Thermochromism in yttrium iron garnet compounds.
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.
Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS , 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex, France.
Hélène
H
Serier-Brault
Lucile
L
Thibault
Magalie
M
Legrain
Philippe
P
Deniard
Xavier
X
Rocquefelte
Philippe
P
Leone
Jean-Luc
JL
Perillon
Stéphanie
S
Le Bris
Jean
J
Waku
Stéphane
S
Jobic
D003116:Color; D058085:Iron Compounds / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010087:Oxides / Q000737:chemistry*; D013696:Temperature; D014961:X-Ray Diffraction; D015019:Yttrium / Q000737:chemistry*
D058085:Iron Compounds; D010087:Oxides; D015019:Yttrium