CONFIGURATIONAL ARRANGEMENTS IN CHALCOGENIDE GLASSES - A NEW PERSPECTIVE ON PHILLIPS CONSTRAINT THEORY

High purity chalcogenide glasses with an average covalent coordination number, [r], in the Ge-Se binary and Ge-Sb-Se ternary systems between 2 and 2.8 were prepared by vacuum melting pre-distilled elements. To understand the effects of [r] on glass-forming capability, properties such as thermal expansion coefficient, molar volume and heat capacity were studied as a function of [r]. Prior authors have searched, genera lly fruitlessly, for extremum behavior either in glassy state properti es or in liquid state properties of glass-forming compositions at [r] = 2.4 to support Phillips' constraint theory. The missing link is prov ided if one examines the configurational changes during glass transiti on at ordinary cooling rates. The configurational contributions to the heat capacity and thermal expansion, in addition to the molar volume, show distinct minima at [r] = 2.4, suggesting that the structural rea rrangements for the [r] = 2.4 liquid in the glass transition region ar e minimized. If such a liquid possesses minimized accessible structura l rearrangements in the supercooled liquid region as well, then it may be concluded that such a liquid would display a poor crystallization tendency. This, then, renders support to Phillips' argument that [r] = 2.4 solid with degrees of freedom equal to the number of constraints marks the 'best' glass.