ELECTRON-DIFFRACTION STUDY OF ALPHA-ALMNSI CRYSTALS ALONG NON-CRYSTALLOGRAPHIC ZONE AXES

The structure of crystalline alpha-AlMnSi is examined by electron diff raction. Six distinct Bone axes are examined, including both normal cr ystallographic and non-crystallographic zone axes, allowing the space group symmetry of alpha-AlMnSi to be studied. a method for indexing th e non-crystallographic zone axis diffraction patterns, which involve r eflections from several nearby crystallographic zone axes, is describe d and applied to electron diffraction patterns of the quasi-5-fold, 3- fold and a-fold axes of the icosahedral building units of cubic alpha- AlMnSi. These are compared with electron diffraction patterns from the corresponding 5-fold, 3-fold and a-fold axes of the quasicrystalline phase i-AlMnSi, from which we may make some conclusions concerning the occupancies of the icosahedral units in i-AlMnSi. Electron diffractio n patterns characteristic of Pm (3) over bar were obtained for thicker specimens. However, for thin specimens, as used for HRTEM imaging, th e electron diffraction patterns were characteristic of Im (3) over bar space group symmetry. This unusual behaviour arises because the struc tural basis for the Pm (3) over bar to Im (3) over bar phase transitio n is a weak effect, involving changes in occupancy of the icosahedral structural elements located at the corners (double-MacKay icosahedra) and body-centers (MacKay icosahedra) of the cubic unit cell. The effec ts of changing the occupancies of the outer shells of the MI and DMI s tructural units on the diffraction intensities of the weak reflections were examined. Thus, calculation of the dynamical diffraction amplitu des shows that in fact the weak reflections characteristic of Pm (3) o ver bar only develop sufficient intensity if two conditions are satisf ied: namely (1) the crystal thickness exceeds approx. 50 nn and (2) if a significant proportion of Im (3) over bar occupancies are included in the structural model. By fitting the observed thickness variation o f the diffraction intensities we propose a new set of occupancies for alpha-AlMnSi, which is consistent with the electron, X-ray and neutron diffraction data.