Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 309
532.4 eV (7.5 %) in fig. (7.8) can be attributed to the O1s level in OH groups at the
cleaved glass surface.
The [TeO4] tbp has a lone pair of electrons at one of the equatorial sites of the Te sp 3 d
hybrid orbitals, and the [TeO3] tp also has a lone pair at one of the Te sp 3 hybrid orbitals
(see fig. (2.2)). Himei et al. [8] showed that the O1s and Te3d5/2 binding energies
decreased with increasing Na2O. Tellurite glasses have lower O1s binding energies than
silicates due to the higher electron density of tellurium compared to silicon. Some of the
electrons from the lone pair on the Te atom in [TeOn] polyhedra are donated to the ligand
oxide ions through Te-O σ bonds. In alkali silicates two peaks are seen in the XPS
spectra: one for bridging oxygens (BO) and one for non-bridging (NBO). However in
alkali tellurites, this is not the case, only one peak is observed. Himei et al. [8] propose
that this is due to the fact that the electron density of the valence shell of BO atoms is
almost equal to that of NBO atoms. This can be explained by electrons of the NBO atoms
in [TeOn] being back donated to Te atoms and the electrons delocalised through pπ-dπ
bonds between O2p and empty Te5d orbitals. As a result the electronic density of the
valence shell between BO and NBO atoms is equalised to give one component of the O1s
peak. In this study, the main O1s peak could not be resolved into two similar sized peaks
close in energy (fig. (7.8)), as in the work of Chowdari et al. [15-20], who deconvoluted
the main O1s peak into two components. These two components were attributed to non-
bridging oxygens (531.1 eV) and bridging oxygens (533.0 eV), with the NBO peak
growing at the expense of the BO peak with increasing modifier (Ag + ) content. The NBO
peak was found at a lower energy than the BO peak as in the NBOs, the electron density
is higher around the oxygen atom, resulting in a lower nuclear potential and hence