Rare-earth Laves phases

Nuclear magnetic resonance in the magnetically ordered state of rare-earth Laves phase compounds. 

The bulk moduli for EUAI2 at room temperature were found to be Bq = 53.8 1.8 and B o = 4.2 0.4GPa (Gleissner 1992) which -as shown in fig. 27-is lower than Bq of typically trivalent rare earth Laves phase systems like ErAl2 and DyAl2 but still larger than Bq of the IV compound YbAl2. From this point of view... 

Typically, binary Laves compounds AM2 are formed in several systems of A metals such as alkaline earths, rare earths, actinides, Ti, Zr, Hf, etc., with M = Al, Mg, VIII group metals, etc. Laves phases are formed also in several ternary systems either as solid solution fields extending from one binary phase (or possibly connecting the binary phases of two boundary systems) or as true ternary phases, that is forming homogeneity fields not connected with the boundary systems. 

The structure of YCu4.74Pbo.26 is shown as an example in Figure 19. The copper atoms on the 16c site build up a three-dimensional network of corner-sharing tetrahedra. This network leaves voids of coordination number 16 which are filled by the rare earth and Pb/Cu atoms on the four-fold sites. The large coordination number of this void readily explains the higher lead content on this site. The atoms occupying the 16c site (mainly copper in the RCus-xPb series) have coordination number 12 in the form of a distorted icosahedron. For more crystal chemical details on the Laves phases and related compounds we refer to review articles (Simon, 1983 Nesper, 1991 Johnston and Hoffmann, 1992 Nesper and Miller, 1993). 

Only rare-earth system (AB5-type) and zirconium-titanium-vanadium system (AB2 Laves phase-type) hydrogen storage alloys have been used as negative electrode materials for the commercial production of Ni-MH batteries [3, 7, 8], However, these materials have a low hydrogen storage capacity resulting in a low electrode energy density. 

The magnetic moments of rare earth elements are caused by the unpaired electrons in their 4f shells. These shells" are shielded by the outer shells so that chemical bonding has relatively little effect on the magnetic moments of these elements. The rare earths form a series of Laves phases of composition MB2 (M = rare earth, B = precious metal) which share the characteristic of ferromagnetic coupling at low temperatures (7). Figure 13 shows the Curie temperatures of a series of com-... 

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. 

Table 2.1. Structures of Laves-phase Fe2-rare-earth compounds (CIS) exposed to hydrogen gas at a pressure of 5 MPa and various temperatures for 86.4 ks, including the crystallization temperatures Tx of the hydrogen-induced amorphous alloys [2.33]...

The present monograph was first written as a chapter for Volume 8 of the series Materials Sdence and Technology A Comprehensive Treatment , edited by Robert W. Cahn, Peter Haasen, and Edward J. Kramer (Volume Editor Dr. Karl Heinz Matucha). Its aim is to give an overview of intermetallics, which is both detailed and comprehensive and which includes the fundamentals as well as applications. The result is an extended, critical review of the whole field of intermetallics with an emphasis on those intermetallic phases which have already been applied as functional or structural materials or which are currently the subject of materials developments. A historical introduction and a discussion of the relationship between atomic bonding, crystal structure, phase stability and properties is followed by a discussion of the major classes of intermetallics. The titanium aluminides, nickel aluminides, iron aluminides, copper phases, A15 phases. Laves phases, beryllides, rare earth phases, and siliddes are reviewed. In particular, the crystal structures, phase diagrams, and physical properties as well as the mechanical and corrosion behavior are treated. The state of developments as well as prospects and problems are discussed in view of present and future applications. The publisher has decided to publish the review as a separate monograph in order to make it accessible to a wider audience. 

As a property of anomalously high magnitude in cubic Laves phase compounds of rare earth metals such as Fe2(Tbo.3Dyo.7), which are the basis of magnetostrictive materials serving as magnetostrictive transducers and sensors [3.3]. 

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. 

One of the unique features of the rare earth amorphous alloys is the existence of many crystalline counterpart compounds (e.g. the Laves phase RFez com pounds) which allow direct comparison of structural and magnetic properties in both amorphous and crystalline phases. 

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