Laves phases structures

N. Hanada, S. Orimo, H. Fujii, Hydriding properties of ordered-/disordered-Mg-based ternary Laves phase structures, J. Alloys Compd. 356-357 (2003) 429-432. 

Powder X-ray diffiactograms of the alloys are taken using Fe Ka radiation. The X-ray powder patterns are indexed on the basis of hexagonal C14 Laves phase structure. The lattice constants are evaluated by least square refinement. 

RNi2 compounds crystallize in the cubic Laves phase structure. Because of the simplicity of structure and the ease of preparation and characterization these materials have been extensively studied. From magnetic studies of this family of compounds, Skrabek and Wallace (55) established that nickel moment is zero in the ordered state and moreover that the moment of the rare earth atom is considerably reduced in comparison to that expected for a free trivalent rare earth ion. Bleaney (86) and Skrabek and Wallace (55) have interpreted this decrease in the saturation moment as arising from partial crystal field quenching of the orbital contribution to the total moment. In this respect the RNi2 compounds behave like the RA12 compounds described in an earlier section above. 

TiBe2 has the C15 cubic Laves phase structure and is an itinerant antiferromagnet with a Neel temperature near 10 K and a magnetic moment of 1.64jub. The Laves phase can be maintained when Cu is substituted for Be over the range 0 < x < 0.5 and a transition to ferromagnetism is observed at xcrit 0.115 (Acker et al. 1981) near which the SEW behaviour, i.e. the Landau parameter in the magnetic equation of state... 

The transition from antiferromagnetism to ferromagnetism at stoichiometric composition is ascribed to Fe atoms entering Ti sites in the C14 (Laves phase) structure... 

Fig. 3.7. Schematic drawing of the cubic Laves-phase structure (MgCu 2-type) adopted by a large number...

This section refers to intermetallics of type MX2 that do not take the Laves-phase structure. Two materials crystallizing in the orthorhombic CeCu2 structure and some R-hydrides have been studied by xSR. 

Under a much higher pressure of 15 GPa, CaAl2Si2 decomposed to Ca(Al,Si)2 with the Laves phase structure [60]. The Laves phase compounds with transition metals are well known for superconductors with the C15 structure, which also belongs to space group Fd3m isotypic with the type II silicon clathrate compounds. Solid solutions Ca(Ali cSi c)2 (0.35 < x < 0.75) can be directly prepared under HPHT conditions of 1,000 °C and 13 GPa. The stoichiometric Laves phase compound CaAlSi shows superconductivity at = 2.6 K. This is the hrst Laves phase compound composed of only commonly found elements. The compound can... 

The RAI2 rare earth compounds possess the cubic Laves-phase structure, which is shown in fig. 58. CeAl2 provides a good example of a Kondo-lattice system with the Fy ground state and an incommensurate sinusoidally modulated antiferromagnetic structure below 3.8 K (Barbara et al. 1979), as shown in table 14. 

In the rhombohedral structure, the R atoms are situated at two different crystallographic sites, 3c(I) and 6c(II), whereas the 3d atoms are distributed over three different sites 3d(I), 6c(II) and 18h(III), see fig. 59. In the first approximation, the local arrangement of the Ri site is that of the hexagonal RC05 structure whereas the local arrangement of the Rii site resembles that of the cubic RC02 Laves phase structure. Thus, different magnetic behaviour can be expected for R ions placed at different sites. The same holds for the... 

For the notation of the structure types we follow Teslyuk here, but in the case of MnjCujAl and MnInCu4 there is no complete proof that these compounds really have an ordered Laves phase structure. In the case of Mn2Cu3Al, there exists an independent investigation according to which Cu and A1 are not ordered and the real structure is therefore cubic (JoH, 68). 

Fig. 16. The four ternary ordered Laves phase structure types showing the atom arrangement in or close to each side of the (1120) plane of the hexagonal or triple hexagonal cells. Of the atoms positioned away from the (1120) plane only B atoms are indicated which form a triangle with a B atom in the (1120) plane with the same z value.

Fig. 36. Block stacking for the hexagonal and rhombohedral nT4+5 structural series, after Parthe and Lemaire (1975). 24 denotes a R2T4 block as found in the Laves phase structures and 15 a RT5 block with an atom arrangement as in the CaCuj-type.

Teslyuk, M.Yu., 1969, Metallic Compounds with Laves Phase Structure (Nauka, Moscow) (in Russian). 

---