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Q-Glasses Could Be a New Class of Solids

Posted on: 08/06/2013
"Very weird. The strangest material I ever saw," commented Lyle Levine. TMS member and materials physicist at the National Institute of Standards and Technology (NIST) on a potential new category of solids that his team has discovered that are neither pure glasses, crystals nor even exotic quasicrystals.

The research team from NIST and Argonne National Laboratory has analyzed a solid alloy that they found in small discrete patches of a rapidly cooled mixture of aluminum, iron, and silicon. 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 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, says Levine, but regardless, the atomic arrangement apparently is not random. "As the nodule grows, every atom still knows where to go," he noted.

The q-glass seems to have a strict chemical composition, according to Levine. 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," says Levine. "It's amazing. Everything you can think about this thing behaves like a crystal, except it isn't."

The team used a variety of sophisticated techniques at Argonne's Advanced Photon Source 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," says Levine.

One possibility, say the researchers, 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 three-dimensionally ordered configuration of atoms that possesses neither translational nor rotational symmetry," says Levine. "Such structures have been theorized by mathematicians, but never before observed in nature."

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