The Atomic Distribution in Red and Black Phosphorus and the Crystal Structure of Black Phosphorus

No satisfactory crystal structure determination has been made of any of the forms of phosphorus. Five samples of black phosphorus gave identical powder patterns different from those reported by Linck and Jung, whose rhombohedral structure gives improbable interatomic distances and coordination. A sample which had been prepared at room temperature and 35,000 atmospheres gave the diffuse rings of an ``amorphous'' x-ray pattern. Another sample, prepared at 300°C and 8000 atmospheres, is a new form of phosphorus, having the same density as black. Atomic distribution curves of crystalline and ``amorphous'' black and red phosphorus were obtained by the method of Fourier analysis. All four gave practically the same curve, showing three neighbors at 2.28A and about twelve at 3.6A. With the aid of the atomic distribution curves and the fact that certain lines showed a preferred orientation, it was possible to determine the structure of black phosphorus. It consists of double layers; the cell is side-centered orthorhombic with a=3.31A, b=4.38A and c=10.50A. There are eight atoms in the cell, giving a calculated density of 2.69 compared with 2.70 observed. The space group is Vh18—Bmab. The black phosphorus structure allows a closer packing than the arsenic structure, where the atomic environment is similar, and so is favored by the high pressure.