Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 27
and Zn-O) distances are also preserved in the glass from crystalline α-TeO2 and zinc
tellurite [22]. One of the axial (Te-Oax) bonds can elongate in the [TeO4] units with
second component addition due to lack of central symmetry, forming [TeO3+1]. This
elongation can be so extreme, such as in K2TeO3.3H2O with three Te-O bonds of 1.85 Å
and a fourth of 3.87 Å that the structural unit should be considered [TeO3]. The zinc
tellurite glass was found to be composed of 35 % [TeO4], and 65 % [TeO3] / [TeO3+1]
[22].
Neutron diffraction peaks > 3 Å were thought to be due to weak cation bonds (Te-Te
and Te-Zn via oxygen atoms) between chains in the glass. Distances between chains were
not preserved from crystalline materials on glass formation due to incorporation in
between chains of [ZnO4+1+1] polyhedra [22]. These zinc structural units can link the α-
TeO2 like chains, terminate chains, or bond onto the side of chains sitting in the channel
in between chains.
ZnO enters tellurite glass network as [ZnO5] and [ZnO6] units as opposed to [ZnO4]
units, which make up crystalline ZnO. This type of coordination in TeO2-ZnO containing
glasses bears a closer resemblance to the structure of crystalline ZnTeO3 (orthorhombic)
and Zn2Te3O8 (monoclinic) than to ZnO [23]. The TeO2-ZnO system contains a eutectic
point at 21 % ZnO at a temperature of 590°C [22].
Smimizugawa et al. [23] studied the structure of a series of TeO2-ZnO glasses by
EXAFS. α-TeO2 is composed of deformed [TeO6] octahedral rather than [TeO4] as in the
glass. This 6-coordination of TeO2 with two remaining weak Te-O in the pure form is
what prevents glass formation [23]. The presence of a second component lowers the CN
to 3 or 4, and Te +4 satisfies the criteria for a network former after Zachariasen [6]. With