Viscous Flow and Melt Allotropy of Phosphorus Pentoxide

The viscosity of liquid phosphorus pentoxide at temperatures both above and below the melting point of the tetragonal crystal has been determined by the falling sphere method. At the melting temperature of 580°, the viscosity is 5.15×106 P and the activation energy for viscous flow is 41.5 kcal/mole. These values suggest that similar to liquid B2O3 or SiO2, the melt of tetragonal P2O5 can be classified as a network liquid. From qualitative observations on melting behavior and a consideration of the refractive indices of the crystalline and glassy phases, hexagonal P2O5, on the other hand, gives a molecular liquid on fusion. The ``hexagonal'' liquid, however, is unstable and undergoes rapid polymerization to give the network liquid.