The basic building blocks of crystalline and amorphous phosphates are the P-tetrahedra that result from the formation of sp3 hybrid orbitals by the P outer electrons (3s23p3). The fifth electron is promoted to a 3d orbital where strong π-bonding molecular orbitals are formed with oxygen 2p electrons (see, e.g., [15] and [16]). These tetrahedra link through covalent bridging oxygens to form various phosphate anions. The tetrahedra are classified using the Qi terminology [17], where ‘i’ represents the number of bridging oxygens per tetrahedron (shown schematically in Fig. 1). The networks of phosphate glasses can be classified by the oxygen-to-phosphorus ratio, which sets the number of tetrahedral linkages, through bridging oxygens, between neighboring P-tetrahedra [2].