Structural and magnetic ordering in Cd-based crystalline approximants

Abstract

Complex compounds can be found in a large variety of intermetallic systems. They are usually categorised according to the number of atoms per unit cell, which amounts to hundreds or even thousands in some cases [1]. Several quasicrystal approximants and some modulated structures can be found among these compounds, which occur preferably in highly-symmetric lattices. Therefore we are concentrating on cubic [2] and hexagonal structures. In order to better understand complex intermetallics, we describe their structures in different ways, for example in terms of the cluster approach, as superstructures, or as layered compounds. When these compounds are interpreted as cluster packings, two recurring structural motifs are most prominent: multiple-shell fullerene-like clusters packed closely in different ways and blocks of smaller polyhedra (mainly Friauf polyhedra and similar clusters) filling the remaining gaps. Another general property of complex intermetallics is a complex diffraction pattern, which contains a small subset of strong reflections, indicating that the structure can be regarded as a superstructure. The periodicity of the underlying basic structure corresponds also to the number of flat and puckered layers, which can be regarded as building the structure by simple stacking. By comparing geometrically similar but chemically different structures, we hope to reveal the building principles responsible for the formation of highly complex intermetallic structures. In order to do this, we juxtapose the mentioned complementary methods of structure description.