Abstract
In network glass including chalcogenides, the network topology of microscopic structures can be tuned by changing the chemical compositions. As the composition is varied, an intermediate phase (IP) singularly different from the adjacent floppy or rigid phases on sides has been revealed in the vicinity of the rigidity onset of the network. Glass formers in the IP appear to be reversible at glass transition and strong in dynamical fragility. Meanwhile, the calorimetry experiments indicate the existence of a first-order liquid-liquid transition (LLT) at a temperature above the glass transition in some strong glass-forming liquids. How are the intermediate phase and the liquid-liquid transition related? Recent molecular dynamic simulations hint that the intermediate phase is thermodynamically distinct that the transitions to IP as varying the chemical composition in fact reflect the LLT: out of IP, the glass is frozen in vibrational entropy-dominated heterogeneous structures with voids; while inside IP, energy dominates and the microscopic structures of liquids become homogeneous. Here we demonstrate such first-order thermodynamic liquid-liquid transition numerically and analytically in an elastic network model of network glass and discuss possible experimental approaches to testify the connection.
Highlights
In network glass, the material properties relying on structures can be tuned by changing the chemical compositions that have different abilities to make covalent connections with its neighbor atoms
We will show that these heterogeneous and homogeneous structures correspond to two distinct thermodynamic liquid phases that are separated by a first-order liquid-liquid transition at a critical temperature TLLT
We will further argue that depending on the relation between TLLT and the glass transition temperature Tg, the liquid can be frozen into different thermodynamic phases, which could be the origin of the singular intermediate phase in network glass
Summary
The material properties relying on structures can be tuned by changing the chemical compositions that have different abilities to make covalent connections with its neighbor atoms. First pointed out by Maxwell (1864), a general network will lose rigidity as the network connectivity is reduced to below certain critical connectivity when the average number of constraints per atom, n, is equal to the degrees of freedom, i.e., nc = d in spatial dimension d This rigidity loss applies to the chalcogenides when selenium concentration is high, predicted by Phillips in Phillips (1979) and Thorpe (1985), where he showed that counting both radial and angular constraints of covalent bonds gives nSe = 2, nAs = 9/2, and nGe = 7, indicating a chalcogenide glass is marginally rigid at a composition with average number of covalent bonds rc = 2.4. One of the most interesting discoveries is the intermediate phase (IP) near rc (Boolchand et al, 2001; Wang et al, 2005; Rompicharla et al, 2008; Bhosle et al, 2012), which remains a big puzzle
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