Laves compounds, cubic

L. Pauling, Electron transfer and the valence states of cerium and platinum in the cubic Friauf-Laves compounds with the platinum metals. Phys. Rev. Lett. 47, 277-281... 

Anlr2 compounds (cubic Laves phases) are known for all five stable light actinides (Th-Am) and for Cm. Electronic properties have not been studied for the latter two compounds. 

CeRu2. The compound (cubic Laves phase) was originally used as a nonmagnetic host for R doping in early studies of the interaction of magnetism and superconductivity (see for example Matthias et al. 1958, Hillenbrand and Wilhelm 1972). In recent years, it has been established that the superconducting properties of CeRua itself are most unusual. 

When the atomic size ratio is near 1.2 some dense (i.e., close-packed) structures become possible in which tetrahedral sub-groups of one kind of atom share their vertices, sides or faces to from a network. This network contains holes into which the other kind of atoms are put. These are known as Laves phases. They have three kinds of symmetry cubic (related to diamond), hexagonal (related to wurtzite), and orthorhombic (a mixture of the other two). The prototype compounds are MgCu2, MgZn2, and MgNi2, respectively. Only the simplest cubic one will be discussed further here. See Laves (1956) or Raynor (1949) for more details. 

An especially hard and stable Laves-type compound is cubic HfW2. Its melting point is 2650 °C, and its hardness at room temperature is 1900kg/mm2 (Stone, 1977). However, it has a high mass density, so its usefulness is limited. 

Alloys with the 7th to 9th group elements. Several Laves phases are formed either as the hexagonal MgZn2 type (with Re, Fe, Ru, Os) or as the cubic Cu2Mg type (with Fe, Co, Rh, Ir). CsCl-type compounds are given mainly by Yb. 

There are several dozen metallic AB2 compounds called Laves phases that are superconducting they have either cubic or hexagonal crystal structures. Some have critical temperatures above 10 K and high upper critical magnetic fields Bc2- For example, Zri/2Hfi/2V2 has rc = 10.1K, B 2 = 24 T, and a compound with a different Zr/Hf ratio has similar and Bc2 values with the critical current density Jc 4 X 10 A/cm. These materials also have the advantage of not being as hard and brittle as some other intermetallics and alloys with comparable transition temperatures. 

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. 

For cubic Laves-phase compounds with RpJR > 1-35, the Z2/rj ratio becomes less than 1. In this case, each g site has only one nearest neighbor lying at the adjacent hexagon. Such a transformation of the g-site sublattice may lead to a qualitative change in the microscopic picture of H jump motion the faster jump process is expected to be transformed into the back-and-forth jumps within pairs of g sites belonging to adjacent hexagons. The results of recent QENS experiments... 

AnT%. All compounds AnT2 with An = U and Np posses the cubic C15 structure with the exception of UNi2 (hexagonal C14 Laves phase). To our knowledge, no pSR data exist for the uranium alloys. The only actinide material studied by pSR thus far is NpCo2. 

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