Abstract
The Mössbauer spectroscopy of Gd 2.2Pr 0.8Fe 5O 12 garnet, which exhibits higher electronic conduction with respect to Gd 3Fe 5O 12 due to the presence of Pr 4+ cations, showed that praseodymium doping decreases the coordination of Fe 3+ in octahedral sites. Penta-coordinated Fe 3+ ions, in combination with small quantities of Fe 4+, are also formed in the lattice of Gd 2.5Ca 0.5Fe 5O 12 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 Gd 3Fe 5O 12-based garnets.
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