Spectrochemical investigations of ultrafine amorphous and crystalline rare earth iron garnets

Abstract

The absorption bands caused by the lattice vibrations and electronic transitions due to tetrahedral and octahedral Fe 3+ in ultrafine submicron-sized rare earth iron garnets (RIG) prepared by the thermal decomposition of citrate precursors are studied to elucidate the local molecular structure. The ultrafine garnet samples consisting of 1.0- to 1.5-nm sized crystallites are X-ray amorphous and show evidence for greater specific volume compared to crystalline garnets in XRD and IR studies. An energy level scheme of 3 d 5 for Fe 3+ in tetrahedral and octahedral coordination in these ultrafine crystallites is proposed. The ligand-field parameters suggest a decrease of octahedral Fe O distance and an increase of tetrahedral Fe O distance. The Mo¨ssbauer isomer shift values show interesting correlations with Racah parameters and confirm the variation of Fe O distances in the amorphous state. The ultrafine crystalline materials comprising 10- to 35-nm sized crystallites exhibit absorption frequencies and ligand-field parameters comparable to crystalline garnets.