Uniform approach to preventing avalanches in glassy materials

An international team of researchers has used computer simulations to study the aging mechanism that can cause an amorphous glassy material to turn into a crystal. They found that removing tiny irregularities in local densities can help prevent the atomic 'avalanches' that trigger the formation of an ordered, crystalline structure, potentially leading to more stable glassy materials, including for pharmaceutical applications. The researchers report their findings in a paper in Physical Review Letters.

Glasses are highly unusual solids, as they lack an organized crystal structure. In fact, they are more like 'frozen liquids', because their atoms can get stuck in non-equilibrium positions. Over time, however, some glasses experience an aging process known as devitrification, in which the amorphous configuration settles into a more regular, crystalline structure.

In most cases, devitrification is undesirable, as it can turn previously transparent materials opaque. In addition to aesthetic considerations, the lack of stability can cause damage to glass panes, as well as to pharmaceuticals or optical media. However, the physical mechanism that triggers aging or devitrification remains poorly understood.

The team of researchers, led by Hajime Tanaka at the University of Tokyo in Japan, first used computer simulations to study the process of devitrification. This then led them to take a novel approach to preventing devitrification, by starting with a 'conventional glass' and progressively removing inhomogeneities in density until a 'uniform glass' was formed. This new state showed unprecedented stability against devitrification.

“This uniform glass state shows there is a clear connection between the stability of a glass state and its mechanical homogeneity,” said lead author Taiki Yanagishima from the University of Oxford in the UK.

As with other forms of crystal growth, devitrification starts when a tiny 'seed' crystal called a nucleus recruits neighboring atoms to arrange themselves in the same pattern. But unlike in liquids, the 'seeds' in glasses only grow when they are given a shake by large intermittent coordinated movements of atoms known as 'avalanches'.

In previous work, the researchers showed that these sudden events corresponded to destabilizations of the load-bearing network that spans the entire sample, and hypothesized that 'avalanches' were the only mechanism by which atoms may rearrange themselves. Making the glass more uniform prevents this relaxation and devitrification, even when nuclei are present.

“When avalanches are prevented, nuclei that normally would grow into crystals are instead frozen in their glassy state,” said Tanaka. This leads to the possibility that by making the number of load-bearing atomic neighbors more uniform across the sample, ultra-stable glasses may be formed.

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.