Convergent-beam and small-area-parallel-beam electron diffraction of icosahedral quasicrystals of a melt-quenched Al-Mn alloy

Abstract

Intensity distribution in convergent-beam electron diffraction (CBED) patterns obtained from icosahedral quasicrystas of a melt-quenched Al-Mn alloy reveal that the quasicrystals do not have fivefold, threefold and twofold rotation axes and have no inversion center, although ordinary diffraction patterns obtained thus far showed these rotation symmetries. CBED patterns taken from specimen areas of about 3 nm in diameter show a deviation in geometry in spot positions from the fivefold rotation symmetry. Ring patterns due to higher-order Laue zone reflections are not observed in CBED patterns. Kikuchi bands are composed of two sub-bands in the five equivalent directions, and each band has a different intensity profile. Parallel-beam (3 × 10 -5 rad) electron diffraction patterns obtained from specimen areas less than 100 nm in diameter also show a deviation from the fivefold symmetry in spot positions and make clear that each Bragg reflection consists of many fine spots which show no fivefold symmetry. It is proven experimentally that all the observed reflections occur already in the approximation of kinematical diffraction, although their intensities may be modified by dynamical diffraction effect.