We studied the glass-formation range of tellurite systems containing TeO₂. The experiments were made in the same way as in previous reports on the borate, silicate and germanate systems. The crucibles employed were made of Au-alloy containing 15% Pd. Except for TeO₂, the Oxides used were of 16 kinds of a-group elements, namely K, Na, Li, Ba, Sr, Ca, Mg, Be, La, Al, Th, Zr, Ti, Ta, Nb and W, and 5 kinds of b-group elements, namely, Tl, Cd, Zn, Pb and Bi. The results of binary and ternary systems which include all the combinations of these oxides are shown in Table 1 and Fig. 1-49 and 42-73, except for the systems with a narrow glass-formation range or none at all, which systems are listed in Table 3.
According to Brady's data on the X-ray analysis of TeO₂ glass, 4 oxygen atoms are ranged around Te at a distance of 1.95 Å and two other oxygen atoms are ranged at a distance of 2.75 Å. Therefore, the coordination of the Te⁴⁺ ion lies in an intermediate state between 4- and 6-coordination. TeO₂ itself is not vitrified. In the crystal state of TeO₂, the coordination number of Te⁴⁺ is 6. If a small amount of a modifier ion is introduced, however, TeO₂ may be vitrified. In these tellurite glasses, we consider that the Te-O distance shrinks somewhat; therefore, the 4-coordination of Te⁴⁺ becomes more stable than the 6-coordination. On the other hand, there is a series of ions which have no vitrifying range in any binary system with TeO₂. It is notable that the values of their ionic radii are within a narrow rage; as the valency of ions increases, the radius range shifts somewhat to the smaller side. These facts can be explained as follows: if a modifier ion has the structure of 6-coordination and if the size of MO₆ (M=modifier ion) is nearly the same size as TeO₆, the 6-coordination state of Te⁴⁺ may be stable.
The remarkable features of the glass-formation range of the tellurite system also include the following: (1) the range of modifier ions in the tellurite system is wider than in the borate or the silicate system and (2) there is no immiscible range in the tellurite system. These properties very much resemble those of P₂O₅ systems. The first one may be explained by the electronegativity (see Table 2). The electronegativity of Te is 2.1, which is the same value as P; therefore, the O-M bond may be ionic except for small and high valent ions. The second property is probably a problem arising from the polymerization power of glass-forming oxides.
Because many oxides are classified as modifiers, according to (1), ternary systems of the tellurite are classified largely as A-type (consisting of one glassformer and two modifier components). The hatched area in Fig. 1 shows the expected glass-formation range of the A-type.
WO₃ cannot be considered to be a modifier component. In the tellurite systems generally, the glass-formation ranges of WO₃-containing systems are large very unlike those of borate, silicate, etc. According to Gelsing the coordination number of W⁶⁺ is 4 in WO₃R₂O system (R=alkali ion). However, we consider that the coordination number of W⁶⁺ is 6 in the tellurite systems, as in borate, silicate, etc. Most of the actual glass-formation ranges of WO₃ are above the A-WO₃ line (see Fig. 35-39). In these regions the network structure contains WO₃ without a modifier ions. On the contrary, the regions within the AD line contain a network of WO₃ with modifier ions.
It is difficult to consider that Nb⁵⁺ is a network-former of the 4 coordination. The hatched area of Fig. 30-34 consists of two parts. In the one Nb⁵⁺ is a modifier, while in the other Nb⁵⁺ is a networkformer of 6 coordination. The