On the Mechanism of the Glass Transition in Electron-Rich Intermetallic Compounds

This work focuses on the chemical origin of complex free energy landscapes that emerge in a technologically important class of electron-rich intermetallic compounds called the chalcogenide alloys. Similarly to many other inorganic compounds, the chalcogenides exhibit a puzzling multiplicity of bonding configurations that underlies a very viscous, activated transport already above the melting point and readily vitrify when thermally quenched. The rate of the decrease of the fluidity and the configurational entropy alike, with lowering temperature, is however surprisingly slow: Molecular motions persist at temperatures that are well below characteristic bonding energies, despite the covalent bonds forming a contiguous network at all times. This puzzling decoupling of liquid thermodynamics and the energetics of bonding—in many inorganic systems and in the chalcogenides in particular—is of basic interest in itself; it also complicates first-principles prediction of even the most basic properties of substances of practical interest, such as the melting point or the heat of fusion. We approach this challenge by first introducing a chemically-motivated spin model on a fixed lattice. We show that even in the presence of stereochemical interactions characteristic of realistic bonding patterns, the thermodynamics of a fixed-lattice model can be reduced to that of a set of non-interacting defects, implying essentially a one-body physics. The energy scale of these effective defects is in the electronvolt range and whose configurational entropy is thus much too low to account for liquid motions in actual materials. Consequently, we argue that the landscape complexity of the chalcogenides stems not from a variety of defected configurations on a uniquely defined lattice but, instead, stems from the multiplicity of distinct lattice types and the myriad ways such distinct lattices can join in physical space, a many-body effect. To explicitly demonstrate this inference, we implement a direct off-lattice simulation of a model liquid mixture. We introduce a novel, artificial force field that allows one to emulate the highly directional, octahedral bonding characteristic of the chalcogenide alloys in an efficient way that avoids using computationally expensive quantum-chemical calculation. In contrast with state-of-the art model liquid mixtures, the present model liquid has a well-defined crystal state and allows for unambiguous determination of the melting point and the entropy of the glassy liquid, for the first time. We show that the model liquid indeed shows several distinct regimes of activated dynamics. The distribution of local coordination patterns indeed shows a slow temperature dependence that is consistent with the behavior of the configurational entropy in actual materials. This, then, furnishes a constructive argument that glassy dynamics are not due to defects in a globally-defined lattice that corresponds to a unique vibrational ground state. Instead, glassy dynamics should be thought of relaxation among an exponential multiplicity of distinct vibrational vacua; thus a lattice can be defined only locally. The present results also suggest that the activated dynamics observed in currently used liquid models do not correspond to equilibrium processes, but, instead, are transient relaxations toward phase-separated solid that is not glassy but has a unique vibrational ground state.