Gem-quality diamonds

All static studies at pressures beyond 25 GPa are done with diamond-anvil cells conceived independently by Jamieson [32] and by Weir etal [33]. In these variants of Bridgman s design, the anvils are single-crystal gem-quality diamonds, the hardest known material, truncated with small flat faces (culets) usually less than 0.5 nun in diameter. Diamond anvils with 50 pm diameter or smaller culets can generate pressures to about 500 GPa, the highest static laboratory pressures equivalent to the pressure at the centre of the Earth. 

The crystal figures shown in Fig. 9.2 were selected from sketches that appeared in a series of books published between 1913 and 1923, and all represent forms of single or twinned crystals larger than a few millimeters. They are mostly gem-quality diamonds. Other than these forms, there are translucent or opaque... 

Research. A significant impact on research at high pressure has come about with the use of gem quality diamonds as Bridgman-type anvils in a small compact high pressure device (40—42). With this type of apparatus, pressures greater than those at the center of the earth (360 GPa = 3.6 Mbars) have been reached, and phase transformations of many materials have been studied. Because of the x-ray transparency of diamond, it is possible to determine the structure of the phases under pressure. Because of the strenuous environment, crystals selected for this application have to be of very high quality. 

AH = 2.9 kJ mol-1 at 300 K and 1 atm. Diamond is the hardest material known, although a compound containing boron and nitrogen, borazon, has almost equal hardness. Because of this property, diamond has a great many industrial uses in tools and abrasives. Because gem quality diamonds are prohibitively expensive for such uses, there has been a great deal of interest in producing diamonds synthetically. Industrially, this process is carried out under extreme conditions (3000 K and 125 kbar) to produce several tons of diamond annually. 

Natural gem-quality diamonds are expensive and difficult to machine and fabricate into useful shapes for general industrial purposes. Except for a few cases (i.e., diamond-studded rotary drill bits, dressers, diamond-tipped glass cutters, fine superabrasive powders... 

Because diamonds are so much more valuable than the other forms of carbon, it has long been the dream of entrepreneurs to make gem-quality diamonds from other carbon-based substances. In fact, we have been able to make artificial diamonds for about 50 years. One of the first crude diamonds was actually made by compressing peanut butter at high pressures and temperatures ... 

In an attempt to understand further the structures of coals, the n.m.r. spectra of a series of four coals were compared with the corresponding spectra of gem quality diamonds (chemical shift 156 3 p.p.m. from CS and powdered natural graphite (chemical shift 35 p.p.m. from CSg). The spectra were rationalized in terms of increasing carbon aromaticity with increasing coal rank, leading finally to the formation of graphite-like structures. The chemical shift in diamond was found to agree well with empirical... 

In addition, the CVD process is now being used to synthesize gem-quality diamonds. Companies have now been formed that can grow gem-quality diamonds as large as several carats. These synthetic diamonds are virtually indistinguishable from diamonds formed by natural processes. This has tremendous implications for the natural diamond industry, which is worth more than 60 biUion per year. 

Figure 3.6 Raman spectra of carbon coatings. Spectrum (a) is from a natural gem-quality diamond. Spectrum (/>) is from a high-quality diamond film. Spectrum (c) is from a diamond-like (amorphous) carbon coating. Spectrum (d) is from a glassy carbon film. ...

The basic principle of the DAC is very simple. As was first reported in 1959 in the description of the original opposed-anvil diamond high pressure cell developed at the National Institute of Standards and Technology (NIST) (formerly the National Bureau of Standards) [1], two miniature diamond anvils (single-crystal, gem-quality diamonds about 1/3 carat each) are in an opposed-anvil configuration as shown in Fig. 1, similar in concept to Bridgman anvils. 

Lindblom J, Hosla J, Papunen H et al (2003) Differentiation of natural and synthetic gem-quality diamonds by luminescence properties. Opt Mater 24 243-253 Lindblom J, Holsa J, Papunen H (2005) Luminescence study of defects in synthetic as-grown and HPHT diamonds compared to natural diamonds. Am Miner 90 428-440 Mayer I, Layani J, Givan A et al (1999) La ions in precipitated hydroapatites. J Inorg Biochem 73 221-226... 

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