Correlation between structure and physical properties of chalcogenide glasses in the As_xSe_1-x system

Physical properties of chalcogenide glasses in the As_x Se _1-x system have been measured as a function of composition including the Young's modulus E, shear modulus G, bulk modulus K, Poisson's ratio ν, the density ρ, and the glass transition T_g. All these properties exhibit a relatively sharp extremum at the average coordination number 〈r〉 =2.4. The structural origin of this trend is investigated by Raman spectroscopy and nuclear magnetic resonance. It is shown that the reticulation of the glass structure increases continuously until x=0.4 following the "chain crossing model" and then undergoes a transition toward a lower dimension pyramidal network containing an increasing number of molecular inclusions at x>0.4. Simple theoretical estimates of the network bonding energy confirm a mismatch between the values of mechanical properties measured experimentally and the values predicted from a continuously reticulated structure, therefore corroborating the formation of a lower dimension network at high As content. The evolution of a wide range of physical properties is consistent with this sharp structural transition and suggests that there is no intermediate phase in these glasses at room temperature.