TABLE OF CONTENTS The structure of vitreous phosphoric oxide is also based on a tetrahe-dral building block. Since phosphorus is a pentavalent ion, the formation of a phosphorus-oxygen tetrahedron with 4 bridging oxygens would result in a unit with a net positive charge of +1. A charge balanced tetrahedron can be created, however, if one of the oxygens forms a double bond with the P 5+ ion, while the other three oxygens form BO with adjacent tetrahedra. The 2-dimensional network formed by the connection of these tetrahedra at 3 corners thus has the same connectivity as that of vitreous boric oxide, even though the building blocks are 4-cornered tetrahedra. This network is easily disrupted, resulting in a very low glass transformation temperature for vitreous phosphoric oxide.
Details of the structure of vitreous phosphoric oxide are dependent upon the source of P 2O 5 used to produce the melt. Crystalline phosphoric oxide exists in three polymorphic forms: hexagonal, orthorhombic, and tetragonal. These crystals all contain phosphorus-oxygen tetrahedra, but contain different intermediate range units in the form of rings with different numbers of tetrahedra per ring. Glasses produced using different starting materials retain some of the structural details of the crystalline form for short melting times, with only gradual convergence of properties toward equilibrium values after extended time at high temperature.
Addition of alkali and alkaline earth oxides to phosphoric oxide cause the breaking of rings and the conversion of the network to a system of...
