Chemical Bonding in Laves Phases Revisited: Atom Volumina in Cs-K System
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1128-U08-01
Chemical Bonding in Laves Phases Revisited: Atom Volumina in Cs-K System Yu. Grin1, A. Simon2, A. Ormeci1 P
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Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187, Dresden, Germany. 2 Max-Planck-Institut für Festkörperforschung, Heisenberg Str. 1, 70569, Stuttgart, Germany. P
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ABSTRACT Laves phases comprise a large group of binary and ternary intermetallic compounds with general composition AB2. The crystal structures of Laves phases are often regarded as closest packing of differently sized spheres. This observation, beginning with very early work on Laves phases, has led many researchers over the years, to emphasize the role of geometrical factors in the formation of Laves phases. In order to develop a firm understanding of chemical bonding in Laves phases and assess the importance of geometrical factors, we undertake a first-principleselectronic structure-based chemical bonding analysis for several representatives. As a first step towards this goal we concentrate on the K-Cs system which contains the Laves phase CsK2 and the hexagonal compound Cs6K7. In such alkali-metal-only compounds it is generally expected that chemical bonding-caused energy effects are minimal. Atom volumina and charge transfer investigations reported here, however, suggest that even in alkali metal-alkali metal Laves phases chemical bonding plays a non-negligible role. B
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INTRODUCTION Laves phases form one of the largest groups among the intermetallic compounds. More than 1000 binary and ternary representatives are known [1]. The membership in this group is defined by a special structural feature: three-dimensional framework of the majority component (B) based on the so-called Kagome pattern with cavities in form of a truncated tetrahedron where the minority atoms (A) are located. The Laves phases are found in three main types of crystal structure: MgCu2 (C15, cubic), MgZn2 (C14, hexagonal), MgNi2 (C36, hexagonal). The ideal stoichiometry is AB2, and the constituent elements come from almost any part of the periodic table. This large variety of Laves phases makes them very interesting from a chemical bonding viewpoint, because one expects different chemical bonding patterns depending on the particular A and B elements. It seems to be not obvious for this group of compounds taking into account the fact that they all have the same structural features. The crystal structures of Laves phases are often interpreted as closest packing of differently sized spheres, giving rise to a demand to explain how the expected different chemical bonding patterns derived from the different chemical nature of the components yield closest packing of atoms at the end. Another related question is: are factors responsible for formation of more than 1000 Laves phase compounds the same for all, or are there sub-categories with different dominant factors? Two rules, one geometric, other electronic, are discussed in regard to stability of Laves phases. The geometric rule is based o
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