Borides, rare-earth metal

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Figure 2. Formation of ternary borides MreMt-B4 (Mre = rare-earth metal, = transition-metal element) and different structure types. , YCrB4 type OD, ThMoB4 type O, YjReB type H, CeCr2Bj type B, Er4NiB,3 type H, ErNiB4 type , compound formation observed but structure type unknown. Refs a , b, c", d" e f , h i k", 1, m , n, o p, q , r , s t . u , v ", w , x ,...

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

The solubility of rare-earth metals in /3-rh boron is unknown. Rare-earth-boron systems are cubic borides - with an composition (E = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb). The occurrence of this phase excludes extensive solid solutions in... 

Table 1. Sintering Characteristics of Borides of Rare-Earth Metals... 

This soft, silver white metal reacts with air and water. The oxide is applied in optical glasses with high refractive indices (special lenses for powerful cameras and telescopes). Used for special effects in optoelectronics and electronics. Lanthanum exhibits catalytic properties. It is a component of flint and battery electrodes. Lanthanum boride (LaB6) is the superior electron-emitter for electron microscopes. Lanthanum is the first of the series of 14 lanthanides, also called the "rare-earth" metals, whose inner N shells are filled with electrons. They do not belong on the "red list" of endangered species they are neither rare nor threatened with depletion. China is particularly rich in lanthanide ores. 

The ionic radii of the rare-earth metals have a dominant effect on the structure types of their crystalline borides, as shown in Table 13.3.2. 

The author considered it best not to include in the reference book the properties of certain little-studied compounds rarely used in practice. Thus, in the presentation of the information on carbides, borides, nitrides, and other classes of metal-like compounds, no data are given on the refractory compounds of metals of the platinum group for the sulfides, data are given only for the class of sulfides of the rare-earth metals and actinides, in most of which the properties of refractory compounds in the wide sense are most clearly expressed, the proportion of ionic bond, in particular, being small. It was, however, found e qpedient to consider also the properties of oxysulfides of the rare-earth metals and actinides, which are very similar to the properties of sulfides and are obtained simply by replacement of two atoms of sulfur in a sesquisulfide by two atoms of oxygen. This is one of the few exceptions where the tables of the reference book give the properties of ternary and not binary compounds. 

G. V. Samsonov, Yu. B. Paderno. Borides of the Rare-Earth Metals, Izd. AN UkrSSR, Kiev, 1961. 

Because they exhibit interplay of magnetic and superconducting properties, the formation and crystal chemistry of MRgMy4B4 compounds have been examined. Ternary rare-earth and actinide (Th, U, Pu)-transition metal borides of the approxi-... 

Figure 2. Formation of ternary borides MREMT4B4 (Mre = rare-earth element, Mj = transition metal) and different structure types. B, CeCo4B4 type B, LURU4B4 type a, NdCo4B4 type H, YOS4B4 type 8, Sm,4.eFe4B4 type MRcRe4B4 type , LuRh4B4type. Refs b, c , d", e, f , g , h, il l ...

Rare-earth (and actinide)-B-carbon compounds resemble metal borides in B-rich carboborides, whereas the physical and structural properties of C-rich borocarbides tend to a more earbide-like behavior (which will not be covered in this context). 

Among the tetraborides, UB4 has the smallest volume and hence the smallest effective radius. Thus an actinide element having a metallic radius of 1.59 A (Pu) or smaller forms a diboride, while those having larger radii do not. As in the rare-earth series, the actinides able to form MB4, MBg and MB,2 borides form also MB2 diborides (Table 1). 

Experimental work can be difficult owing to both the high volatility of the metals and their reactivity toward O2 (e.g., alkali-metals, alkali-earth metals, rare earths). Once reaction begins it often is highly exothermic judging by the enthalpies of formation of borides. ... 

The problems raised by the preparation of some rare-earth borides such as SmB4, YbB4 and TmB2 are comparable to those found for the alkali borides from the point of view of the volatility of the metals. They dissociate through metal evaporation, yielding boron-rich borides as indicated in 6.7.2.4. 

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... 

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 . 

Parth6, E. and Chabot, B. (1984) Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues. In Handbook on the Physics and Chemistry of Rare Earths, ed. Gschneidner Jr., K.A. and Eyring, L. (North-Holland, Amsterdam), Vol. 6, p. 113. 

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