Studies of the Glass-formation Range of Tellurite Systems

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 A and two other oxygen atoms are ranged at a distance of 2.75 A. 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