What distinguishes a crystal from a glass? Physically, the difference lies in the internal structure, which is significantly influenced by the different external conditions under which two objects solidify. A crystal must cool slowly and in a controlled manner. Then the molecules can arrange themselves in a regular lattice in which their forces cancel each other out. Thus, a crystal has the most ordered state possible. Its internal structure is very stable and therefore it will not change, no matter how long it is left standing.
The situation is quite different with glass. When cooling rapidly, the molecules do not have time to arrange themselves regularly and they solidify in the same disordered state as they were in liquid form. The molecules have not yet reached their optimal position and so unbalanced forces between them allow the glass to change its structure slightly in order to reach a more stable internal structure. However, it will never reach the state of a crystal, because to do so it would have to break the already formed stable bonds between molecules by investing energy which it does not have.
Paradoxical order: Can glass be more ordered than a crystal?
In physics, the degree of order is quantified by an entropy term. The lower the entropy, the higher the order. Through its structural changes, a glass strives to approach a more and more ordered state. Entropy calculations predict that after a certain time the glass entropy should fall below that of an ordered crystal. By definition, this is not possible because the greatest order, and therefore the lowest entropy, is possessed by the crystal structure which is unattainable for a glass. This is the so-called Kauzmann paradox. To solve this, it has long been hypothesized that a glass undergoes a phase transition to an ideal glass before reaching the entropy of its corresponding crystal. In this state, it will have the minimal entropy that it can still reach as a glass while still lying above that of a crystal.
TOFTOF reveals the signature of ideal glass for the first time
Using the TOFTOF instrument at the MLZ, a research group from Spain has not only demonstrated the existence of the ideal glass, but also linked it for the first time to observations in inelastic neutron scattering. Glasses, because of their higher disorder, are able to absorb the energy of incident neutrons better than an ideal glass. This energy absorption by an ideal glass should become apparent by a certain disappearing intensity at the maximum of the scattering spectrum, providing a unique signature of ideal glass. “The transmitted energy of the neutrons to the glass lies perfectly within the observable energy spectrum of our neutron spectrometer TOFTOF. It registers even the most minimal changes of the scattering maximum and is thus perfectly suited for this measurement,” says Marcell Wolf, instrument scientist at TOFTOF at the Heinz Maier-Leibnitz Center.
Instead of several centuries, production took 100 hours
The greatest challenge physicists faced in their search for the ideal glass was the long time scales over which conventional glass changes its structure. Under these normal circumstances, the ideal glass state would only be reached after a time between several centuries and the age of the earth itself – unthinkable for laboratory experiments. Therefore, a research group from Spain has taken advantage of the fact that the aging process is sped up as the surface area relative to volume of the glass is increased. For their studies, they used glass powder consisting of small spheres with a diameter of about 150 nanometers. By doing this, their sample reached the state of ideal glass in under 100 hours.
Xavier Monnier, Juan Colmenero, Marcell Wolf and Daniele Cangialosi. Reaching the Ideal Glass in Polymer Spheres: Thermodynamics and Vibrational Density of States. Physical Review Letters 126, 118004 (2021). DOI: 10.1103/PhysRevLett.126.118004