The remaining polymorph is a glass or amorphous form; it can be made by fusing any of the others.

Preparation

P₄O₁₀ is prepared by burning white phosphorus with a sufficient supply of oxygen:

: P₄ + 5 O₂ -> P₄O₁₀

The dehydration of phosphoric acid to give phosphorus pentoxide is not possible, as on heating it forms various polyphosphates