Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 20
crystallisation was suppressed due to the competition between these two phases. This
idea might be extended to other novel glasses which have a low viscosity at the liquidus.
2.3. Tellurite glasses
2.3.1. Early studies
The trioxide of tellurium (TeO3) decomposes when heated to dull redness with the
formation of the dioxide (TeO2). Tellurium dioxide, is a white crystalline solid which can
be melted at 733°C, without the gain or loss of oxygen [9]. Melts of pure TeO2 do not
solidify to form a glass [2], but glass formation has been shown with systems of more
than 90 mol. % TeO2 (eutectic region of TeO2-rich area of the phase diagram) and no
other glass former present [2]. Since the early work of Berzelius in 1834 [10], very little
was contributed to work on tellurite compounds, until Lenher et al. in 1913 [11] studied
the chemistry of the so-named metallic tellurites, and observed that sodium di- and tetra-
tellurite fuse below red heat to form a clear glass. In this work sodium tellurite was also
found to be readily soluble in water (important when considering glass durability). In
1952, Stanworth [12, 13] performed the first major systematic studies on tellurite glasses
on the basis Te 4+ has an electronegativity in the range of other good glass forming oxides
(1.7 to 2.1) as shown in table (2.1) with some network formers for comparison. This is
known as Stanworth’s electronegativity criterion [1].