Mercury(II) iodide displays thermochromism; when heated above 126 °C (400 K) it undergoes a phase transition, from the red alpha crystalline form to a pale yellow beta form. As the sample cools, it gradually reacquires its original colour. It has often been used for thermochromism demonstrations. A third form, which is orange, is also known; this can be formed by recrystallisation and is also metastable, eventually converting back to the red alpha form. The various forms can exist in a diverse range of crystal structures and as a result mercury(II) iodide possesses a surprisingly complex phase diagram.