L a v e s   P h a s e s

The initiative concentrates on a particular class of intermetallic phases, the so-called Laves phases. These phases, which form the largest group of intermetallics with more than 1400 representatives, have the ideal composition AB₂. An intermetallic compound is classified as a Laves phase purely on the basis of the geometry of the crystal structure. The Laves phases crystallize in three structure types, which are named after the representatives cubic MgCu₂ (C15), hexagonal MgZn₂ (C14) and hexagonal MgNi₂ (C36). They are polytypes with a common underlying structure principle. In the first half of the last century it was shown by J.B. Friauf [1,2], F. Laves [3,4], G.E.R. Schulze [5], F.C. Frank and J.S. Kasper [6,7] that the Laves phases can be regarded as tetrahedrally close packed structures from atoms A and B with ideal ratio of the radii of r_A/r_B = (3/2)^1/2. The A atoms are surrounded by a Z16 Frank-Kasper polyhedron (16 vertices decorated with 12 B atoms and 4 A atoms) and the smaller B atoms are surrounded by an icosahedron formed by 6 A atoms and 6 B atoms. Many ternary and multinary representatives of the Laves phases have been observed with A or B in excess. In addition, ternary Laves phases are known in systems with no corresponding binary Laves phases. Laves phases can be formed by main group metallic elements, transition metals as well as by lanthanides and actinides. The large variety of possible components for both A and B as well as the large interval of experimentally observed enthalpies of formation (0 down to -350 kJ mol^-1) indicate a considerable diversity in the chemical bonding situation.

In the past decades several heuristic approaches to describe the stability have been applied to the family of Laves phases. In particular geometrical concepts relying on symmetry, packing density or electronic factors like valence electron concentration or the difference in electronegativity have been considered. However, these concepts are of limited value [8]. They have turned out to be of low predictability even in binary systems and they are inapplicable for extrapolation, e.g., by introducing more components.

Laves phases have not only attracted much interest in basic research. Especially, in the last decade many metallurgists and engineers have studied these materials after becoming aware that Laves phases with transition metals as components show remarkable mechanical and physical properties. This has induced considerable efforts in the development of novel structural materials based on Laves phases for extremely high temperatures. Current research on turbine steels, e.g., is aimed on the use of fine precipitates of Laves phases in order to improve fatigue strength. An important application of Laves phases as functional material is their use as hydrogen storage materials in nickel-metal hydride batteries. Nevertheless, a systematic approach to the development of materials based on Laves phases is still lacking. The exploration of Laves phases as materials components has revealed many problems in our understanding of intermetallics. A break-through in the targeted development of materials with superior properties based on intermetallics compared to steel alloys can only be expected after a much better understanding of their stability and chemical bonding.


References:

[1] J.B. Friauf, The crystal structure of two intermetallic compounds,  J. Am. Chem. Soc. 49 (1927) 3107-3114.

[2] J .B. Friauf, The crystal structure of magnesium di-zincide, Phys. Rev. 29 (1927) 34-40.

[3] F. Laves, H. Witte, Die Kristallstruktur des MgNi₂ und seine Beziehung zu den Typen des MgCu₂ und MgZn₂, Metallwirtschaft 14 (1935) 645-649.

[4] F. Laves, H. Witte, Der Einfluß der Valenzelektronen auf die Kristallstruktur ternärer Magnesium-Legierungen, Metallwirtschaft 15 (1936) 840-842.

[5] G.E.R. Schulze, Zur Kristallchemie der intermetallischen AB₂-Verbindungen (Laves-Phasen), Z. Elektrochem. 45 (1939) 849-865.

[6] F.C. Frank, J.S. Kasper, Complex alloy structures regarded as sphere packing 1: Definitions and basic principles, Acta Crystallogr. 11(3) (1958) 184-190.

[7] F.C. Frank, J.S. Kasper, Complex alloy structures regarded as sphere packing 2: Analysis and classification of representative structures, Acta Crystallogr. 12(7) (1959) 483-499.

[8] F. Stein, M. Palm, G. Sauthoff, Structure and stability of Laves phases Part I - Critical assessment of factors controlling Laves phase stability, Intermetallics 12 (2004) 713-720.