3.2 Inorganic Pigments

In contrast to the thermochromic effect of organic composites, thermochromism of inorganic materials, such as metal salts and metal oxides, has been well known for a long time [29 31]. The appearance of thermochromism in inorganic pigments has its origin in changes of crystal structure. For example, the red to brown colour change of copper mercury iodide, Cu ₂(HgI ₄), is caused by a change from an ordered to a disordered structure. In both modifications the iodide ions form a face-centred cubic cell with tetrahedral holes which are partly occupied by the copper and mercury ions. In the low-temperature modification definite tetrahedral holes are occupied, whereas in the high-temperature modification the cations are randomly distributed throughout all of the tetrahedral holes. Another example is mercury(II) iodide. The transition from the ? to the ?-modification at 127 C is accompanied by a colour change from red to yellow. Colour changes and respective transition temperatures of several inorganic pigments are listed in Table 3.1 In most of the inorganic pigments considered the thermochromic effect appears at temperatures above 100 C and is irreversible. Only the last three examples listed in Table 3.1 exhibit reversible thermochromic behaviour.