Configurational arrangements in chalcogenide glasses: A new perspective on Phillips' constraint theory

Abstract

Abstract 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, generally fruitlessly, for extremum behavior either in glassy state properties or in liquid state properties of glass-forming compositions at 〈 r 〉 = 2.4 to support Phillips' constraint theory. The missing link is provided if one examines the configurational changes during glass transition 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 rearrangements for the 〈 r 〉 = 2.4 liquid in the glass transition region are minimized. If such a liquid possesses minimized accessible structural 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.