An-Najah Staff

The characteristic temperatures such as the glass transition temperature (T_g), the onset temperature of crystallization (T_c) and the melting temperatures, (T_m) have been determined for glasses belonging to the Ge_xSe_{100 − x − y}Fe_y (y = 2, 4 and 6 at.%). Differential scanning calorimetry and differential thermal analysis measurements have been used for their determination. These temperatures have been used to evaluate the thermal stability of the investigated glassy alloys using Dietzel (ΔT) and Hruby (H_r) criteria. The variations of ΔT and H_r with the average coordination number, n, have been specified. It is found that both ΔT and H_r exhibit a maximum at n = 2.4. This observation is a realization of Phillips'–Thorpe threshold where the maximum stability of the network is just obtained if the percolation threshold limit is reached. The overall mean bond energies of the studied compositions have also been calculated and their correlation with the glass transition temperature is discussed.

The compactness of the structure of glassyGexBi6S94-x (14 ≤ x ≤ 26 at %) is determined from the measured densities. The peculiarities observed in the compactness-composition dependence are understood using Phillips’-Thorpe and Tanaka’s topological models proposed for the structure of these covalently bonded solids.

Bulk glasses with the chemical composition Ge_y Se_94−yIn₆ (8 ⩽ y ⩽ 30 at%) have been prepared from high purity constituent elements. Fragments of the bulk glasses are used to deposit thin films by vacuum thermal evaporation. The optical band gaps (E_g s) of the as-deposited films have been measured. The allowed optical transition is found to be indirect. The relation of E_g to the covalent coordination number, Z, is demonstrated by varying the composition parameter y of the thin films. A maximum in the compositional dependence of E_g is attained at Z = 2.63. The cohesive energies (CE) of the investigated samples have also been calculated using the method suggested by the chemical bond approach. It is found that the composition dependence of the CE also possesses a peak at Z = 2.63.A plausible explanation based on the bonding considerations between the constituents has been provided for the understanding of the E_g-Z and CE-Z dependences for these Ge–Se–In thin films.
PACS 61.43.Dq;78.66.Jg