A defect model for the optical and EPR activity of the T-center in yttrium-stabilized zirconia

Abstract

Direct quantitative correlations have been recently observed between the T-centre EPR signal and an optical absorption band at 370 nm, in X-irradiated YSZ, suggesting that the colour centre and the paramagnetic centre might be the same entity. In order to confirm this hypothesis, theoretical modeling was undertaken, where by appropriate refinement the known electronic structure of the T-centre is shown to be compatible with an optical transition, having an oscillator strength of the correct magnitude.

Theoretical investigations of the g factors and the local structure for the trigonal Zr3+ center in X-ray irradiated YSZ

“…In X-ray irradiated YSZ, the Zr 3+ center was observed and attributed to the original Zr 4+ capturing an electron on irradiation, associated with two V O along the [1 1 1] (or C 3 ) axis [12,14,16], i.e., the trigonal (D 3d ) [ZrO 6 ] 9− cluster. Since the effective charge of the V O is positive, the oxygen ions in the ligand triangles near the V O may be expected to shift towards the latter along the [1 0 0] directions by an amount X due to the electrostatic attraction.…”

“…1, the local structural parameters R j and β j can be determined from the corresponding effective impurityligand distance R 0 and angle β 0 (in absence of perturbation due to the V O ) and the displacement X. For the Zr 4+ site in YSZ, the Zr 4+ O 2− distance is about √ 3a/4 (≈2.226Å [16,17]), and the angle β 0 of the Zr 4+ O 2− bond related to the C 3 axis is about 70.53 • .…”

“…E 1 is the energy difference between the ground state 2 A 1 and the original 2 E state of cubic case, and E 2 is that between 2 A 1 and the excited state 2 E due to trigonal splitting of the cubic 2 T 2 state [23,24]. They can be written in terms of the cubic field parameter D q and the trigonal field parameter V as [9,16] …”

“…Recently, investigations have been reported on YSZ as electrolyte in solid oxide fuel cell (SOFC) [1][2][3][4], high-T c superconductor thin film substrates [5][6][7], as well as the properties of optical absorption and upconversion luminescence [8][9][10], large room temperature magnetoresistance in YSZ doped La 0.67 Ba 0.33 MnO 3 composite [11]. Particularly, studies on the paramagnetic defects in YSZ due to X-ray irradiation have been carried out by means of electron paramagnetic resonance (EPR) and optical absorption techniques [12][13][14][15][16]. For example, the EPR g factors g // ≈ 1.989, g ⊥ ≈ 1.852 and absorption band (370 nm) were measured for the electron-type trigonally distorted octahedral defect (T-center), which were attributed to the hexacoordinated Zr 3+ ion (i.e., the original Zr 4+ ion capturing an electron due to X-ray irradiation), midway between two oxygen vacancies (V O ) at the opposite ends of the coordinated cube (see Fig.…”

“…For example, the EPR g factors g // ≈ 1.989, g ⊥ ≈ 1.852 and absorption band (370 nm) were measured for the electron-type trigonally distorted octahedral defect (T-center), which were attributed to the hexacoordinated Zr 3+ ion (i.e., the original Zr 4+ ion capturing an electron due to X-ray irradiation), midway between two oxygen vacancies (V O ) at the opposite ends of the coordinated cube (see Fig. 1) [12,14,16].…”