Dynamics of the Perovskite-Based High-Temperature Proton-Conducting Oxides: Theory and Experiment
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C. KARMONIK('), T. YILDIRIM(', 2), T.J. UDOVIC( 1), J.J. RUSH(1) and R. HEMPELMANN(3) (1) NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-0001, USA (2) University of Maryland, College Park, Maryland, 20742, USA (3) Physikalische Chemie, Universitiit Saarbriicken, Germany ABSTRACT The dynamics of the doped perovskite-based high-temperature protonic conductors (HTPC) were studied by means of neutron vibrational spectroscopy (NVS) and firstprinciples pseudopotential supercell calculations. Vibrational spectra from hydrogencharged samples with different rare-earth dopants revealed three well-defined vibrational bands in the energy ranges 20-60, 60-90, and 100-140 meV. The two lowest-energy bands were insensitive to the dopants. First-principles phonon calculations indicate that they are mainly associated with oxygen modes. In contrast, the high-energy band was very sensitive to the dopant, and in this case, calculations indicate that it is associated with OH bending modes. INTRODUCTION In recent years, scientific interest in perovskite-based high-temperature protonic conductors (HTPC) has increased strongly because of the promising industrial applications of these materials. [1] For large-scale stationary applications, intense materials research is
underway on solid oxide fuel cells (SOFC) operating at temperatures considerably lower than those oxygen-conducting, yttrium-stabilized zirconia as the solid electrolyte. With decreasing temperature, which is one of the main aims of current developments, protonconducting oxides become competitive. To have a better understanding of the proton dynamics and thus the proton conductivity mechanism, we have performed first-principles supercell phonon calculations in combination with an inelastic neutron scattering study of the vibrational dynamics of various HTPC's. As a model system, we chose the extensively studied , doped strontium cerates SrCeo. 95M0.05HllO 3 -, where M=Nd, Ho, and Sc. Together with the experimental results, preliminary calculations for the similar system SrZrO 3 and its Sc-doped derivative are presented, since Zr 4+ ions are much easier to model theoretically than Ce 4+ ions. Extension of these calculations to the doped strontium cerate analogs are in progress and will be reported elsewhere. EXPERIMENTAL DETAILS The preparation of the ceramic powder and the charging with hydrogen were carried out as described elsewhere [2]. The powder samples (approx. 35 g each, - 90 % neutron transmission ) were sealed under a helium-containing atmosphere in aluminum cans with a diameter of approximate 25 mm. The hydrogen concentrations were determined using prompt-gamma activation analysis [3] at the cold neutron PGAA spectrometer situated on 199 Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
the Neutron Beam Split-core Reactor (NBSR) at the NIST Center for Neutron Research (NCNR). Sample Sc Ho Nd Undoped mol % H 1.77 ± 0.13 1.70 ± 0.09 1.06 + 0.08 0.2 ± 0.1 Ionic radii r(Sc 3 +) - 0.732
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