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A new crystalline form of carbon—hexagonal diamond—has been synthesized in the laboratory under conditions of static pressure exceeding about 130 kbar and temperature greater than about 1000°C. It is necessary to start with well-crystallized graphite in which the c axes of the crystallites are parallel to each other and to the direction of compression. There is electrical evidence that the transformation starts at room temperature but hexagonal diamond is not retrieved unless a setting temperature exceeding about 1000°C is applied. The electrical and crystal characteristics have been studied. The crystal structure is hexagonal with a=2.52 Å and c=4.12 Å. The theoretical density is 3.51+g/cm3, same as cubic diamond. It has also been prepared recently in another laboratory from crystalline graphite by a method involving intense shock compression and strong thermal quenching. More recently it has been discovered to be present to the extent of over 30% in the Canyon Diablo meteorite diamonds.

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Hexagonal, graphitelike boron nitride may be changed directly into the zincblende cubic form reported earlier or into a new wurtzite form by the application of static high pressures. No catalyst appears to be necessary. At high temperatures, between about 2500° and 4000°K, the zincblende form is favored; at lower temperatures, down to 300°K, the wurtzite form is favored; frequently both forms appear together. The minimum pressure required for the transformations is about 115 kbar at 2000°K; somewhat higher pressures, of the order of 130 kbar, suffice at higher and lower temperatures. The crystallites of the dense phases are small but give good x-ray diffraction patterns from which the crystal structures can be determined.

The direct transition behavior among the graphitic (hBN), wurtzitic (wBN), and zincblende (zBN) crystal forms of boron nitride is investigated as a function of temperature for pressures up to 130 kbar. At pressures in the 45 to 70 kbar range, direct transformation of both the hBN and wBN forms to the zincblende form are observed and at higher pressures (85 kbar and above) direct transformation of hBN to wBN is also observed. A pressure/temperature phase diagram is presented for pressures up to 130 kbar. In this pressure range, the thermodynamically stable solid phases are hBN and zBN, the experimental behavior indicating that wBN is not thermodynamically stable over this range. From temperature/time data, the activation energy for both the hBN to zBN and wBN to zBN transitions is estimated to be about 200 kcal/mole. From these high activation energies it is concluded that the direct conversion processes essentially require disruption of the hBN and wBN lattices before the atoms can re-form into the zincblende structure.

Static pressure apparatus has been developed which is capable of pressures up to 200 kbar and transient temperatures up to about 5000°K, using an electric flash-heating technique. At pressures above about 125 kbar and temperatures in the 3000°K range it is found that graphite spontaneously collapses completely to polycrystalline diamond which may be retrieved quantitatively. The threshold temperature of the transformation is several hundred degrees lower than the melting temperature of graphite. The diamond/graphite/liquid triple point is found to be located at about 125 (+10, —0) kbar and 4100 (±200)°K. The shock compression results of DeCarli and Jamieson, and of Alder and Christian, are linked with the present results to construct a phase diagram of carbon extending to 5000°K and 800 kbar.

Diamond or cubic boron nitride particles can be sintered into strong masses at high temperatures and very high pressures at which these crystalline forms are stable. Most of the desirable physical properties of the sintered masses, such as hardness and thermal conductivity, approach those of large single crystals; their resistance to wear and catastrophic splitting is superior. The sintered masses are produced on a commercial scale and are increasingly used as cutting tools on hard or abrasive materials, as wire-drawing dies, in rock drills, and in special high-pressure apparatus.

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