Page 106 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

framework of covalent or ionic interactions. Since visible light (350–700 nm) is not
energetically sufficient to cause bond rupturing and/or electronic transitions of the
constituent metal atoms/ions, this energy is not absorbed by pure crystals, giving rise
to a colorless state. However, when an impurity is added to the lattice, visible
radiation may be suitably energetic to cause lattice alterations and/or electronic
transitions, yielding an observable color change.
Colored crystals need not be gemstones; in fact, a colorless crystal of potassium

chloride may be suitably altered to exhibit color. When solid KCl is heated to 500 C
in the presence of potassium vapor, the crystal becomes a violet color. This occurs

due to the ionization of gaseous potassium atoms that abstract a Cl anion from the
crystal lattice. The electron formed in the oxidation process becomes trapped in
the anion vacancy, as this will rebalance the overall charge of the crystal (Eq. 33):

ð33Þ [(KClÞ (KCl)] þK ðgÞ ! [(KClÞ (K)(e ފ
n ðsÞ n ðsÞ + KCl ðsÞ
Another process that may be used to generate an anion vacancy is through irradiation
of the crystal with ionizing radiation such as X-rays. This high-energy radiation will
cause the removal of a halide ion from the lattice and will excite some of the lattice
electrons from valence to conduction bands (see Section 2.3.7). At this point, the
electrons are free to diffuse through the crystal, where they remain mobile until they
find an anion vacancy site. At low temperatures (e.g., in liquid nitrogen), electrons
may even become localized by polarizing their surroundings; that is, displacing the
surrounding ions, to give self-trapped electrons. For each type of electron trap, there
is a characteristic activation energy that must be overcome for the release of the
electron. As an irradiated crystal is heated, electrons are released from their traps by
thermal activation, leading to a change in the observed color. The free electrons are
able to migrate once again through the crystal until they recombine with an anion
hole. This phenomenon has been studied in detail for aptly named “chameleon
diamonds”, which undergo color changes from greyish-green to yellow when they
are heated/cooled (thermochromic behavior) or kept in the dark (photochromic
behavior). In these diamonds, the color change is thought to arise from electron
traps created by the complexation of H, N, and Ni impurities. [50]
Everyone is familiar with the coloration phenomenon of gemstones such as ruby.
In these crystals, the brilliant colors are due to the presence of transition metal
dopants. Table 2.11 lists some common gemstones, and the respective host crystal

Table 2.11. Active Dopants in Gemstone Crystals
Gemstone Color Host crystal Impurity ion(s)
Ruby Red Aluminum oxide Cr 3+
2+
Sapphire Blue Aluminum oxide Fe ,Ti 4+
Emerald Green Beryllium aluminosilicate Cr 3+
Aquamarine Blue-green Beryllium aluminosilicate Fe 2+
Garnet Red Calcium aluminosilicate Fe 3+
Topaz Yellow Aluminum fluorosilicate Fe 3+
Tourmaline Pink/red Calcium lithium boroaluminosilicate Mn 2+
Turquoise Blue-green Copper phosphoaluminate Cu 2+