Lanthanide borides

CH2CN)4Yb[( J.-H)2BH]2, and (CgH N)4Yb[( J.-H)2BH4]2 have been stmcturally characterized by x-ray crystallography and shown to contain ytterbium to boron hydride Yb—H—B linkages. Thermal decomposition of lanthanaboranes can be used to generate lanthanide metal borides. 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

The existence of MBj, MB, MB,2 and MB borides depends on the metal atom size (see 6.7.2.4.6.). The di-, dodeca- and hectoborides are known only for the smaller lanthanides and actinides, and the hexaborides are known only for the larger members of each series. Thus high T (1100-1250°C) and pressures (65 X 10 N m ) are used to prepare SmB2, GdB,2 and ThB,2, phases unobtainable as products of the reactions carried out at high T only. ... 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

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 main difficulty is to prevent pyrohydrolysis over the long experimental periods. (Moisture is removed by "gettering with hot lanthanides or actinides.) The potentials of M/MF reference electrodes have also been used to study activities of metals in alloys, carbides, and borides (208). The accuracy is limited by reproducibility of potentials (2 1 mV) as a fraction of the cell potential and indeterminate errors. The latter can be avoided by checking for internal consistency (e.g., measuring emfs of two cells against a common third electrode). 

Borothermic reduction of oxides enhanced by electron-beam bombardment. Light lanthanide borides have been prepared from the oxides according to a reaction such as the following one (Latini et al. 2002) ... 

Borides from metallic fluxes. The preparation of several binary borides of lanthanides, vanadium, tantalum, chromium has been performed by using aluminium as a flux. Aluminium has been used also as a reactive flux in the preparation of alumino-borides (of Mo, Fe) which are stable in concentrated HC1 solutions. 

It has been reported that ScBi2 has a tetragonal modification of the UBi2-type structure with lattice parameters of a = 5.22 A and c = 7.35 A with the space group of I4/mmm (Hamada et al., 1993 Paderno and Shitsevalova, 1995). Due to the small size of scandium as compared to the other rare earth atoms, scandium phases have been observed to form anomalous higher boride structures compared to the heavy lanthanides and yttrium, as will be discussed later in Sections 9 and 11. Small amounts of metal replacement for Sc in Sci xMxBi2 (x as small as 0.1, M = Y, Tm, Lu) have been reported to stabilize the structure in the normal cubic UBi2-type. 

The MB6 borides, formed by a number of metals including Ca, Sr, Ba, Sc, Cr, Y, Th, and several lanthanides (Eu, Yb), adopt the structure shown in Fig. 5-1 consisting of interconnected B6 octahedra with metal atoms in interstitial locations. 

Various borides, sulfides, carbides, nitrides, etc., have been obtained by direct interaction of the elements at elevated temperatures. Like other actinide and lanthanide metals, thorium also reacts at elevated temperatures with hydrogen. Products with a range of compositions can be obtained, but two definite phases, ThH2 and TlqH, have been characterized. 

AXe Compounds. There also exist several binary borides with the ABs formula. The structure can be visualized as a body-centered CsCl lattice with the Cl ions being replaced by Bs octahedra, while the body-center cation may be Na, K, Rb, Cs, Ca, Sr, Ba, Sc, Y, Zr, La, lanthanide, or actinide. The A Bs and A Bs borides have a high metallic conductivity (10" -10 cm ) at room temperature, but the other borides are... 

The interstitial structures comprise the compounds of certain metallic elements, notably the transition metals and those of the lanthanide and actinide series, with the four non-metallic elements hydrogen, boron, carbon and nitrogen. In chapter 8 we discussed the structures of a number of hydrides, borides, carbides and nitrides of the most electropositive metals, and these we found to be typical salt-like compounds with a definite composition and with physical properties entirely different from those of the constituent elements they are generally transparent to light and poor conductors of electricity. The systems now to be considered are strikingly different. They resemble... 

Later developments of linear methods have been in the direction of self-consistent calculations of ground-state properties utilising local spin-density-functional formalism [1.51,52] for exchange and correlation. The basis of the self-consistency procedure was given in papers by Madsen et al. [1.53], Vouisen et al. [1.54] and Andersen and Jepsen [1.55], and was soon followed by results for the magnetic transition metals [1.56], the noble metals [1.57], some lanthanides [1.58], the actinides [1.59,60], and the 3d transition metal monoxides [1.61,62]. In this context one should also mention calculations of the electronic structure in transition metal compounds [1.63,64], A15 compounds [1.65,66], rare-earth borides [1.67], Chevrel... 

Shortly after acceptance of my manuscripts on the Chevrel phases (15) and the lanthanide rhodium borides (16) the initial scientific reports on the high copper oxide superconductors... 

---