Diamond As The King Of Materials

How did diamond acquire its royal quality above all materials? The secret lies in its ability to form 4 covalent bonds that defines the simplest three-dimensional volume — a tetrahedron. Each atom in a crystal lattice can be surrounded by 0,1,2,3,4,6,8, and 12 neighbors. The increasing trend of this "kissing number" (coordination number) marks the gain in order (enthalpy) of a pile of atoms at the expense of the chaos (entropy). There is only one type of extreme order and one type of extreme chaos, so the compromise between them would allow diversification and complexity. The diversification of the tetrahedral bonding in diamond has helped enrich the field of organic chemistry, whereas its complexity has prompted the birth of biology.

A kissing number less than four define a point (e.g. gas molecules), line (e.g. polymer molecules), or plane (e.g. foliated molecules) relationship. This relationship lacks either a structure or it defines a loosely attached structure. A low kissing number will highlight the local relationship and hence its exhibits insulating properties. On the other hand, with a larger kissing number beyond 4, the structure of the solid will become tighter so that the global property of metal becomes dominating. The delicate balance between a loose and tight structures lies exactly at the kissing number of 4. The flexibility of the tetrahedral bonding in diamond reflects this well. This intermediate structure makes diamond's property fall between an insulator and a metal, and therefore, allows diamond to be a semiconductor.

Moreover, the packing between a loose and tight structure can allow for flexibility of atomic positions that are not possible in insulators or metals. Carbon atoms with the capability to form tetrahedral diamond bonding (e.g. methane and its polymers) and planar graphitic bonding (e.g. benzene and its derivatives) have become the corner stone of the organic chemistry. The open tetrahedral bonding also serves as the building block of many versatile semiconductors that power indispensable consumer products

The Wonder of Tetrahedral Bonding

Fewest Atoms to Form Volume (Tetrahedron) • Abundance Flexible Structure to Allow Complexity [Information) — Life

<4 Bonds Chaos (Entropy)

4 Ronds Complexity (Information)

>4 Bonds Order (Enthalpy)

Noble Common Discrete Layered Gas Gas Molcculc Structure (He) (02) (HjO) (Graphite)

Organic Chemistry (Most Compounds)

4 Ronds Complexity (Information)

Noble Common Discrete Layered Gas Gas Molcculc Structure (He) (02) (HjO) (Graphite)

Simple Semi Ductilc

Salt Metal Metal

Diamond Structure

Simple Semi Ductilc

Salt Metal Metal

Semi-Conductors (Most Computation)

Biochemistry (Most Intelligence)

Diamond Structure

Nano Devices (Most Technology)

Figure 3.3. Locally connected atoms with a kissing number less than 4 cannot form an integral solid. On the other hand, the globally dominated structures with a kissing number greater than 4 do not allow for many variations in structure. The compromise between local and global allows a versatile tetrahedral bonding that makes complex structures possible. This complexity has prompted the formation of organic compounds and the evolution of living organisms. Tetrahedral bonding has also enabled semiconductors for the making of integrated circuitry.

such as computers, lasers, and other electronic devices. Alternatively, the versatility of tetrahedral bonding can be coupled with closer packed metal atoms to form advanced composite materials for micro electro-mechanical devices (Fig. 3.3).

Although tetrahedral bonding is the common feature of all semiconductors, diamond has the smallest atoms and therefore it possesses the highest atomic density (176 atoms/nm3) of all the semiconductors, and in fact, of all materials. The combination of the highest atomic density with the largest number of covalent bonds (4) has lead to diamond's concentrated bond energy (7.4 eV). This strong bonding of atoms has imparted diamond with its unique properties that single this material out as the King of Materials.

Although diamond's "royal status" stems from its ability to form tetrahedral bonds, this ability is actually the result of carbon's "crown position" in the periodic table of elements (Fig. 3.4). Carbon is located at the center column (Group IV) and top period

Li

Be

B

C

N

O

F

Na

MG

Al

Si

P

S

Cl

K

Ca

Ga

Ge

As

Se

Br

Rb

Sr

In

Sn

Sb

Te

I

Cs

Ba

Tl

Pb

Bi

Po

At

Figure 3.4. Periodic table showing solid materials with atomic bonding. This table excludes hydrogen and the noble gases that form solids only with weak metallic bonds (e.g. metallic H) or with polarized (van der Waals) electrostatic force (e.g. cryogenic He). The transitional metals have inner filling elements of d and f orbitals so they are hidden in the parental Group II. This periodic table highlights carbon's crown position at the center column and top row. The center column gives carbon the highest number of covalent bonds per atom (4). The top position makes its atoms the smallest among all center column elements. The combination of these two attributes imparts diamond with the highest concentration of bond energy, and thus lends it unsurpassed hardness and other useful properties.

of all elements capable of forming chemically bonded solids. At part of the center column, carbon is impartial between valence electron deficient metals (kissing number less than 4) on the left side of the periodic table, and valence electron surplus insulators (kissing number greater than 4) on the right side. The atoms of the left side elements (Groups I, II, and III) have many electron vacancies so they can attract more neighbors (kissing number = 6, 8, 12) to share their few valence electrons. The consequence of sharing few valence electrons by many neighbors is the formation of isotropic metallic bonds. On the other hand, the atoms of the right side elements (Group V-VII) possess many valence electrons that repel one another. As a result, they can allow for only a few neighbors (kissing number = 1, 2, and 3) to share their valence electrons. The consequence of sharing many by few is the formation of directional covalent bonds. Compounds formed between indiscriminate metallic elements and selective covalent elements possess intermediate ionic bonds.

Covalent bonds have the most concentrated electron cloud between atoms so they are much stronger than metallic bonds or ionic bonds. The center column of elements of diamond and silicon possess four covalent bonds, the most before these bonds become metallic with further increases of kissing number. As a result of this high density of covalent bonds, carbon or silicon possesses the most rigid crystal lattice among all structures formed by the same or higher period elements.

Carbon sits at the top of the middle column of elements so its atoms are the smallest. Hence, diamond, which is comprised of carbon, has the highest bond energy density of all materials. Diamond is the miracle material with so many extreme properties that it overshadows all the other materials.

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