Electron anions (center green blob) pair up in the center of molecular cages and lower the temperature at which glass forms in C12A7 electride. Image: Johnson/Sushko/PNNL.
Electron anions (center green blob) pair up in the center of molecular cages and lower the temperature at which glass forms in C12A7 electride. Image: Johnson/Sushko/PNNL.

A material known as mayenite, made up of aluminum, calcium and oxygen atoms, has several useful properties. Not only can it be turned from an insulator to a transparent conductor and back again, but it is used in the industrial production of chemicals such as ammonia and as a semiconductor in flat panel displays.

The secret behind mayenite's magic is a tiny change in its chemical composition, but researchers hadn't been sure why this tiny change had such a big effect on the material, also known as C12A7. In a new study, researchers now show how specific components of C12A7 known as electron anions can help the material to transform from a crystal into a semiconducting glass.

The study, published in the Proceedings of the National Academy of Sciences, uses computer modeling to zoom in at the electron level, along with lab experiments. These revealed how the small change in composition results in dramatic changes in the material’s glass properties and, potentially, allows for greater control of the glass formation process.

"We want to get rid of the indium and gallium currently used in most flat panel displays," explained materials scientist Peter Sushko of the US Department of Energy's Pacific Northwest National Laboratory (PNNL). "This research is leading us toward replacing them with abundant non-toxic elements such as calcium and aluminum."

More than a decade ago, materials scientist Hideo Hosono at the Tokyo Institute of Technology in Japan and colleagues plucked an oxygen atom from a crystal of C12A7 oxide, transforming the transparent insulating material into a transparent conductor. Such transparent conductors are rare: most conductors are not transparent (think metals) and most transparent materials are not conductive (think window glass).

This transformation is all due to the departing oxygen atom leaving behind a couple of electrons and creating a material known as an electride. The C12A7 electride is remarkably stable in air, water and ambient temperatures, whereas most electrides fall apart in these conditions. Because of this stability, materials scientists want to harness the structure and properties of C12A7 electride. Unfortunately, its crystalline nature is not suitable for large-scale industrial processes, so they needed to make a glass equivalent of C12A7 electride.

And several years ago, they did. Hosono and colleagues converted crystalline C12A7 electride into a glass that shares many of the properties of the crystalline electride, including its remarkable stability.

Crystals are neat and tidy, like apples and oranges arranged orderly in a box, but glasses are unordered and messy, like that same fruit in a plastic grocery bag. Researchers make glass by melting a crystal and cooling the liquid in such a way that the ordered crystal doesn't reform. With C12A7, the temperature at which the electride forms a glass is around 200°C lower than the temperature at which the oxide forms a glass.

This temperature – when the atoms stop flowing as a liquid and freeze in place – is known as the glass transition temperature. Controlling the glass transition temperature allows researchers to control certain properties of the material. For example, how car tires wear down and perform in bad weather depends on the glass transition temperature of the rubber they're made from.

Sushko and his PNNL colleague Lewis Johnson, together with Hosono and others at Tokyo Tech, wanted to determine why the electride's glass transition temperature was so much lower than the oxide's. They suspected that components of the electride known as electron anions were responsible. Electron anions are essentially freely-moving electrons that take the place of the much larger negatively-charged oxygen atoms that urge the oxide to form a tidy crystal.

The researchers simulated Hosono's lab experiments using molecular dynamics software that could capture the movement of the atoms and electron anions in both the melted material and the glass. They found that that the negatively-charged electron anions paired up with positively-charged aluminum or calcium atoms, replacing the negatively-charged oxygen atoms that would typically be found between the metals.

The bonds that the electron anions formed between the metal atoms were weaker than the bonds between the metal and oxygen atoms, and these weak bonds could also move rapidly through the material. This movement allowed a small number of electron anions to have a greater effect on the glass transition temperature than the much larger quantities of minerals typically used as additives in glass production.

To rule out other factors as being responsible for the lower transition temperature – such as the electrical charge or change in oxygen atoms – the researchers simulated a material with the same composition as the C12A7 electride but with the electrons spread evenly through the material instead of packed in as electron anions. In this simulation, the glass transition temperature was no different to that of the C12A7 oxide. This result confirmed that the network of weak links formed by the electron anions is responsible for the change in the glass transition temperature.

According to the researchers, electron anions form a new type of weak link that can affect the conditions under which a material can form a glass. They join the ranks of typical additives that disrupt the ability of a material to form long chains of atoms, such as fluoride, or promote the formation of weak, randomly-oriented bonds between atoms of opposite charge, such as sodium. The work suggests researchers might be able to control the transition temperature of glasses by changing the amount of electron anions they use.

"This work shows us not just how a glass forms," said PNNL's Johnson, "but also gives us a new tool for how to control it."

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