handbook of carbon, graphite, diamond and fullerenes

Structure and Properties of Diamond 255
material being tested as accurately as possible in order to obtain a true
evaluation of the measured properties.
Types of Impurities. Diamond, synthetic or natural, is never completely
free of impurities. These impurities are divided into two different types:
1. Lattice impurities which consist of foreign elements
incorporated in the lattice, the foreign atom replacing a
carbon atom.
2. Inclusions, which are separate particles and not part of
the lattice, usually consist of silicates of aluminum,
magnesium, or calcium such as olivineJ 11 '
The two major lattice impurities found in diamond are nitrogen and
boron. These two elements are the neighbors of carbon in the periodic table.
They have small atomic radii and fit readily within the diamond structure.
Other elemental impurities may also be present but only in extremely small
amounts and their effect on the properties of the material is still uncertain
but probably minorJ 12 !
Nitrogen. Nitrogen impurity in diamond is detected and characterized
by IR absorption and paramagnetic resonance. The majority of nitrogen
atoms within the diamond structure are arranged in pairs as shown in Fig.
11.8J 11 ' Isolated nitrogen atoms are rarer. Nitrogen platelets are also
present and can be represented as a quasi-planar structure within the cube
{100} plane of the diamond crystal. Their exact form is still controversial.
Carbon atom
Nitrogen atom
Figure 11.8. Schematic of a nitrogen pair impurity in the lattice structure of
diamond.' 9 ]