Thermochromism in Yttrium Iron Garnet Compounds

International audiencePolycrystalline yttrium iron garnets (Y 3 Fe 5 O 12) were synthesized by solid-state reaction and characterized by X-ray diffraction, Mössbauer spectroscopy and UV-vis-NIR diffuse reflectance spectroscopy. The green color of the compounds have been assigned to the presence of a O 2-Fe 3+ ligand-to-metal charge transfer together with d-d transitions, the absence of Fe 2+ in samples being verified with Mössbauer measurements. By increasing the temperature to around 300°C, the YIG material shows a thermochromic behavior with a continuous color change from greenish to brownish due to a red-shift of the ligand-to-metal charge transfer. Y 3 Fe 5 O 12 host lattice was also doped with chromium ions and the same red-shift phenomenon was observed although the effect on the crystal structure is opposite. Indeed, Cr 3+ ions were proved to play a major role on the color either by an inductive effect or, more probably, by a direct effect on the electronic structure of the material as discussed in this work.