Binary structures Laves phases

According to Rajasekharan and Girgis on a A t>, A ws1/3 map, considerable resolution is obtained among the binary systems in which different structure types occur. The points corresponding to the systems in which the Laves phases (or the phases of types such as Cr3Si, TiAl3, etc.) occur show linear relationships on the... 

A systematic analysis of structure and stability of binary and ternary Laves phases, or Friauf-Laves phases, has been performed by Stein et al. (2004, 2005). By reviewing a large number of experimentally determined phase diagrams with Laves phases, a number of general conclusions have been obtained. These may be summarized in the following points ... 

Laves phases occurring in ternary systems, the binary subsystems of which contain no Laves phases, generally pertain to the hexagonal MgZn2-type structure. 

Alloys with the 10th group elements. Typical compositions and structure types of the binary phases formed with Ni, Pd, Pt are 1 1 (CsCl type or FeB or CrB), 1 2 (Laves phases), 2 7 (Gd2Co7 or Ce2Ni7 type), 1 5 (CaCu5), etc. 

Considering then the phase composition as a significant parameter, we obtain the histogram shown in Fig. 7.1(a) for the distribution of the intermetallic phases according to the stoichiometry of binary prototypes. For instance, the binary Laves phases, the A1B2, Caln2, etc., type phases are all included in the number reported for the 66-67.99 stoichiometry range, even if the real stoichiometry of the specific phase is different, see Fig. 7.1(b). We may note the overall prevalence of phases and, to a certain extent, of structural types, which may be related to simple (1 2, 1 1, 1 3, 2 3, etc.) stoichiometric ratios. 

Among the actinide compounds the interest is concentrating on binary compounds of simple structure (e.g. 1 1 compounds with elements of the groups V and VI of the periodic table) for which the theoretical treatment is rather advanced, and on intermetal-lic (e.g. Laves-) phases. 

Pseudo-binary Laves phases. The relatively easy formation of solid solutions of binary Laves phases has resulted in an intensive investigation of various pseudo-binary series, especially those for U-3d compounds. They provide us with the possibility to trace the formation or the destruction of moments which accompanies the variations of the electronic structure in a continuous way. The pseudo-binary series can be divided into four groups which will be discussed separately below. 

The Laves phases - sometimes designated as Friauf-Laves phases - with an AB2 composition in the binary case form a very large group of intermetallics which crystallize with the hexagonal C14 structure, the cubic Cl 5 structure, or the dihexagonal C36 structure (Laves, 1967 Wernick, 1967 Livingston, 1992). These structures are topologically close-packed (tcp) structures (Wernick, 1967 Schulze etal., 1973 Watson and Bennet, 1984), i.e. the Laves... 

Structure data are summarized in Table 2. No ternary phase has been reported in the Cr-Fe-Zr system. However binary Laves phases show extended solubihty ranges along the ZrFc2-ZrCr2 quasibinary section. The variation of the lattice parameters with composition along the section are reported in Fig. 1 according to [1970Kan]. 

An important and numerous class of intermetallic compounds AM2 are the cubic Laves phases (MgCu2 type, C15). Many representatives with A or M being one of the transition metals forming stable binary hydrides (see in.B.l) absorb considerable amounts of hydrogen up to compositions AM2H7. Hydrogen occupies tetrahedral interstices of the crystal structure, thereby expanding it. 

Ste] Stein, R, Palm, M., Sauthoff, G., Structure and Stability of Laves Phases Part II - Structure Type Variations in Binary and Ternary Systems , Intermetallics, 13, 1056-1074 (2005) (Review, 249)... 

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