Rare earth hexaborides

The hexaborides for which the oxidation state of the metal is 2 -f- are CaB, SrBg, BaB5, EuB, YbB. They are diamagnetic except EuB, which is ferromagnetic (Tc = 12.5 K) . The other rare-earth hexaborides are characterized by the 3-1-oxidation state, except for SmB, in which samarium is in a mixed valence state. They exhibit antiferromagnetic order at low T except LaB, YB and SmB . ... 

Figure 1. Metallic radii of rare-earth metals and cubic lattice parameters of the rare-earth hexaborides.

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

Two types of formation mechanism for rare-earth hexaboride are distinguished, depending on the hexaboride. ... 

This method is used extensively in the laboratory because it is particularly suitable for preparing borides of rare or expensive metals, e.g., the transition-metal-rich borides CrB, Cr3B4, CrB2 (except Cr3B2 and Cr4B), the diborides ScB2, TiB2 the rare-earth hexaborides, dodecaborides and MB -type borides. 

The reduction of a metal oxide by a mixture of B and C is easier than the reduction by the borothermic process described above. The rate of reduction depends on the removal of CO, so operation under vacuum increases the rate and allows the reaction to proceed at a lower T than the borothermic process. The metal oxide may be volatile and the borides can be contaminated by C. Accordingly, this method is not suitable for preparing pure alkaline-earth and rare-earth hexaborides because in all cases borocarbides of formula MBg C, (e.g., M = Sr, Eu, Yb) are formed . 

It can be used for hexaborides as well. Reduction with boron carbide can also be used for diborides as well as hexaborides, although in that case alkaline-earth and rare-earth hexaborides will dissolve some carbon forming RBs-xCx borocarbides. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

Kunii, S., K. Takahashi, and K. Iwashita. 1999. Phonon and specific heat analysis in rare-earth hexaborides. ht Abstracts of the 13th International Symposium on Boron, Borides and Related Compounds, Dinard, France, p. 73. 

The rare earth hexaborides, well known for their superior electron emitting properties [38], are also quite hard materials. The hardness of YBg is reported to be comparable to that of TiB2 [32] however, such borides are quite expensive. YBg is isotypic with CaBg and LaB [38 40], where the boron atoms form B ... 

The rare-earth hexaborides This class of compounds has been... 

The rare earth hexaborides RBg crystallize in the cubic (CaBe type) structure which possesses a CsCl type arrangement of R atoms and Be octahedra. Figure 7 shows the crystal structure of RBe and its simple cubic Brillouin zone. LaBe is a reference non-f compound. CeBg is a typical Kondo-lattice compound undergoing two magnetic ordering... 

Some of the early crystal growth experiments on the rare earth elements employed arc zone melting. Carlson et al. (1975) grew a Lu crystal (7.8 cc) and subsequently Schmidt and Carlson (1976) an Er crystal (13 cc) by this method. Verhoeven et al. (1976) describe the application of arc zone melting to LaB, producing rods —20 x 6 mm at rates 50p,ms , and this approach has been adopted by other workers for rare earth hexaborides (Gruhl and Winzer 1986). 

M Kasaya, F Iga. Preparation and physical properties of rare earth hexaborides and dodecaborides. In D Emin, Aselage, CL Bekkel, lA Howard, C Wood, eds. Boron-Rich Solids, AIP Conference Proceedings 140. Albuquerque AIP, 1985, p 11. 

Tarascon et al. (1980) investigated the valence transition of Sm in the hexaboride solid solutions Sm, M B (M = Yb, Sr +, La ", Y, Th +). Samples were prepared by borothermal reduction of the mixed oxides under vacuum and high temperatures. The exact values of x have been determined by x-ray fluorescence analysis and checked by density measurements. Density measurements. X-ray and chemical analysis of SmB indicate an atomic ratio B/Sm x 6. From X-ray absorption measurements at the L, edge at 300 K the Sm + Sm + atomic ratio was obtained as a function of x. (The L,n absorption spectrum of Eu + in EuB was used as reference.) Y + substitution of Sm decreases the average Sm valence towards Sm in accordance with estimations of the average Sm valence in the hexaborides (Sm, Sm +), M B from lattice parameter measurements, fig. 34c. Lattice parameters of Sm +B a = 4.186, and Sm + Bg a = 4.115, were derived from interpolations of neighboring divalent and trivalent rare earth hexaborides. 

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