SEMINAR @ SISSA, Fabio Petrucci dd 29/03/06

[ICTP info point]

From:  corzani at sissa.it

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                             SEMINAR
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Time and Place: 29 March 2006 at 12:00, Room A, SISSA



                       Fabio Pietrucci

    Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca


Title: "Ab-initio study of oxygen vacancies in yttria-stabilized 
zirconia"


Zirconia (ZrO2) stabilized in the cubic fluorite phase by the addition 
of
yttria (Y2O3) is a fast oxygen conductor widely used in solid oxide fuel
cells and oxygen sensors. Although the number of carriers (oxygen
vacancies)
increases with yttria content, the conductivity reaches a maximum for a
dopant concentration of 9 mol%, decreasing beyond this value. This 
complex
behaviour is believed to stem from interactions among defects, including
the
formation of a complex of two vacancies aligned along á111ñ directions.


However, a compelling experimental evidence of this defect, which has 
also
been tentatively associated to prominent signals in EPR and optical 
spectra
of reduced samples, is still lacking. In fact, other extrinsic defects
(such
as Ti) have also been claimed responsible for the spectroscopic 
signals. In
this work we have studied the electronic properties of paramagnetic
di-vacancy complexes by computing the EPR g-tensor from density 
functional
perturbation theory [1]. The paramagnetic defects are modeled in 
periodic
supercells of zirconia with 10 or 14 mol% yttria containing up to 93 
atoms.
The calculated EPR g-tensors of both a Zr3+ or a Ti3+ ion at the center
of a
di-vacancy aligned along the á111ñ directions are compatible with the
experimental EPR signal. However, comparison of experimental optical
absorption spectra with calculated optical excitations at the B3LYP 
level
allows us to decide in favor of the Ti3+ di-vacancy complex [2].
Preliminary
results will be also presented on the diffusion processes of oxygen
vacancies. The results show that the activation barrier for the
diffusion of
a single vacancy does not depend only on the vacancy-yttrium 
interaction as
customarily assumed, but also on the position of the other vacancies, 
which
might also promote concerted multiple jumps.

[1] C. J. Pickard and F. Mauri, Phys. Rev. Lett. 88, 86403 (2002).

[2] F. Pietrucci, M. Bernasconi, C. Di Valentin, F. Mauri, and C. J.
Pickard, Phys. Rev. B, in press.
 
 
 
 
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