Diamonds are hard and beautiful crystals, and they are known to be the most ancient material. Other than the un-parallel sparkle that makes it the best choice for jewels, its chemistry and compactness make it the strongest of all the natural substances. Due to their wonder properties, they are applicable in many fields like industry and research. Its unique chemistry, composition, and crystal structure is discussed below
Chemistry of Diamond and Carbon Composition
The chemical composition of a diamond can be understood by knowing its constituent element, Carbon. Carbon atoms possess six protons and six neutrons in the nucleus, which is balanced by six electrons. The electronic configuration of Carbon is defined by 1s2, 2s2, 2p2. Carbon has four valence electrons that can be used to fill the 2p orbital. The diamond structure is composed of repetitive units of carbon atoms linked with four additional carbon atoms through the strongest chemical bond, covalent bonds.
The carbon atoms are a rigid tetrahedral association that is equidistant from the adjacent carbon atoms. The structure of a diamond is composed of eight atoms, essentially linked in the form of a cube. The bond is rigid and stable; that’s why the diamonds are extremely hard and possess a high melting point.
Crystal System of Diamonds
There are a total 7 crystal systems that explains the structure and symmetry of different atoms. Diamond falls in the category of “Cubic crystal system” also known as “The Bravais lattice”. Diamond is the allotropic form of carbon and the space lattice of the diamond is face centered cubic (FCC) and two atoms in the basis.
The Bravais Lattice
It is implemented to characterize the crystals without any imperfections and defects, and they have regular atomic arrangements. It is a mathematical set of guidelines that are equivalent to one another. The different symmetry operations for a 3D set of points form 14 different Bravais lattices. Each lattice shows a set of points in space which creates a periodic structure. Each point possesses the same environment. The lattice points are characterized by a lattice vector R as follows:
R = n1a1 + n2a2 + n3a3
Where n1, n2, and n3 are integers, whereas a1, a2, and a3 are the three independent vectors, respectively.
Of the 14 Bravais Lattices, a valuable role is played by the face-centered cubic lattice, as this is the structure that is visible in various semiconductors. It is noteworthy to be aware of the arrangement of adjacent atoms, specifically the nearest neighbors (NNs). The number of nearest neighbors of the atoms is referred to as the coordination number.
Crystal Symmetry
An ideal crystal structure has a specific symmetry. Crystal symmetry is characterized by a set of operations that leave the crystal unchanged. Operations that are applied via fixed points in the unit cell are known as the point group operations. These operations comprise rotations, reflections, inversions, and translations. In the crystals, rotations are only permitted for specific angles ϴn = 2π/n, where n = 2, 3, 4, 6, consistent with translations of lattice vectors. Such axis symmetry is the nth fold axis and is represented as Cn. It consists of
- 3 C4 axes, which pass through centers of opposite faces.
- 4 C3 axes are found along diagonals
- 6 C2 axes, which pass through midpoints of opposite sides.
Each axis has clockwise and anticlockwise rotations and the same rule applies to the other rotational axis.
Diamonds’ Crystal Structure
Some of the important features of the face-centered cubic diamond structure are listed below:
- Crystal Structure: Diamond
- Bravais Lattice: Face- Centered Cubic (FCC)
- Pearson Symbol: cF8
- Space group: 227 (F d 3m)
- Strukturbericht: A4
- Point group: m3m (Oh)
- Six 2-fold rotations; four 3-fold rotations; three 4-fold rotations; nine mirror planes, inversion
- Lattice Constant: α, αC = 0.3567 nm, αSi = 0.531 nm, αGe = 0.5658 nm, αα-Sn = 0.646 nm
- Atomic density natoms = 8/a3
Diamond cubic structure (a) the unit cell illustrates all the atoms (b) (001)-plan view of the structure in which positions labeled represents the position in the z-direction only whereas x and y-axis positions are self-explanatory
The diamond structure is a combination of two identical penetrating FCC lattices. One of the two sublattices is moved along the body diagonal to the cubic unit by one-quarter of the length of the diagonal. The diamond crystal structure is, therefore, FCC with the basis that contains two identical atoms.
