Electron microscopy of crystalloid structure in Ni-Cr small particles

Abstract

The crystalloid structure, an aperiodic arrangement of small structural units, observed in Ni-Cr small particles was studied by selected-area electron diffraction, dark-field imaging, high-resolution electron microscopy and optical diffraction analysis using the high-resolution micrographs. The electron and optical diffraction patterns show an arrangement of sharp diffraction spots with twelvefold rotational symmetry and an intensity distribution which is (at least approximately) consistent with this symmetry. The high-resolution electron microscopy revealed that the crystalloid structure is non-periodic but has bond orientational order over a range of 60 nm. From these experimental results, as well as from those for crystalline phases coexisting in the same particles, a random tiling model of three local structural units (A15-type, Zr₄Al₃-type and 30° rhombus prism type) is proposed as a possible structural model of the crystalloid structure. An indexing scheme using five integers is also proposed for the crystalloid reflections. Using these indices, a precise analysis of the crystalloid diffraction pattern has been carried out.