Pyrite Type of Silicon Diphosphide

Rabenau, H. Rau, and G. Rosenstein, Z. Anorg. Allgem. Chem., in press. 

Rau and A. Rabenau, III Ait Conf. Solid Compds. Transition Elems., June 16-20,Oslo, Norway, 1969. 

The synthesis of black, pyrite type of silicon diphosphide was first accomplished at high pressure1 (15-50 kb.). It has since been prepared by vapor-transport techniques.2 3 A low-pressure, red form of SiP2 is produced by sublimation from 900-500°C. in a 

The chemical bonding in the two forms is quite different. The low-pressure form has the GeAs2 type of structure, in which the coordination of the atoms is normal, i.e., fourfold for Ge and threefold for As. In the pyrite type silicon is octahedrally coordinated, and phosphorus is tetrahedrally coordinated. The difference is reflected in the density and properties. The density of the GeAs2 type is 2.47 g./cc., whereas for the pyrite type it is 3.22 g./cc. The pyrite type is a metallic conductor, whereas the GeAs2 type is a semiconductor. 

Reactants should be weighed in the atomic ratio of 1 Si/ 2.0-2.5P. The presence of excess phosphorus has been found to enhance reaction and impede the formation of silicon monophosphide. Since a large excess of phosphorus increases the chance of explosion from its vapor pressure, an optimum ratio is 

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