Space resolved optical spectroscopy on Fe-doped SrTiO3 single crystals

R. Merkle

In Fe-doped SrTiO3 perovskite single crystals, the oxidation state of iron (substituting for Ti) depends sensitively on equilibration conditions (T, pO2). At increasing temperature or decreasing pO2, oxygen is gradually excorporated according to

                        OOx + 2 FeTix ↔ 1/2 O2 + VO.. + 2 FeTi'                   (1)

leading to the formation of oxygen vacancies VO.. and reduction of FeTix (corresponding to Fe4+) to FeTi' (Fe3+). Since Fe3+ in SrTiO3 is colorless while Fe4+ has a characteristic optical absorption at 590 nm, the kinetics of oxygen stoichiometry change can be monitored on a slice of Fe-doped SrTiO3 single crystal (larges faces sealed by glass) by in-situ optical spectroscopy. This allows us to analyze oxygen surface exchange kinetics, bulk chemical diffusion of oxygen and transport across grain boundaries (Fig. 1).

Fig. 1: Concentration profiles and false-color images (insets) of a Fe-doped SrTiO3 single crystal sample after pO2 change at elevated temperatures. (a) surface reaction (b) bulk diffusion (c) transport across grain boundary is limiting.

Not only oxygen, but also water can be incorporated into oxygen vacancies leading to protonic defects OHO. (OH- on the site of O2-):

                        H2O + VO.. + OOx ↔ 2 OHO.                                               (2)

Interestingly, water uptake into Fe-doped SrTiO3 occurs by a 2-step process as shown in Fig. 2: Firstly, fast diffusing protons are taken up at the expense of electron holes (corresponding to a reduction of the sample), followed by slower oxygen uptake (reoxidation).

Fig. 2: Increase of pH2O from 4 to 20 mbar, monitored by optical spectroscopy and electrical conductivity
Publications:
  • R. Merkle and J. Maier: How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid-State Reaction with SrTiO3 as a Model Material  Angewandte Chemie International Edition47(21), 3874–3894 (2008). DOI: 10.1002/anie.200700987
  • R. Merkle and J. Maier: Oxygen incorporation into Fe-doped SrTiO3: Mechanistic interpretation of the surface reaction Physical Chemistry Chemical Physics 4(17), 4140–4148 (2002). DOI: 10.1039/b204032h
  • M. Leonhardt, R. A. De Souza, J. Claus, and J. Maier: Surface Kinetics of Oxygen Incorporation into SrTiO3 Journal of The Electrochemical Society 149(2), J19–J26 (2002).
    DOI: 10.1149/1.1430723
  • M. Leonhardt, J. Jamnik, and J. Maier: In situ Monitoring and Quantitative Analysis of Oxygen Diffusion Through Schottky-Barriers in SrTiO3 Bicrystals Electrochemical and Solid-State Letters 2(7), 333–335 (1999). DOI: 10.1149/1.1390827
  • Denk, F. Noll, and J. Maier: In-situ Profiles of Oxygen Diffusion in SrTiO3: Bulk Behavior and Boundary Effects Journal of the American Ceramic Society 80(2), 279–285 (1997). DOI: 10.1111/j.1151-2916.1997.tb02827.x
  • J. H. Yu, J.-S. Lee, and J. Maier: Formation of protonic defects in perovskite-type oxides with redox-active acceptors: case study on Fe-doped SrTiO3 Physical Chemistry Chemical Physics 7(20), 3560–3564 (2005). DOI: 10.1039/b509164k
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