9

Diamond

There is no mineral, other than diamond, consisting of only sp3 covalent bonds which fits perfectly the purpose of this book. Diamonds are formed deep in the Earth under high-temperature and high-pressure conditions, and are uplifted during geological events to the Earth’s surface at a speed of 100 km/h. The history of natural diamond crystals, ranging from subduction of plates, growth of crystals in the upper mantle, plastic deformation associated with rapid uplifting, and partial dissolution experienced in the ascent process, is recorded within the crystal. Diamond crystals synthesized under high-temperature, high-pressure conditions, but from the solution phase with a different solvent component from that in natural crystallization, and those synthesized from the vapor phase under labile conditions, show different morphological characteristics, to the extent that a distinction between natural and synthetic diamonds can easily be made.

9.1Structure, properties, and use

Diamond C is the high-pressure phase of carbon, and the C–C bonding is of sp3 pure covalent nature. The structure has a three-dimensional framework as indicated in Fig. 9.1, and is different from the low-pressure phase, graphite, which has a sheet structure consisting of sp2 covalent bonds and Van der Waals bonds connecting the sheets. Other polymorphs called lonsdalite, fullerene, and carbon nanotube, which consist of mixed sp2 and sp3 bonds, are also known.

In diamond C, it is the fact that the structure consists of sp3 covalent bonds only that accounts for its unique physical properties, such as the highest hardness of any material (Mohs hardness 10), small compressibility (1.7 10 7cm2/kg), the highest elasticity among any known material (4 6 1012 dynes/cm2, bulk elasticity), large thermal conductivity (9 26 W/(deg cm)), and small thermal expansion coefficient (0.8 0.1 10–6 at 20 °C, comparable to the value of invar). As a result,

170Diamond

since the first diamond was found in India during the fourth to fifth centuries BC, diamonds have been used as a tool to process other hard materials, and even now they are used uniquely in a wide range of industrial processes, such as wire dies, tools to manufacture ultra-fine machining tools, surgical knives, high-pressure diamond anvils, heat sinks for compound semiconductors, optical windows for spacecraft, etc. Semiconductor diamond intended for use under extreme conditions is expected to become available in the near future.

The reasons why diamond became the most highly valued gemstone are (1) that the anisotropic properties in the hardness of diamond single crystals were recognized and a technique was created to polish diamond to a high brilliance using the powder, and (2) that the primary mother rock of diamond was found in the middle of the nineteenth century, which enabled the mining of the deposit on a large scale, resulting in a great increase in production. In turn, the brilliant-cut style was invented, which enhanced the optical properties (colorless, high transparency, high refractive index n 2.144), which promoted the position of diamond above that of other gemstones. Before this, colored stones, like emerald and ruby, were esteemed higher than diamond.

Owing to its single composition and pure covalent bonding, diamond is a standard solid material, and takes the role of the most appropriate sample in explaining the effects upon structure-sensitive properties when the structure of solid material deviates from the ideal state.

Natural diamonds are classified into Types I and II based on their transparency to ultra-violet rays. The two types have different physical properties, such as the absorption of ultra-violet and infra-red rays, the presence and absence of satellite reflections on X-ray diffraction, the roughness of cleavage surface, and the plastic deformation temperature; these differences arise principally from the amount of nitrogen that is present as an impurity and the state that the nitrogen takes. In Type I, which contains up to 0.2% N, the nitrogen usually presents itself in the form of clusters or precipitated platelets. This is because the crystal experienced an annealing process. In contrast, there are some diamonds, such as synthetic diamonds, which have not experienced an annealing process, and so the nitrogen appears in a substitutional form. The two forms are classified as Type Ia and Ib, respectively. Synthetic diamonds are mostly Type Ib, and most natural diamonds belong to the Type Ia group. Type II diamonds contain nitrogen at less than one part per million, and so they are crystals of purer carbon than Type I. When a small amount of Al or B is present in this type, the diamonds show semiconductor properties, and so are called Type IIb, discriminating them from the insulator Type IIa. Depending on the state of nitrogen clustering, Type I is further subdivided into Type Iaa, Type Iab, etc. Recently, the effect of another impurity, hydrogen, has attracted interest, and extensive studies have been made.

Diamonds are unique in existence as the standard sample for solid state physics,