Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 31
[TeO4] and [TeO3] units with non-bridging oxygens (NBOs) were shown to increase in
the glass with alkali addition at the expense of [TeO4] units with no NBOs [26].
McLaughlin et al. [27, 28] showed by neutron diffraction, XRD, and NMR that all
five polyhedra shown in fig. (2.5) are present in TeO2-Na2O glasses.
n
Fig. (2.5): Variety of polyhedra present in TeO2-Na2O glasses, where Q m denotes the
structural unit Q, with m oxygens bonded to a central tellurium atom, n of which oxygens
are bridging [27].
A continuous cleavage of the TeO2 network occurs with Na2O addition, with an
equilibrium population and type of polyhedra depending on composition, frozen in from
the liquid [27]. In crystalline sodium tellurites, dimers linked by four member rings are
seen. However, in TeO2-Na2O glasses, only a few percent of the tellurium atoms engage
in this type of ring structure (around 3 %). This again points to a high degree of structural
rearrangement needed for devitrification, and may explain glass stability. This study also