Local Structures in the ZrO 2 –15 mol% Fe 2 O 3 System Obtained by Ball Milling Santiago Figueroa, Judith Desimoni,** Patricia C. Rivas, and Marı´a C. Caracoche* , *** Departamento de Fı´sica, FCE, Universidad Nacional de La Plata, IFLP, 1900 La Plata, Argentina Oscar de Sanctis* ,w Laboratorio de Materiales Cera´ micos, IFIR-CONICET, FCEIA, Universidad Nacional de Rosario, 2000 Rosario, Argentina The Mo¨ ssbauer effect and perturbed angular correlation tech- niques have been used to investigate the nature and thermal sta- bility of Zr and Fe surroundings in a solid solution of zirconia and 15 mol% hematite obtained by mechanical mixing. Hema- tite incorporates completely into the zirconia lattice, resulting in a substitutional cubic structure that remains stable over a wide thermal range. Upon annealing at 9001C, a 20% decrease in the iron solubility occurs, accompanied by the cubic to tetragonal zirconia transformation. Both the cubic and the tetragonal crys- talline solid solutions exhibit two local coordinations for each one of the probes. The presence of the charge-balancing oxygen vacancy, which appeared after Fe 31 substitution for Zr 41 and located as a nearest neighbor to host and dopant cations, is re- flected by the most disordered interaction, which shows similar characteristics for the cubic and the tetragonal polymorphs. The other local coordination, described at Zr sites by an electric field of phase-dependent intensity, has been thought to depict a direct interaction between very near Fe and Zr cations. I. Introduction I T is established that the transition between high-temperature tetragonal and room-temperature (RT) monoclinic phases of zirconia involves a volume change that causes the ceramic crack- ing on cooling. 1 One means to overcome this unavoidable tran- sition during a thermal cycling is to produce zirconia at RT in any of its high-temperature structures, cubic or tetragonal, i.e., in a metastable phase. This can be accomplished by addition to zirconia of given amounts of some stabilizing oxide such as CaO, MgO, Y 2 O 3 , CeO 2 , Fe 2 O 3 2 , leading to a cubic or tetrago- nal (c/t) solid solution of variable stability. In the case of dopant cations of lower valence than zirconium localized at Zr sites, oxygen vacancies appear for charge compensation, thus enhanc- ing the anion diffusion and improving the performance of me- tastable zirconia as a solid electrolyte. 3 In addition, the hematite (aFe 2 O 3 )—zirconia solid solution is also known for its good catalytic properties. 4 Investigations reported by various authors on the zirconia– hematite system 5–10 lead to the conclusion that the position of the Fe 31 dopant cation in zirconia lattice is still an open ques- tion. While Berry et al., 5 using Mo¨ ssbauer effect (ME) spectros- copy, reported that Fe 31 incorporates at interstitial sites of a t/c zirconia solid solution, Li et al., 6 Narwankar et al., 7 and Jiang et al. 8 interpreted their results assuming the substitutional re- placement of Zr 41 ions by the Fe 31 ions. In the last work, ball- milled alloys of different concentrations were investigated using XRD and ME spectroscopy. The hyperfine description of the solid solution consisted of a quadrupole interaction identified as corresponding to high-spin Fe 31 that was an average of two components of similar relative areas, quite broad line widths, nearly equal isomer shifts (around 0.35 mm/s), and different quadrupole splittings (1.5 and 0.95 mm/s). The same quadrupole doublets were also determined by Stefanic et al., 9 who suggested the possibility of two non-equivalent distributions of iron in the substitutional tetragonal (or cubic depending on iron content and annealing temperature) solid solution. The present authors, using mainly the hyperfine techniques of ME and perturbed an- gular correlations (PAC), have recently investigated a solgel- derived solid solution of ZrO 2 15 mol% Fe 2 O 3 . 10 The resulting product was tetragonal zirconia with two non-equivalent local coordinations for substitutional Fe 31 cations in zirconia. One of them was interpreted as the well-established array involving the charge-balancing oxygen vacancy derived from the substitution of a lower valence dopant cation for Zr 41 . In the case of un- dersized dopant cations (i.e., Fe 31 , of smaller ionic radius than the Zr 41 host cations), the vacancy has been reported to be a nearest neighbor to both cations—defect 6 — and thus, its effect could be unambiguously sensed by PAC at Zr sites and ME at Fe sites. The other local coordination was assumed to include Fe 31 and Zr 41 in close proximity within the highly distorted cation lattice. Zirconia-based ceramics are extremely suitable for the appli- cation of the PAC technique. 11 As natural zirconium contains a few percent of hafnium impurities randomly distributed at cat- ion lattice sites, the thermal neutron irradiation of the ceramic leads to the activation of 181 Hf nuclei. These decay by b  to the 181 Ta ‘‘probe’’ nuclei, giving rise to the 133—482 keV g 1 –g 2 cas- cade. Its intermediate level, of known quadrupole moment and life time, interacts with the electric field gradient (EFG) pro- duced by the charge distribution at the probe’s nearest environ- ment (EFG p r 3 ). The hyperfine interaction causes the splitting of the nuclear level into the magnetic sublevels and the transitions among them, in turn, the precession of the nu- clear spin around the major component of the EFG tensor. The precession ‘‘perturbs’’ the angular correlation existing between the two g rays in cascade, thus providing knowledge on the ex- tranuclear EFG. This local structure information is drawn from the experimental A 2 G 2 (t) spin rotation curve or PAC spectrum, obtained at the laboratory by measuring the g 1 –g 2 coincidences as a function of the time and angular separations between both photons. Each EFG is conventionally described by three quadrupole parameters drawn from a fitting process of the experimental A 2 G 2 (t) function: the quadrupole frequency o Q proportional to the EFG intensity, the asymmetry parameter Z that measures its departure from the axial symmetry (Z 5 0), 3759 J ournal J. Am. Ceram. Soc., 89 [12] 3759–3764 (2006) DOI: 10.1111/j.1551-2916.2006.01290.x r 2006 The American Ceramic Society J. Hellmann—contributing editor *Member, American Ceramic Society. **Member of CONICET, Argentina. ***Member of CICPBA, Argentina. w Author to whom correspondence should be addressed. e-mail: oski@fceia.unr.edu.ar Manuscript No. 21728. Received April 20, 2006; approved July 9, 2006.