Thermochromism - Inorganic Materials

Inorganic Materials

Virtually all inorganic compounds are thermochromic to some extent. Most examples however involve only subtle changes in color. For example, titanium dioxide and zinc oxide are white at room temperature but when heated change to yellow. Similarly indium(III) oxide is yellow darkens to yellow-brown when heated, due. Lead(II) oxide exhibits a similar color change on heating. The color change is linked to changes in the electronic properties (energy levels, populations) of these materials.

More dramatic examples of thermochromism are found in materials that undergo phase transition or exhibit charge-transfer bands near the visible region. Examples include

• Cuprous mercury iodide (Cu₂HgI₄) undergoes a phase transition at 55 °C, reversibly changing from a bright red solid material at low temperature to a dark brown solid at high temperature, with intermediate red-purple states. The colors are intense and seem to be caused by Cu(I)-Hg(II) charge-transfer complexes.
• Silver mercury iodide] (Ag₂HgI₄) is yellow at low temperatures and orange above 47–51 °C, with intermediate yellow-orange states. The colors are intense and seem to be caused by Cu(I)-Hg(II) charge-transfer complexes.
• Mercury(II) iodide is a crystalline material which at 126 °C undergoes reversible phase transition from red alpha phase to pale yellow beta phase.
• Bis(dimethylammonium) tetrachloronickelate is a raspberry-red compound, which becomes blue at about 110 °C. On cooling, the compound becomes a light yellow metastable phase, which over 2–3 weeks turns back into original red.
• Bis(diethylammonium) tetrachlorocuprate is a bright green solid material, which at 52–53 °C reversibly changes color to yellow. The color change is caused by relaxation of the hydrogen bonds and subsequent change of geometry of the copper-chlorine complex from planar to deformed tetrahedral, with appropriate change of arrangement of the copper atom's d-orbitals. There is no stable intermediate, the crystals are either green or yellow.
• Nickel sulfate is green at room temperature but yellow at 155 °C.
• Chromium(III) oxide:aluminium(III) oxide in 1:9 ratio is red at room temperature and grey at 400 °C, due to changes in its crystal field.
• Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infrared transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductor at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungsten lowers the transition temperature to 29 °C.

Other thermochromic solid semiconductor materials include Cd_xZn_1–xS_ySe_1–y (x=0.5...1, y=0.5...1), Zn_xCd_yHg_1–x–yO_aS_bSe_cTe_{1–a–b–c} (x=0...0.5, y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), Hg_xCd_yZn_1–x–yS_bSe_1–b (x=0...1, y=0...1, b=0.5...1).

Some minerals are thermochromic as well; for example some chromium-rich pyropes, normally reddish-purplish, become green when heated to about 80 °C.