Page 110 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

Figure 2.63. Crystal structure of the beryl lattice, showing the location of Al octahedra. Reproduced with
permission from http://www.seismo.berkeley.edu/~jill/wisc/Lect9.html.

(Morganite and Red Beryl), the color changes from pink to red, respectively. As a
general rule, as the oxidation state of the transition metal ion increases, the ligand
ions are drawn in closer to the metal center. This will result in more electron–
electron repulsions between the metal and ligand, and a larger D o . The increase in
the energy gap between d-orbitals causes the absorption of higher energy wave-
lengths, and a corresponding red shift for the observed/transmitted color. As you
might expect, it should be possible to change the color of such a crystal through
heating in an oxidizing or reducing environment. This is precisely the operating
principle of “mood rings” that respond to differences in body temperature. The color
change resulting from a temperature fluctuation is referred to as thermochromism.
When the application of an external pressure causes a color change, the term
piezochromism is used.
Another factor that must be mentioned relative to our discussion of color is the
3þ
wavelength of light used to irradiate the crystal. Alexandrite, Cr -doped chyso-
beryl (BeAl 2 O 4 ), has two equivalent transmission windows at the red and blue-green