handbook of carbon, graphite, diamond and fullerenes

266 Carbon, Graphite, Diamond, and Fullerenes
Effect of Impurities and Structural Defects. As seen above,
diamond would be the ideal transparent material if it were totally free of
impurities, particularly nitrogen, and had a perfect structure. Howeverthese
conditions are never completely achieved, and impurities and crystal-lattice
defects and other obstacles to the free movement of photons affect its
transmittance. These obstacles add a number of absorption bands to the
two mentioned above, particularly in the IR region as shown in Table 11.4.
In spite of this, diamond remains the best optical material.
Lattice vacancies (missing atoms) may considerably alter the valence
bonds and cause electrons to be exited by a much smaller amount of energy
(such as produced by a photon of red light) that would normally be required
in a perfect lattice. A diamond containing such lattice vacancies appears
blue since the red components of light (the one with less photon energy) are
absorbed. A minimum of one vacancy per 10 5 atoms is necessary for the
blue color to be noticeable.! 11 )
Table 11.4. Optical Absorption of Diamond by Type
Type Optical absorption bands
la IR: 6-13 microns
UV: