Phosphorus pentoxide forms three solid modifications, of which the metastable M form is the ordinary commercial P2O5. This modification crystallizes as rhombohedra with a molecular lattice (P4O10) and sublimes readily at 250°C and 10 mm. Above 260°C and even more quickly above 500°C, form M changes into form R. The latter crystallizes in a three-dimensional atomic lattice of PO4 tetrahedra and is less volatile. A form S crystallizes in a layer lattice, and a few vitreous forms are also known. Modification M is brittle; R and S form hard crystals, which deliquesce after a few hours in the air and undergo considerable swelling in water. The density of R is 2.72, and that of M is 2.30 (calculated from x-ray data).
