THE CRYSTAL STRUCTURE OF THE NEW SILICON CARBIDE POLYMORPH 69R,
FOREIGN TECHNOLOGY DIV WRIGHT-PATTERSON AFB OHIO
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The 69R polymorph is one of the 32 silicon carbide polymorphs recently discovered by us. The space group is R3m and the unit cell is hexagonal with cell dimensions a 3.0806 A c 173.85 A z 69. This new polymorph was discovered in crystals prepared by three laboratory sublimation methods and in two industrial silicon carbide crystal plates. They all pair with the fundamental type 6H. The five ways of pairing are 6H 69R 87R, 6H 69R ., 6H 21R 39R 69R, 6H 69R and 6H 15R 15R 69R. The crystals are thin, almost hexagonal plates, 3 x 3 microns in size and about 0.2-1.0 micron thick. For 69R there are more than a hundred thousand ways of close stacking. Based on the known SiC structural analyses and considering the characteristics of the SiC crystal structure, it is possible to form the zigzag sequence from two or three of the pairs of numbers. These two or three pairs come from the eight pairs 22, 21, 23, 32, 33, 34, 43 and 44. Between twenty and a hundred different zigzag sequences can be formed from two numerical pairs, and several hundred kinds from three pairs. The crystal structure of polymorph 69R can be represented by the zigzag sequence 333333323.
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