Physical property changes associated with glass transition in chalcogenide glasses

High purity chalcogenide glasses were prepared in the binary Ge-Se and ternary Ge-Sb-Se systems by vacuum melting of pre-distilled samples. To understand the effects of average coordination number, {dollar}langle {lcub}rm r{rcub}rangle{dollar}, on glass-forming capability, connectivity and rigidity percolation properties such as thermal expansion coefficient, molar volume, heat capacity, viscosity, hardness, elastic modulus, acoustic attenuation and isothermal compressibility were studied as a function of {dollar}langle {lcub}rm r{rcub}rangle{dollar} in chalcogenide glasses. In the early eighties, Phillips proposed the constraint theory for covalent glasses, suggesting that ideal glass-forming condition should occur when the number of constraints match the degrees of freedom. This condition is satisfied at {dollar}langle {lcub}rm r{rcub}rangle{dollar} = 2.4 for covalent glasses. Most of the earlier experiments, in this field, to support Phillips' constraint theory of an existing percolation threshold at {dollar}langle {lcub}rm r{rcub}rangle{dollar} = 2.4 concentrated on locating extremum behavior either in glassy state properties or in liquid state properties of chalcogenide glasses and, hence, were inconclusive. The present work shows that the configurational contributions to the thermal expansion, heat capacity and isothermal compressibility, in addition to the molar volume, show distinct minima at {dollar}langle {lcub}rm r{rcub}rangle{dollar} = 2.4, suggesting a minimization in structural rearrangements during liquid to glass transition. We believe that if such a liquid possesses minimized accessible configurations in the supercooled liquid region it ought to display a poor crystallization tendency. Thus the key to the problem of percolation threshold is embodied in configurational changes associated with glass transition. A thermodynamic argument based on free energy is put forth to correlate constraint theory with ease of glass-forming ability. The study of thermodynamic order parameters indicates maximized connectivity for the glass at {dollar}langle {lcub}rm r{rcub}rangle{dollar} = 2.4. This renders additional support to the hypothesis that ease of glass-forming ability is associated with a balance between constraints and degrees of freedom in a glass-forming system.