Shaking up quasicrystals.

Shaking up quasicrystals

Normally, a crystalline material is made up of identicalbuilding blocks that stack neatly in a regularly repeating pattern. Thus, atoms occupy particular, well-defined positions within a crystal. In an amorphous material like glass, atoms tend to be scattered more or less randomly throughout a sample. Quasicrystals, discovered only two years ago, fall somewhere in between. Their building blocks don't appear to sit in a regular array, yet the sharp diffraction spots evident from X-ray or electron scattering experiments indicate a high degree of order and a fivefold symmetry (SN: 3/23/85, p.188). Recent experiments show that the physical properties of quasicrystals have unique features that are quite unlike those of the corresponding crystalline forms.

One important property is how the material shakes or bendswhen it is "excited.' These crystal lattice vibrations, which are called phonons, have a range of wavelengths and frequencies. To study this property, physicist Hartmut Zabel of the University of Illinois at Urbana-Champaign and his colleagues fired neutrons at samples of a quasicrystalline aluminum-manganese alloy. They observed changes in the energy and momentum of the scattered neutrons and used that information to deduce phonon characteristics. For comparison, they then heated each of their samples to transform them into their crystalline state and repeated the measurements.

The researchers found that for lower energies (or frequencies),both the crystalline and quasicrystalline forms had similar vibrational densities, indicating that their response at low temperatures to heat and their elastic properties or bending motions are about the same. At higher energies, however, the quasicrystalline structure shows a significantly higher density of vibrational states. Apparently, quasicrystals can have local lattice vibrations that either don't spread or are restricted to small regions of the lattice.

Zabel and his group also explored the magnetic properties ofquasicrystals. A material's measured tendency to become magnetized (its magnetic susceptibility) is a good reflection of the arrangement and distribution of electrons within the material. The researchers observed that magnetism is much stronger in the quasicrystalline state than in the crystalline. "This enhancement could again be the result of an increased amount of localized electronic states,' says Zabel. Now, further research is needed on larger crystals (SN: 11/15/86, p.309) and at higher resolutions to obtain a more detailed understanding of the differences noted in these experiments.