Q-glass, a new solid alloy that has been discovered in a rapidly cooled mixture of aluminum, iron and silicon, is neither a pure glass, a crystal, nor even a quasicrystal, according to a team of researchers from the National Institute of Standards and Technology (NIST) and Argonne National Laboratory.
“Very weird. Strangest material I ever saw,” said Dr Lyle Levine, a materials physicist with the NIST and a co-author of a paper published in the Physical Review Letters.
The material appears to have none of the extended ordering of atoms found in crystals, which would make it a glass, except that it has a very defined composition and grows outward from ‘seeds’ – things that glasses most assuredly do not do.
The solids catalog used to be pretty straightforward. Solid stuff was either a crystal or a glass. Crystals fill up space with atoms or molecules in specific, fairly rigid patterns. The positions of the atoms are fixed such that if you take any section of pure crystal and slide it up, down, in, out or sideways a given distance, it will fit perfectly in the new position. That’s translational symmetry. You can also spin the crystal through certain angles and the atoms also will line up; that’s rotational symmetry.
Glasses have neither symmetry. They’re just a random arrangement of their components, as if you’d taken a liquid and suddenly frozen everything in place without giving the atoms a chance to get in order. Which, in fact, is how metallic glasses are made.
“The new material, which the research team has provisionally dubbed a ‘q-glass,’ can be shown by X-ray diffraction to have neither rotational nor translational symmetry, just like a glass, but regardless, the atomic arrangement apparently is not random. As the nodule grows, every atom still knows where to go,” Dr Levine said.
“For one thing, the q-glass seems to have a strict chemical composition Seen under a microscope, it’s clear that, like a crystal, the spherical q-glass regions grow outward from a seed during cooling and exclude atoms that don’t fit. It’s rejecting atoms that aren’t fitting into the structure, and if there’s no structure, it’s not going to be doing that. It’s amazing. Everything you can think about this thing behaves like a crystal, except it isn’t.”
The NIST-Argonne team used a variety of sophisticated techniques to rule out other possibilities.
The material might, for example, be a mass of randomly arrayed crystals so small they don’t show up individually under the X-ray probes. But if such crystals were there, they’d grow slowly as the stuff is annealed. That doesn’t happen.
“We went through the laundry list of possibilities and disproved them, one by one,” Dr Levine said.
“One possibility is ‘frustration’ – two or more incompatible crystal orderings may start growing from the seeds and continually interfere with each other, destroying any long-range order. But, one exciting possibility is that the q-glass is the first example of a 3D ordered configuration of atoms that possesses neither translational nor rotational symmetry. Such structures have been theorized by mathematicians, but never before observed in nature,” the scientist concluded.
Bibliographic information: Gabrielle G. Long et al. 2013. Highly Ordered Noncrystalline Metallic Phase. Phys. Rev. Lett., vol. 111, no. 1; doi: 10.1103/PhysRevLett.111.015502