الفهرس 
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Abstract
A method which has been recently used to correlate the structure and properties of various types of chalcogenide glasses is the chemical bond approach. It can be successfuly used to rationalize the observed properties of these materials, as well as to predict criteria. . for
selecting materials with desirable properties.
In the present study the system Se Ge Te was chosen. The energies of chemical bonds expected to ,be present in the system have been estimated. The cohesive energy, CE, of different compositionswas calculated. Three compositions of high values of CE were prepared in such a way that all heteropolar bonds are satisfied. The chosen compositions have the general formula Se40 Te where x=23.33 ,26.67 and 30
X-ray diffraction patterns indicate that all prepared samples at room temperature have amorphous structure. Diferential Thermal
Analysis D.T.A. technique reveals that Tg, T and Tm are depending on the composition of the glasses as well as values of cohesive energies CE.
D.c conductivity, a of all prepared samples were measured as a function of ambient temperature, T. Often the relation between in a and l/T could be divided into two parts separated by a kink at temperature T*, which reveal that there is more than one conduction mechanism. The observed trend has been accounted for in terms of Mott and Davis model. The variations in activation energies.