Cation Transport and Surface Reconstruction in Lanthanum Doped Strontium Titanate at High Temperatures
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Cation Transport and Surface Reconstruction in Lanthanum Doped Strontium Titanate at High Temperatures Karsten Gömann1, Günter Borchardt1, Anissa Gunhold2, Wolfgang Maus-Friedrichs2, Bernard Lesage3, Odile Kaïtasov4, Horst Baumann5 1 Institut für Metallurgie, Technische Universität Clausthal, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany 2 Institut für Physik und Physikalische Technologien, Technische Universität Clausthal, Leibnizstr. 4, D-38678 Clausthal-Zellerfeld, Germany 3 Laboratoire d'Etude des Matériaux Hors Equilibre, Université Paris-Sud, F-91405 Orsay Cedex, France 4 Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Université Paris-Sud, F-91405 Orsay Cedex, France 5 Institut für Kernphysik, J. W. Goethe-Universität, August-Euler-Str. 6, D-60486 Frankfurt, Germany ABSTRACT Tracer diffusion experiments were carried out in synthetic air at 1573 K in SrTiO3(100) and (110) single crystals, which were either undoped or doped with up to 1 at.% La, respectively. Tracer sources of 139La and 142Nd were applied by ion implantation. The resulting depth profiles were measured by SIMS. The reconstruction of the surface was monitored ex-situ using microscopic and spectroscopic methods including SEM, EPMA, and AFM. The measured tracer diffusivities show no dependency on orientation. The tracer diffusion takes place via cation vacancies. Under oxidizing conditions the dopant is compensated by Sr vacancies. Hence the diffusion is increasing strongly with La concentration. The observed time dependency of the diffusivities may be related to a space charge layer postulated by the current defect chemistry model for donor doped SrTiO3. At high dopant concentrations annealing leads to segregation of bulk La to the surface. La is not significantly incorporated into the secondary crystallites at the surface which consist almost entirely of Sr and O. INTRODUCTION Donor doped strontium titanate SrTiO3 is a promising material for resistive oxygen sensors operated at high temperature. Changing the ambient oxygen partial pressure p(O2) under high temperature leads to an undesirable surface reconstruction and the formation of secondary phases. Though an overall consistent model is still lacking, many of the phenomena can be explained by the bulk defect chemistry model of donor doped SrTiO3 [1]. The defect chemistry is dominated by the following reaction (see [2] for defect notation): 1 VO•• + 2e'+ O 2 ( g ) ⇔ O Ox (1) 2 Upon oxidation of O deficient crystals, VO•• and free electrons are consumed by the incorporation of O into the lattice. Finally, Sr vacancies are generated and a subsequent change in
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the donor compensation mechanism occurs. The excess Sr migrates to the surface where secondary SrOx phases are formed on top of the surface (e.g. [3,4]), and Ruddlesden-Popper phases (SrO·nSrTiO3, [5]) are formed at the surface between the islands [6]: 1 (2) SrSrx + 2e'+ O 2( g ) ⇔ VSr'' + SrO sec .phase 2 The amount of VSr'' produced is fixed by the donor content [D • ] = 2[VSr'' ] , (3) expl
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