A denser, more perfect diamond.

Scientists have created a diamond that packs in more atoms per cubic centimeter than any other material known to exist at atmospheric pressure. Chemists from the General Electric Research and Development Center in Schenectady N.Y., made the synthetic diamond using carbon atoms containing an extra neutron. Natural diamond usually contains no more than 1 percent of this carbon-13 isotope, but carbon-13 represents 99 percent of the new gem, says William F Banholzer, a GE chemical engineer.

High-resolution X-ray diffraction measurements of one natural diamond and five of Ge's synthetic stones show that the heavier carbon atoms crowd a little more closely together than do carbon-12 atoms, says Harry Holloway, who carried out the analysis with his colleagues at Ford Motor Co. in Dearborn, Mich. In the Oct. 1 PHYSICAL REVIEW B, the Ford and GE scientists also describe the superior crystal quality of these synthetic diamonds. The quality almost matches that of silicon semiconductors, which include some of the least-flawed crystals known, Banholzer says.

The GE researchers produce the gems by first making clusters of very tiny diamonds that contain the desired amount of the heavier carbon. To do this, they use a process called chemical vapor deposition and control the diamond's composition by varying the isotopic content of the methane gas used as the carbon source. Then the researchers apply very high pressure to dissolve and recrystallize a cluster into a single gem.

Previously, GE scientists had discovered that by lowering the carbon-13 content below that of natural diamonds, they could make synthetic diamond that conducts heat more efficiently and withstands more laser energy than the best natural diamonds (SN: 7/21/90, p.37). The researchers are now testing the properties of the carbon-13 diamonds, and they expect the new synthetics to surpass the hardness of natural diamonds.