Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 311
network. NaF is a stable compound, and fluorides of tellurium exist, but are not as stable
as ZnF2 and NaF. TeF4 is a colorless, hygroscopic solid, which decomposes above 190°C,
forming gaseous TeF6 (sublimes at -39°C), also hydrolysing completely after 1 day at
room temperature [22]. A number of oxyfluorides of tellurium exist, but all are liquids at
room temperature, and form from Te +6 , these include: Te2O2F8, Te2OF10, Te3O2F14, and
Te6O5F26. However, the Te-F bond may be more stable in the solid state when contained
within the glass network, but also may result in volatilisation of tellurium fluorides, as
discussed in chapter 5. Asymmetry in the XPS peaks of tellurite and fluorotellurite
glasses may be as a result of the high degree of asymmetry of the structural units in the
glass. Unlike the [SiO4] unit which forms highly symmetric tetrahedra, the TeO2
structural units are trigonal bipyramidal (tbp), [TeO4], and trigonal pyramidal (tp),
[TeO3], in structure, with a lone pair of electrons from tellurium in the corner of the
polyhedra not occupied by an oxygen atom [23]. This lone pair of electrons will
inevitably affect the distribution of electrons (and as a consequence the bonding) of any
atoms in close proximity, such as fluorine which is highly electronegative. The
asymmetry could also be due to fluorine associated with ZnF2, and Zn(OH)F derived
structural units in the bulk glass. This impurity (Zn(OH)F) would also shift the F1s peak
in the glass away from the value for ZnF2.
The O1s peak for this glass was also asymmetric, as shown by fig. (7.13), which
shows the O1s peak deconvoluted into three main components, at 531.0 eV (81.2 % →
BOs and NBOs), 532.1 eV (14.9 % → OH), and 535.7 eV (3.9 %). The peak at around
535.7 eV could be due to C-O and / or C=O groups from hydrocarbons used in the
vacuum system of the XPS; however this peak was not seen in the XPS spectra of the