Boron rare-earth compounds

Laser isotope separation techniques have been demonstrated for many elements, including hydrogen, boron, carbon, nitrogen, oxygen, sHicon, sulfur, chlorine, titanium, selenium, bromine, molybdenum, barium, osmium, mercury, and some of the rare-earth elements. The most significant separation involves uranium, separating uranium-235 [15117-96-1], from uranium-238 [7440-61-1], (see Uranium and uranium compounds). The... 

Boron has a particular affinity with rare earth elements, and forms rare earth borides which are of particular interest. The rare earth atoms supply electrons to the boron atomic framework to stabilize and form novel structures, while the shell of f electrons supplies further attractive properties like magnetism. Borides with lower boron content, like the hexaborides RB6 and tetraborides RB4 are well known metallic compounds and have been studied throughout the years, revealing interesting magnetic properties (e.g. Gignoux and Schmitt, 1997). 

Up to the early 1990s, the only rare earth boride known with RB (n > 12) was RB66. These compounds have been primarily studied for their interesting structure and structurally derived features like the amorphous behavior of thermal properties. However, in a recent development, new higher borides have been discovered like the RB25 and RB50 compounds. Furthermore, with addition of small amounts of 3rd elements like C, N, Si, the boron cluster framework was found to arrange... 

Seyboldt (1960) first discovered an extremely boron-rich rare earth cubic compound with only 1-2 atomic percent of rare earth. Early notations of this compound have varied from RB ioo to RB 7o to RB so (an early review of the Y-B... 

Theoretically, since these are layered homologous compounds, a numer-ous/infinite number of compounds are possible in the family However, realistically, we have been able to synthesize pure phases of only the three compounds. Compounds which contained more than four layers of the B12 icosahedral and C-B-C chain layers (which is the case for RB28.5C4) always contained a mixture of other number layers also. In the limit of the boron icosahedra and C-B-C chain layers separating the metal layers reaching infinity (i.e. no rare earth layers) the compound is actually analogous to boron carbide. In the opposite limit, a compound with just one boron icosahedra layer is imaginable. And in actuality, such a MgB9N compound was independently discovered by Mironov et al. (2002). However, such a compound with rare earth atoms has not yet been synthesized. 

Doping has been investigated extensively for compounds like /3-boron and boron carbide to try to modify their thermoelectric properties (e.g., Werheit et al., 1981 Slack et al., 1987 Aselage and Emin, 2003). There have not been as many attempts to dope the rare earth higher borides, we are only aware of transition metal doping into YB66 (Tanaka et al., 2000,2006 Mori and Tanaka, 2006). 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Recent investigations have revealed that the intrinsic behavior of RB28.5C4 is also n-type (Mori et al., 2008a). Very small inclusions of boron carbon "B4C" can cause the p-type behavior previously observed in some samples (Mori and Nishimura, 2006). The origin of the striking n-type behavior observed in the homologous R-B-C(N) compounds is not completely resolved yet but indicated to pertain to the two-dimensional rare earth layers (Mori et al., 2008a). 

First of all, a rare earth existence diagram is given in Figure 45 for all the higher boride compounds discussed in this review. As noted before, size constraints on the voids which are created among the boron cluster networks result in different ranges of possible rare earth elements for the different compounds. 

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