Polycrystalline yttrium iron garnet (Y3Fe5O12, hereafter labeled YIG) has been synthesized by solid-state reaction, characterized by X-ray diffraction, Mössbauer spectroscopy, and UV-vis-NIR diffuse reflectance spectroscopy, and its optical properties from room temperature (RT) to 300 °C are discussed. Namely, its greenish color at RT is assigned to an O(2-) → Fe(3+) ligand-to-metal charge transfer at 2.57 eV coupled with d-d transitions peaking at 1.35 and 2.04 eV. When the temperature is raised, YIG displays a marked thermochromic effect; i.e., the color changes continuously from greenish to brownish, which offers opportunities for potential application as a temperature indicator for everyday uses. The origin of the observed thermochromism is assigned to a gradual red shift of the ligand-to-metal charge transfer with temperature while the positioning in energy of the d-d transitions is almost unaltered. Attempts to achieve more saturated colors via doping (e.g., Al(3+), Ga(3+), Mn(3+), ...) remained unsuccessful except for chromium. Indeed, Y3Fe5O12:Cr samples exhibit at RT the same color than the undoped garnet at 200 °C. The introduction of Cr(3+) ions strongly impacts the color of the Y3Fe5O12 parent either by an inductive effect or, more probably, by a direct effect on the electronic structure of the undoped material with formation of a midgap state.

Thermochromism in Yttrium Iron Garnet Compounds. -The compounds Y 3Fe5O12 and Y3Fe5-xCrxO12 are synthesized by solid state reaction of stoichiometric mixtures of Y 2O3, Fe2O3, and Cr2O3 (air, 1350 C, 6 h). The samples are characterized by XRD, Moessbauer spectroscopy, and UV/VIS/NIR diffuse reflectance spectroscopy. With increasing temperature, the color of Y3Fe5O12 changes continuously from a greenish to a brownish tone. Chromium doping tends to simulate the effect of temperature increase with regular red shift with increasing Cr 3+ content. The thermochromism of Y3Fe5O12 offers opportunities for potential application as a temperature indicator for everyday uses. -(SERIER-BRAULT*, H.; THIBAULT, L.; LEGRAIN, M.; DENIARD, P.; ROCQUEFELTE, X.; LEONE, P.; PERILLON, J.-L.; LE BRIS, S.; WAKU, J.; JOBIC, S.; Inorg.