Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 125
depending on the proportion of PbO and PbF2 in the glass, four distinct structural units
occurred. These are illustrated in fig. (4.22).
Fig. (4.22): Structural units in the PbO-PbF2 glass system, with increasing PbO (a) to (d),
where • = lead, ⊗ = oxygen, O = fluorine [55].
PbO is a covalent solid, whilst PbF2 is highly ionic. In the PbO-PbF2 glass system,
distorted octahedral [PbO2F4] units are connected by oxygens which are coordinated to
four lead atoms with a bond length of 220 pm [55]. In order to form octahedral units lead
is sp 3 d 2 hybridised. Also, as PbF2 content in the glass increases, so does the ionicity of
the Pb-O bond. The PbO-PbF2 glass system behaves very differently to conventional
oxide glass systems; thus when halides are introduced into phosphates, they do not
become part of the phosphate coordination polyhedra in the network [55]. A number of