo and +5 to +3 for Ta substitution ions were selected and formation energies were calculated. The work is focused on substitutional defects, after confirmation that substitutional defects produce higher stability than interstitial ones. Comparing the whole calculation set, it was found that Ta charge defects with oxidation state +5 (TaZr∗) exhibiting less available electrons than the replaced Zr ion, presented the minimum formation energy. Kröger-Vink diagrams evidence that TaZr∗has indeed the lowest formation energy values over the entire range of electron chemical potential. Electronic redistribution due to the substitution of Zr by a charged Ta ion results in an enhanced covalent bond between Ta and nearest O ions and charge polarization between Ta ion and the next nearest O ions along 〈111〉 and <11-1> directions. The results are of great technological incidence, asserting that Ta addition to zirconium nuclear alloys could improve its corrosion resistance properties.