Tetraphosphorus Heptasulphide, P₄S₇

Tetraphosphorus Heptasulphide or Phosphorus Tetritaheptasulphide, P₄S₇, was first obtained during the distillation of P₄S₆ in a vacuum, and was separated by heating under pressure with carbon disulphide in which, as distinguished from P₄S₃, it was only sparingly soluble. In the methods of preparation which have been described the phosphorus should be in slight excess over that required for P₄S₇. The ingredients may be heated together in a sealed tube and the product recrystallised from carbon disulphide in pale yellow crystals.

The melting-point was 310° C. and the boiling-point 523° C. A maximum melting-point corresponding to P₄S₇ was found on the thermal diagram of P₄S₆ and P₄S₁₀. The solubility in carbon disulphide was 0.0286 gram per 100 grams solvent at 17° C.

A compound having the empirical formula PS₂ was said to be formed by heating together the elements in the proportions theoretically required, or by distillation or heating with carbon disulphide in a sealed tube at 210° C. Other methods include the exposure to sunlight of P₄S₃ (1 part) with sulphur (2 parts) dissolved in carbon disulphide, or a solution of phosphorus and sulphur with a little iodine in carbon disulphide. Pale yellow transparent needles of the compound are deposited.

The molecular weight deduced from the vapour density was P₄S₈ or P₃S₆. The melting-point was 248° to 249° C. or 290° to 298° C. The boiling-point is given as 516° to 519° C.

The chemical properties are similar to those of the other sulphides of phosphorus, but the compound is less stable, and easily decomposes, giving P₄S₃ with separation of sulphur.