Question 15.5: Impurities in Diamond The canary or fancy yellow diamonds ca......
Impurities in Diamond
The canary or fancy yellow diamonds called Type Ia diamonds contain substitutional nitrogen impurities at a concentration of less than 100 ppm. The presence of nitrogen creates absorption levels in the band structure of diamond that make it possible for electrons to absorb light so that the diamond is no longer colorless (see Chapter 19). When nitrogen is present but exists at levels less than 100 parts per million (ppm), a diamond will appear yellow. If the concentration of nitrogen in diamond is 100 ppm, what is the concentration of nitrogen atoms per unit cell and per cm³? The lattice parameter of diamond is 3.5668 × 10^{-8} cm.
The diamond cubic crystal structure contains eight atoms per unit cell. If there are 100 nitrogen atoms per 10^6 carbon atoms, i.e. 100 ppm, then
(\frac{100 \ N \ \text{atoms}}{10^6 \ C \ \text{atoms}})(\frac{8 \ C \ \text{atoms}}{\text{unit} \ \text{cell}}) = 8 × 10^{-4} N atoms/(unit cell) or 1250 unit cells/(N atom)
Assuming that the presence of nitrogen does not alter the lattice parameter of diamond, then
(\frac{8 × 10^{-4} \ N \ \text{atoms}}{\text{unit} \ \text{cell}}) \left[ \frac{\text{unit} \ \text{cell}}{(3.5668 × 10^{-8} \ cm)^3 }\right] = 1.76 × 10^{19} \ N \ \text{atoms}/cm^3
Diamond also has industrial applications. It is the hardest naturally occurring material. Industrial diamonds are used as abrasives for grinding and polishing as well as abrasion-resistant coatings for many different applications (e.g., cutting tools). Synthetic diamond and diamond-like coatings are prepared using chemical vapor deposition processes (Chapter 19) as well as through high temperature-pressure processing.