When rhombohedral BN, which has a layer structure with a three-layered stacking sequence, was shock-compressed at 40, 60, and 100 GPa, it was converted to cubic BN. Hexagonal BN was converted to wurtzite-type BN under the same conditions. These results indicate that the transformation proceeds by a diffusionless mechanism in which the stacking sequence of the BN layers in the starting materials is retained during the process. EXAGONAL boron nitride, which has a Hlayer structure with a two-layered sequence (MM . . . , Fig. l(A)), is converted to wurtzite-type BN by shock compression, but not to the cubic form.' The conversion is well explained by a diffusionless mechanism, in which the distance between the layers is reduced without mutual positional replacement among the B and N atoms. For rhombohedral BN, which has a three-layered stacking sequence (ABCABC . . . , Fig. 1 ( B ) ) , such a mechanism should result in conversion to cubic BN, which also has a three-layered stacking sequence along the triad axis.In the present work, an attempt was made to produce cubic BN from the rhombohedra] form by means of explosive shock compression. The starting rhombohedral BN was prepared by heating B203 mixed with hexagonal BN powder in a stream of NZ in a carbon resistance furnace.* The product was a white felt-like material composed of whiskers 0.1 to 1 p m in diameter elongated along the c axis (Fig. 2). The sample was mixed with 200-mesh Cu powder in proportions of 4/90 by weight, and the mixture was packed tightly into a stainless steel capsule at a pressure of 400 to 800 MPa to a density >85% of theoretical.An explosive plane-wave "mousetrap" generator3 was used to produce shock compression. Pressures of 40,60, and 100 GPa (estimated by the impedance matching method) were induced in the capsule. For comparison, hexagonal BN was shockcompressed under the same experimental conditions.The shock-compressed samples were treated with dilute HNO, and HCI to remove the Cu. Composition was determined by electron-probe microanalysis; the main components were B and N, except for a small amount of 0 and a trace of Cu. Figure 3 shows the X-ray diffraction patterns of the shock-compressed samples and starting materials. The diffraction peaks of the product of shock compression-of hexagonal B E were identified as the 1010, 0002, and 101 1 peaks of wurtzite-type BN and the peak of the starting material; no cubic BN peaks were formed. This result agrees with that of Sawaoka et al. ' In the case of shock-compressed rhombohedral BN, peaks corresponding to the 1 1 1, 200, and 220 peaks of cubic BN were identified in the diffraction pattern; the 0003 peak of the starting rhombohedral BN was supressed in intensity to <2% of its initial value. Some peaks remained unidentified; their locations and intensities depended on the density of the mixture in the capsule and the pressure induced by shock compression.The X-ray diffraction peaks of cubic BN appear at almost the same locations as those of Cu. Although a trace of Cu was detected in the sho...
