Author:

Levine, Lyle Edward.

Title:

ANALYTICAL ELECTRON MICROSCOPY OF QUASICRYSTALS.

Source:

Dissertation Abstracts International. Volume: 51-08, Section: B, page: 3914.

Abstract:

Although quasicrystals have been studied intensely since their discovery in 1984, little is known about the underlying atomic structure of these non-periodic phases. In this thesis, the local atomic ordering of these phases is examined. Many analytical electron microscopy techniques are used to obtain information from quasicrystals and related crystal phases. Techniques employed include selected area diffraction (SAD), convergent beam electron diffraction (CBED), energy dispersive x-ray spectroscopy (EDS), electron-energy-loss spectroscopy (EELS), high-resolution imaging, and microdiffraction. Icosahedral phase Ti₆₂Mn₃₆Si₂ was transformed by annealing to produce three new crystalline phases. SAD patterns from these phases show intensity modulations that closely resemble the i-phase SAD patterns. Kikuchi bands from two of these crystal phases have icosahedral symmetries. I present structural models based on crystalline packings of icosahedral clusters of atoms that closely predict the observed SAD modulations, the experimentally determined point groups, the unit cell sizes, and the internal symmetries of these phases. Localized diffuse scattering in reciprocal space is studied for two i-phase alloys and two related complex crystal phases. Two existing theoretical models, transition-state theory and the icosahedral glass, are examined. A third model based on non-random disorder of atomic positions within icosahedral clusters is presented. Finally, the amorphous-icosahedral phase transition is studied using EELS. It is shown that the local atomic order in amorphous Al₇₅Cu₁₅V₁₀ and Pd_58.8U_20.6Si_20.6 are similar to that in the respective i-phases. This similarity does not occur in the Al₆Mn system.