Fe-Doped Zirconium Oxide Produced by Self-Sustained High-Temperature Synthesis: Evidence for an Fe-Zr Direct Bond Paolo Ghigna,* ,² Giorgio Spinolo, ² Umberto Anselmi-Tamburini, ² Filippo Maglia, ² Monica Dapiaggi, ² Gabriele Spina, ‡ and Luciano Cianchi § Contribution from the INCM, C.S.T.E./CNR, and Dipartimento di Chimica Fisica, UniVersita ` di PaVia, Viale Taramelli 16, I 27100 PaVia, Italy, Dipartimento di Fisica, UniVersita ` di Firenze, Via S. Marta 3, I 50139 Firenze, Italy, and IROE, Via Panciatichi 64, I 50127 Firenze, Italy ReceiVed July 3, 1998 Abstract: The local structure of iron in Fe-doped cubic ZrO 2 produced by combustion synthesis was studied by Mo ¨ssbauer spectroscopy and Fe-K edge extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES). Iron was found to be in its II oxidation state and to occupy two different sites of the fluorite ZrO 2 structure, both associated with some amount of disorder. One of the sites has been identified with the regular Zr position in 0,0,0, thus giving rise to substitutional Fe′′ Zr (i.e., Fe atoms occupying the regular Zr position, with a net 2-charge, with respect to the lattice) defects, while the other site has been identified with the normally empty position at 1 / 2 , 1 / 2 , 1 / 2 , thus giving rise to interstitial Fe i •• (i.e., Fe atoms occupying an interstitial position, with a net 2+ charge, with respect to the lattice) defects. For this last site, there is a short Fe-Zr distance (2.64 Å). This result, coupled to the quite small value of the Debye- Waller factor for this distance, gives evidence of a direct Zr-Fe metal-to-metal bond. 1. Introduction The cubic form of zirconium oxide (zirconia) is well known for its mechanical, electrochemical, and optical applications. 1 Several preparative methods have been used for synthesizing, doping, and sintering this important material, as well as its composites with other ceramic or metallic phases. We have recently reported 2,3 that the self-sustained high-temperature synthesis (SHS) technique can be used to produce Me-YSZ (yttria-stabilized zirconia) cermets (Me ) Ni, Co, Fe, Cu) and that these cermets show significant performance enhancements in their electrochemical applications with respect to those obtained with the traditional ceramic route based on the chemical reduction with hydrogen at high temperature of a previously sintered mixture of MeO and YSZ. 4 SHS, or combustion synthesis, is a preparative technique based on the large heat effect of a heterogeneous chemical reaction. In this method, an external energy pulse is used to ignite the heterogeneous mixture of the reactant powders so that a (thermal and chemical) reaction wave is produced and propagates through the reactants in a stable self-sustaining manner. 5-10 YSZ-based materials can be produced by SHS either using oxidation with oxygen of Zr (metal) powders 11 or using the thermite reactions, which can be schematically described (for an MeO oxide) by A well-appreciated property of SHS is its ability to produce solid materials in various metastable forms, which either were previously unknown or are accessible through other synthetic routes only with difficulty. 6,10 This ability has been related to the extremely high thermal gradients and reaction rates and temperatures of the SHS process, where the available reaction paths can be completely different from those active in conditions closer to thermodynamic equilibrium. The fluorite-type structure of cubic zirconia is also well known for its excellent solvent properties for a fairly large variety of cations in different oxidation states. Aliovalent cations have been deeply investigated for their effect on defect chemistry and electrical properties of the phases. Lower valence cations replacing Zr (for instance, Ca 2+ , Mg 2+ , and Y 3+ , just to quote the most known) are balanced by extrinsic ionized oxygen vacancies. Thus, doping dramatically increases the amount of oxygen vacancies and therefore increases the performance of the material as solid electrolyte by enhancing the diffusion coefficient of the oxide anion. Another field of interest concerns the ability of various dopants to extend the stability range of cubic zirconia (above 2650 K in the pure compound) down or close to room temperature. From the point of view of crystal chemistry, there is a strong interest in the electronic structure and optical properties of the dopant as well as in various features of the local atomic structure ² Universita ` di Pavia. ‡ Universita ` di Firenze. § IROE. (1) Stevens, R. An introduction to zirconia; Twickenham, England, Magnesium Elektron Ltd.: 1983; pp 1-22. (2) Anselmi-Tamburini, U.; Arimondi, M.; Maglia, F.; Spinolo, G.; Munir, Z. A. J. Am. Ceram. Soc. 1998, 81, 1765-1772. (3) Ghigna, P.; Spinolo, G.; Anselmi-Tamburini, U.; Maglia, F.; Mor- gante, N., to be published. (4) Anselmi-Tamburini, U.; Chiodelli, G.; Arimondi, M.; Maglia, F.; Spinolo, G.; Munir, Z. A. Solid State Ionics 1998, 110, 35-43. (5) Munir, Z. A.; Anselmi-Tamburini, U. Mater. Sci. Rep. 1989, 3, 277- 365. (6) Moore, J. J.; Feng, H. J. Prog. Mater. Sci. 1995, 39, 243-273. (7) Moore, J. J.; Feng, H. J. Prog. Mater. Sci. 1995, 39, 275-316. (8) Merzhanov, A. G.; Boroviskaya, I. P. Dokl. Acad. Sci. USSR (Chem.) 1972, 204, 429-432. (9) Munir, Z. A. Am. Ceram. Soc. Bull. 1988, 67, 342-349. (10) Varma, A.; Lebrat, J.-P. Chem. Eng. Sci. 1992, 47, 2179-2194. (11) Anselmi-Tamburini, U.; Spinolo, G.; Munir, Z. A. J. Mater. Synth. Proc. 1993, 1, 323-333. Zr + 2MeO f ZrO 2 + 2Me 301 J. Am. Chem. Soc. 1999, 121, 301-307 10.1021/ja982335a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/31/1998