Coordination defect concept is widely used to describe new functionality induced by external influences such high-energy γ-irradiation in chalcogenide glasses. These defects appear as pairs of under- and over-coordinated atoms with corresponding excess of negative and positive electrical charges, respectively. One bond is destroyed by external influence, but another one, sometimes of different type (homopolar-to-heteropolar bond switching and vice versa), is formed in its place. Thus, the observed new functionality is explained by additional disordering within initial covalent bond distribution frozen during glass transition and electrical field throughout bulk irradiated glass caused due to local charge excesses of randomly-distributed defects. These defects significantly disturb both atomic and electron sub-systems of chalcogenide glasses.
We examine the high-resolution X-ray photoelectron spectroscopy (XPS) to study induced coordination defects in As(Sb)-Ge-S(Se) glasses. At the example of glassy Ge23.5Sb11.8S64.7 it is shown these defects are revealed as changes in:
- density of chalcogen-localised lone-pair electrons at the top of the valence band due to positively-charged overcoordinated atoms;
- core-level XPS spectra due to coordination defects with modified charge distribution;
- core-level XPS spectra due to "wrong" homopolar bonds and/or nearest neighbouring charged defects.
Simultaneously, these defects were studied with positron annihilation lifetime (PAL) technique, the obtained results being treated within two-state positron trapping model. It is shown that only under-coordinated defects can be satisfactorily tested with this technique. In the case of under-coordinated chalcogen and pnictide atoms, the excess of free volume is quite great to be detectable as additional input in the second defect-related PAL component, while under-coordinated tathogens atoms are practically non-detectable because of low accompanied free volume. It is concluded these are in full agreement with ones obtained by XPS. |