Quasicrystal Structure Solved

The first complete atomic structure solution of an icosahedral quasicrystalline material has been reported by researchers in Japan. The study demonstrates the power of state-of-the-art analytical techniques and may lead to advances in the analysis of other complex and nontraditional ordered materials, according to experts. The study may also deepen understanding of structure-property relationships in a broad class of metallic alloys.

Unlike ordinary crystals, quasicrystals are ordered solids that lack periodicity, meaning their atomic structure cannot be depicted by a geometric pattern of atoms that repeats in three dimensions at fixed intervals. The oddly ordered materials, which include a large number of multicomponent alloys, often exhibit five- or 10-fold rotational symmetry, a condition that's forbidden in conventional crystallography.

Since their discovery in the 1980s, quasicrystals have sparked debate over atomic structure, stability, and other basic science issues. At the same time, however, they've played key roles in a number of commercial applications, including stick- and scratch-resistant coatings for high-end cookware and hardeners for steel used in electric razors and other tools. Yet, efforts to map the exact arrangement of atoms in a quasicrystal have, until now, been only partially successful because of the materials' lack of periodicity and other complicating factors.

Now, on the basis of synchrotron X-ray diffraction and hyperspace crystallography, which is a multidimensional (6-D, in this case) computational and modeling technique, researchers have solved the complete atomic structure of a quasicrystal—icosahedral YbCd_5.7 (Nat. Mater. 2007, 6, 58). The structure is described as a collection of connected icosahedral YbCd clusters held in place by ytterbium atoms that fill the voids between the clusters. The exact positions of the ytterbium atoms in the voids are determined by comparison with the chemically similar conventional crystals YbCd₆ and YbCd_5.8.

The research team includes Hokkaido University physicist Hiroyuki Takakura, Tohoku University materials scientists An Pang Tsai and Cesar Pay Gómez, and their coworkers.

The team points out that a number of factors helped make the structure solution a manageable task. To begin with, unlike nearly every other stable quasicrystalline material, YbCd_5.7 consists of just two elements and therefore offers chemical simplicity. The researchers were also aided by the strong X-ray contrast between cadmium and ytterbium, as well as the material's propensity for yielding high-quality single-grain samples.

In an accompanying commentary, Patricia A. Thiel of Iowa State University, Ames, and Ames Laboratory, notes that "quasicrystals represent a challenging extreme in structural complexity." Yet in the case of YbCd_5.7, she says, the challenge has been answered with "a comprehensive and elegant model." Hopeful that the study will spur additional success in the field, Thiel adds that the new result represents "an essential starting point to find the structures of more complex quasicrystals."