Atomic, electronic and magnetic structure of an oxygen interstitial in neutron-irradiated Al2O3 single crystals
V. Seeman, A. Lushchik, E. Shablonin, G. Prieditis, Denis Gryaznov, Alexander Platonenko, E. A. Kotomin, Anatoli I. Popov
Abstract
Abstract A single radiation-induced superoxide ion $$O_{2}^{ - }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>-</mml:mo> </mml:msubsup> </mml:math> has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×10 18 n /cm 2 ) corundum (α-Al 2 O 3 ) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type $$O_{2}^{ - }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>-</mml:mo> </mml:msubsup> </mml:math> center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole ( S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the $$O_{2}^{ - }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>-</mml:mo> </mml:msubsup> </mml:math> centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F -type centers (mostly with the EPR-active F + centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the $$O_{2}^{ - }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>-</mml:mo> </mml:msubsup> </mml:math> center spin S = 1/2, its orientation along the a axis. The $$O_{2}^{ - }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>-</mml:mo> </mml:msubsup> </mml:math> center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration.