Defect formation in Gd3Fe5O12-based garnets:: a Mossbauer spectroscopy study

The Mossbauer spectroscopy of Gd2.2Pr0.8Fe5O12 garnet, which exhibits higher electronic conduction with respect to Gd3Fe5O12 due to the presence of Pe(4+) cations, showed that praseodymium, doping decreases the coordination of Fe3+ in octahedral sites. Penta-coordinated Fe3+ ions, in combination with small quantities of Fe4+, are also formed in the lattice of Gd2.5Ca0.5Fe5O12 where the variations of ionic and electronic transport properties indicate charge compensation via generation of oxygen vacancies and electron holes. The mechanisms of garnet lattice disorder, induced by acceptor- and donor-type doping, appear thus quite similar; in all cases, the ionic defect formation requires substantial structural reconstruction, probably associated with direct linking of iron-oxygen tetrahedra. Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd3Fe5O12-based garnets. (C) 2004 Elsevier B.V. All rights reserved.