Thermochromic Behavior of Yttrium-Substituted Bismuth Oxides

High-temperature thermochromic materials are poorly explored in fundamental research, let alone applied research, although these materials may be used as low-cost, intuitively usable sensing materials in an industrial environment. Yet, only few of these materials have been described systematically. We describe a series of yttrium-substituted bismuth oxides (Bi_1-Y)₂O₃ (0.05 ≤ x ≤ 0.25) that show thermochromic behavior with a color change from yellow at low temperatures to various brown hues at high temperatures. The compounds were analyzed between 293 and 1050 K by X-ray powder diffraction, UV/vis spectroscopy, and differential scanning calorimetry. A combination of derived absorption spectral fitting and Tauc methods was applied to determine the band gap energies and band gap types from the diffuse UV/vis spectra, respectively. Two types of materials were found: one with x = 0.05 that exhibits the tetragonal β-phase at room temperature, and the defect fluorite-type cubic δ-phase at temperatures above 920 K. This phase showed a reversible, gradual color change upon heating, followed by an abrupt color change at the phase-transformation temperature. The second type of material had higher yttrium contents (x > 0.10); these samples were cubic at room temperature and showed a continuous color change upon heating and cooling. In contrast to the material with x = 0.05, these latter phases show a reduced cycle stability and were gradually annealed to the hexagonal phase-I. The samples with x = 0.10 provided a mixture of the β- and δ-phases, showing both, the reversible behavior for the β- to δ-phase transition and the irreversible behavior concerning the β₂-phase. This points the way toward smart materials that can not only sense the actual thermal stress but also monitor cumulative thermal stresses over a certain material lifetime.