Push materials to their limits, and strange things can occur – such as the discovery of a previously unknown phase of liquid, which has been reported by scientists looking at the development of super-thin, high-density glass.

These types of glass are used in a variety of ways, including in OLED displays and optical fibers, but they can have stability problems. It's through an effort to tackle those problems that this different type of material has come to light.

Crucially, the newly discovered liquid phase promises thin glass that's more stable and denser than the materials that have come before – a progression that could open up different ways of using the glass, and even completely new types of devices.

"There are a lot of interesting properties that came out of nowhere, and nobody had thought that in thin films you would be able to see these phases," says physicist Zahra Fakhraai from the University of Pennsylvania.

"It's a new type of material."

Glass is a very special type of material that typically forms as a liquid solidifies. While its properties become much like a solid, internally the structure of glass doesn't change much from the liquid phase. It remains a fascinating transition for scientists.

In the case of ultra-thin glass, that transition can be hard to manage without running into problems like crystallization, especially at larger scales. Thin glasses retain more of their liquid properties than normal, which can lead to instability and degradation.

In other glasses, a technique known as vapor deposition – where a gas is turned into a solid directly – is used instead of cooling a liquid, but it hasn't been clear whether this would help improve the stability of ultra-thin glasses.

In the new study, researchers spent years working on experiments to establish that vapor deposition would actually reduce some of the liquid-like properties of thin glass.

It was through this process, when dropping down to extremely cool temperatures, that the new phase of high-density supercooled liquid was spotted – one that differed from the liquid phase typically observed when producing ultra-thin glass.

"The two liquids have distinct structures, akin to graphene and diamond which are both solids made of carbon but exist in very different solid forms," says Fakhraai.

Follow-up experiments confirmed the packing of individual molecules into a structure that wasn't a crystal but something else. Based on the geometry of the phase, the researchers think there could be implications for other types of materials too.

What this means is the potential of producing ultra-thin glasses with a much higher density – higher than crystal in some cases – through vapor deposition and the new phase of liquid in glass.

Further studies are planned to establish exactly how this phase transition comes about, including a closer look at the deposition phase, and it could help scientists solve some of the other remaining mysteries of glass.

"Our hope is that this fundamental understanding motivates more applications and a better ability to design thin film glasses with similarly improved properties," says Fakhraai. "If the structure-property relationships are understood in thin films, we can do better by design."

The findings are due to be published this week in PNAS.

Editor's note (19 Aug 2021): An earlier version of this article incorrectly explained the basics of vapor deposition, which turns a gas directly into a solid. The error has now been corrected.