Dielectric responses in polycrystalline rare-earth iron garnets

Rare-earth iron garnets (RIG, R = Y, Tb and Lu) were sintered at 1350 °C. Their crystal structures were refined by the Rietveld method, which showed that the oxygen coordination polyhedra were highly distorted. X-ray photoelectron spectroscopy revealed a similar Fe²⁺/Fe³⁺ratio (∼27/73) in different RIGs. The origins of dielectric responses were identified by cross examination of the permittivity, modulus and impedance presentations. Fitting experimental data with the Debye-type and Maxwell-Wagner models revealed that the dielectric relaxation at room temperature was dominated by the Debye-type process but as temperature increased, the Maxwell-Wagner effect gradually took over. The static permittivity was 750, 785 and 2653 for YIG, TbIG and LuIG, respectively. The particularly large permittivity of LuIG arose from a large difference between the distortions of FeO₆and FeO₄in the structure and therefore, a large dipole moment was created when electron hopping between the octahedral and tetrahedral sites took place.