Article pubs.acs.org/IC
Thermochromism in Yttrium Iron Garnet Compounds Hélène Serier-Brault,*,† Lucile Thibault,† Magalie Legrain,† Philippe Deniard,† Xavier Rocquefelte,† Philippe Leone,† Jean-Luc Perillon,‡ Stéphanie Le Bris,‡ Jean Waku,‡ and Stéphane Jobic*,† †
Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex, France Tefal SA, Chemin des Granges, 74150 Rumilly, France
‡
ABSTRACT: 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 O2− → Fe3+ 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., Al3+, Ga3+, Mn3+, ...) 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 Cr3+ 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.
1. INTRODUCTION Thermochromic inorganic materials change color with a temperature variation (heating or cooling) and offer potentialities for temperature indicators1,2 in different kinds of devices, such as temperature sensors for safety (kitchen tools, hot plates, fridges, ...), laser marking, or warning signals. Color change can be brutal or continuous, reversible or irreversible according to the involved mechanism. An irreversible color change is mainly associated with a chemical transformation, i.e., a decarbonation or a dehydratation reaction,3 for instance. A material with such an effect has restricted applications and is mainly used for the monitoring of temperature-sensitive products. In contrast, materials with reversible thermochromism can give rise to many applications in daily living. Each application requests then specific characteristics in terms of color, transition temperature range, color contrast, or cyclability. For obvious reasons, materials with saturated hues in the low- and high-temperature states should be privileged for an optimal visual rendering and industrial applications. For oxides, reversible thermochromism may originate from a gradual reduction of the band gap4 with a temperature increase (red shift) as commonly observed for semiconductors, a change in the ligand field around the chromophor, or a crystallographic phase transition.5−9 The major restriction in the use of semiconductors lies in their limited choice of colors (namely, white, yellow, orange, red, or black) and the difficulty to assign an accurate temperature to a given hue by the naked eye due to a continuous color change in the chromatic coefficients with temperature. The d−d transitions can lead to interesting colors in crystal with sometimes thermochromic effects (e.g., Cr3+doped Al2O3), but without high color strength.1,2 Materials that undergo a phase transition (namely, first-order transition) may © XXXX American Chemical Society
display very pronounced color contrast in relation to change in coordination of chemical elements, for instance. Unfortunately, the volume variation at the transition may reduce considerably their cyclability8 if too large. In that domain, metal−insulator transitions (e.g., VO210) have received much attention due to their ability to reflect or to absorb IR wavelengths and control the energy exchange through windowpanes in buildings. Nevertheless, no color change is observed in the visible range when the material shifts from metallic to semiconductor. To have access to a large panel of colors (i.e., colors others than those generated by valence band (VB)−conduction band (CB) transition), transition elements have to be present to take advantage of d−d transitions in the visible region. In first approximation, the positioning of these d−d transitions is much less dependent on temperature than ligand-to-metal transfer or VB−CB transitions. Thus, a solution to obtain a large variety of colored materials with thermochromic properties may consist in mixing a semiconductor material with a d−d transition-based compound. In that case, it is possible to fully design various reversible thermochromic effects with a control of the transition temperature, a control of the blend hues with T, and a good cyclability. This mixing rule was already discussed in the literature for Bi2O3−CoAl2O4, Bi2O3−LiCoPO4, and V2O5− Cr2O3 blends.11 However, this technique has some shortcomings especially concerning the color which is not saturated enough because of the subtractive synthesis principle of colors. The ideal way to get a reversible and robust (high cyclability) thermochromic material will be to get a pigment with a color originating from the intrinsic mixing rule (charge transfer Received: July 17, 2014
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dx.doi.org/10.1021/ic501708b | Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
Article
tetrahedral (Wyckoff position 24d) sites of Fe 3+ ions, respectively. Each octahedron (tetrahedron) is linked to six (four) others via corner-sharing tetrahedra14,15 (octahedra) to provide a three-dimensional structure. The YIG structure is mainly described in the literature in the cubic space group Ia3̅d, but Rodic et al.16 used the trigonal space group R3̅ (SG No. 148) to account for its magnetic behavior. In order to clarify the situation, we carried out density functional theory (DFT) calculations using the WIEN2k code17 and the on-site PBE0 hybrid functional.18 Our results show that the magnetic structure (i.e., ferrimagnetism originating from the antiparallel coupling of the two Fe3+ octahedral and tetrahedral sublattices) described in the space group Ia3̅d is more favorable than the one described in the space group R3,̅ by about 480 meV per formula units. Y3Fe5O12 materials were prepared via conventional hightemperature solid-state reactions, and the purity of the samples was checked by X-ray diffraction. All patterns were refined with the cubic structure (JCPDS No. 83-1027), reflecting that the compounds are single-phased. Rietveld refinement were carried out on undoped YIG (Figure 1), and the refined lattice parameter was determined to be equal to 12.3736(5) Å at room temperature, in good agreement with the literature.19,20
located in the visible range together with d−d transitions). Herein, we report the investigation of a well-known compound, the yttrium iron garnet (Y3Fe5O12, hereafter labeled YIG) with a greenish color at room temperature. For the first time, thermochromic properties of YIG are measured and displayed. Unfortunately, the colors of YIG are far from being saturated. This prompted us to investigate the impact of the insertion of dopant in the Y3Fe5O12 host lattice on the color. Several dopants were tried (e.g., Al3+, Ga3+, Mn3+, ...), but only chromium ions get an effect on the color of the parent material.
2. EXPERIMENTAL SECTION Conventional high-temperature solid-state reactions were performed to synthesize Y3Fe5O12 and Y 3Fe5−xCr xO12 compounds. The constituent oxides Y2O3 (Alfa Aesar, 99.9%), Fe2O3 (Alfa Aesar, 99.945%,
