On the Symmetry and Composition of Complex Intermetallics
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On the Symmetry and Composition of Complex Intermetallics Julia Dshemuchadse and Walter Steurer Laboratory of Crystallography, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland ABSTRACT Complex intermetallic structures can be found in many intermetallic systems and exhibit giant unit cells. They feature quite a number of peculiar structural and physical properties and have, until now, barely been treated in a unifying matter. We are following an integrated approach towards their description and categorization and are thereby formulating a fundamental definition of these very interesting and rather diverse compounds. INTRODUCTION Complex intermetallic compounds and phases are found in a large variety of intermetallic systems. They have unit cells containing hundreds or even thousands of atoms. A growing number of scientists take stock in these materials, also due to their useful properties, e.g., as coatings [1], thermoelectric materials [2], or as catalysts [3]. Some of these properties are similar to those of quasicrystals [4], which occur in the same or in related systems – quasicrystal approximants being a part of the versatile group of complex intermetallics. A large part of these structures displays a few distinct features, such as a cluster-based structure, a layered structure, and also an articulate average structure, making it possible to describe the whole structure as a superstructure of a much simpler unit cell. We have previously presented a study on all complex face-centered cubic intermetallic structures [5]. Therein we try to apply an unbiased view on the definition of complex intermetallics, which we will apply here in a similar manner to intermetallic structures of all symmetries. For this purpose we define the number of atoms per unit cell as the commanding parameter to quantify the complexity of a structure. This seems to be a reasonable choice, since the number of atoms per unit cell is a measure of the unit cell size and therefore the repeat unit of the structure. It expresses the distance between two chemically and symmetrically equivalent atoms of any kind. In addition, it is not biased by, for example, the atom size, as the unit cell volume would be. Also the number of atoms per asymmetric unit would render a distorted parameter, since the shape and multiplicity of the asymmetric unit can vary greatly, whereas the repeat unit – the primitive unit cell – of any periodic structure always has the shape of a parallelepiped. One restriction is imposed by this choice of parameter for the complexity of a structure: it excludes aperiodic structures from the considerations, since they do not exhibit a threedimensional unit cell. Modulated structures may still be regarded as having the unit cell size of their average structure assigned, but the inclusion of quasicrystals will have to be accomplished in a different manner.
DATA MINING We used the database “Pearson’s Crystal Data” (PCD) for our general survey of complexity in intermetallic phases [6]. The whole data file comprises 227’145
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