Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 312
ZnF2 powders or cleaved tungsten-tellurite glasses, indicating that the peak is associated
with oxygens within, or on the surface of the fluorotellurite glasses. The peak observed at
535.7 eV in this study was also seen in the spectra of sodium-tellurite glasses by Himei et
al. but not commented on. The O1s peak for urea ((NH2)2C=O) from the NIST XPS
database occurs at 535 eV indicating the C=O bond increases the O1s binding energy
compared to singly bonded oxygen. Te=O bonds are present in some of the tellurite
polyhedra such as the isolated unit [TeO3], which contains three non-bridging oxygens,
one of which is doubly bonded to the tellurium atom, and the remaining two carry a
single negative charge [24]. This structural unit is common in alkali-tellurite glasses.
Therefore, this peak at around 535.7 eV, not seen in the ZnF2 powders could be due to the
Te=O bond in the glass or other M=O species.
Table (7.6) shows the peak positions from the NIST XPS database of TeO2, (Te3d5/2
and O1s), Na2O (Na1s), and ZnF2 (Zn2p3/2 and F1s), batched at. %, and the quantitative
analysis (peak positions, at. and wt. %) of the corresponding high resolution peaks for
glass MOF001_iii, which was melted for 1.75 hours. The actual at. % values were very
close to the batched values, and as expected, the fluorine levels were lower than batched,
and oxygen higher due to volatilisation of fluorides from the melt. Fluorine, which
volatilises from the glass as species such as HF, F2 and MFx (where M is a cation), will
be replaced by oxygen in the vicinity of the melt surface, to ensure charge neutrality.
There will inevitably be a small error in these values, as the Shirley background model
was fitted manually. Sensitivity factors used by CasaXPS to calculate the quantitative
analysis will also be for pure crystalline elements / compounds, not multicomponent
vitreous solids. However, these errors will be small.