Polymorphism and High Pressure

Polymorphism and High Pressure
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Polymorphism and High Pressure

Author H. Tracy Hall

Elements and compounds often exist in more than one solid form. The different solid forms of the same compound or element are called polymorphs. A classic pair of polymorphs are the substances graphite and diamond. These materials have radically different properties. Diamond is the hardest substance known, is transparent, does not conduct electricity, and has a density 36 percent greater than graphite. It is also rare and in its least costly form, that of industrial diamond grit, is valued at $6,000 per pound. Graphite, on the other hand, is soft and unctuous, is black and opaque, conducts electricity, is relatively common (the primary ingredient in the so-called lead-pencil is graphite), and costs but a few cents per pound. In spite of these gross differences, diamond and graphite are both composed of the element carbon. This singular fact was discovered by the French chemist, Antoine Lavoisier, in the year 1792. From that time forth scientists set about to find a way to transform the inexpensive graphite into the expensive diamond. As many of you know, it was my fortune to first effect this polymorphic change. Since the December day in 1954 when the first tiny, sparkling, man-made crystals were observed, more than 10,000 pounds (over 22,000,000 carats) of diamond grit have been manufactured for industrial consumption.

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