A basic method of constructing a diamond crystal lattice supposes it as an FCC structure having a spare atom situated at ¼ a1+ ¼ a2 + ¼ a3 from each FCC atom. The basic component of the structure is a tetrahedron in which the C atom is placed at the center, and the rest of the four nearest neighbors (NNs) are situated at the corners of the cube structure (or vice versa). Atoms present in the diamond-shaped crystals form the covalent bonding. The covalent bonding energy is linked with the shared valence electrons among the atoms and depends on the relative orientation of the atoms.
Carbon atoms are present at FCC positions that are at eight corners and six face centers of the cube.
Position of carbon atoms
- Carbon exterior atoms
- Located at the corners
- At the face centers
- Carbon interior atoms
- 1/4,1/4,1/4
- 3/4,3/4,1/4
- 3/4,1/4,3/4
- 1/4,3/4,3/4
Each atom of the interior makes a tetrahedral bond with one atom at the nearest neighbor corner and three centered on the nearest faces.
Diamond Unit Cell
Atoms in diamond unit cell= 8
At the corner = 1
At face centres = 3
Interior of the cube = 4
Total = 8
Coordination number = 4
As already mentioned, each atom of the interior makes a tetrahedral bond with one atom at the nearest neighbour corner and three centered on the nearest faces. So the nearest neighbours are “4”. Carbon atoms also present at 4 out of 8 tetrahedral positions in FCC unite cell.
Contribution of atoms in one FCC lattice
Corners x (contribution at corners) + Atoms of face centres = Total atoms in the FCC lattice
4 x ¼ + 1 = 2
Thus the basis which consists of carbon atoms has two identical atoms at coordinates. So it shows that it is an FCC structure with the basis of 2 atoms. In diamond, carbon is sp3 hybridized and each carbon atom is bonded to four other carbon atoms in tetrahedral symmetry.
Crystal Habit
In the diamond crystal structure, carbon atoms arranged in a tetrahedron form (triangular prisms). It is due to its cubic and the diamond crystal can be morphed into various shapes. It is referred to as the ‘crystal habits.’ The most widely occurring crystal habit is the eight-edged octahedron or the diamond shape. Diamond crystals can be produced in the form of cubes, dodecahedra, etc.
These structures are the expressions of the cubic crystal system, excluding the two shape classes. One exception is the etched crystals class, which is indicated by rounded surfaces, and the other one is the flat form known as macle, which is, in fact, a composite crystal.
Trigons
Real diamonds do not possess a completely smooth face, rather they may have indented or raised triangular protruding elements called ‘trigons.’ Diamond structures have perfect cleavage in four varying directions, implying that a diamond can be cut neatly along these directions smoothly without breaking in a jagged and random manner. The lines of the cleavages are produced as a result of diamond crystals with lesser chemical bonds along the octahedral plane side instead of other directions. Professional diamond cutters take the benefits of lines of cleavage to feature gemstones.
Stability of Diamond Structure
Graphite is just a tad bit more stable than diamond. However, the activation barrier for transformation needs around as much energy as the destruction of the entire crystal lattice and reconstructing it. Hence, once the diamond is created, it cannot be reconverted since the barrier is too high. Diamonds are considered metastable because they are kinetically stable instead of thermodynamically stable. Under the high temperature and pressure conditions required to produce a diamond, its structure is practically more stable than graphite, and therefore around millions of years later, carbonaceous residues may eventually crystallize themselves into diamonds.
Thus, Diamond’s crystal structure is typically known as the diamond cubic structure. A single unit cell consists of two FCC lattices, one at (0, 0, 0) axis and another at (1/4, 1/4/ ¼) axis. In the case of compounds, FCC crystal lattice can be created by one kind of atom and remaining atoms, commonly from the same group, filling half of the tetrahedron.
The atomic arrangement of the diamond structure is beneficial in explaining the chemical, mechanical, and metallurgical properties. The semiconductor crystal can be morphed along specific atomic planes to form exceptional planar surfaces, for instance, diamonds used in various jewelry items. These surfaces are used as Fabry-Perot reflectors in semiconductor lasers. Thus, chemical reactions among these crystals, like etching, are often preferred in the desired directions.