synthesize elemental boron B, as well as lithium Li and beryllium Be, Nature requires ... B/Me with chemical formulas from Me5B to MeB66 can be counted of ~ 250 ... electron affinity, i.e. boron hydrides, carbides, nitrides, oxides, silicides, and.
Editorial: Why boron based solids? Boron belongs to the least abundant chemical elements. Its content in the Earth’s crust makes up no more than about 0.005 mass %, while in the Ocean water even lesser [1]. Explanation can be found in cosmology: in order to synthesize elemental boron B, as well as lithium Li and beryllium Be, Nature requires an environment that is dramatically different from those which produce the remainder of the Periodic Table [2]. Just because of this, the mentioned triad of elements constitutes only a minute fraction of the total matter in Solar System (Figure 1). Nevertheless, role of boron in forming of the various molecular and solid-state structures is incommensurably great. The relative importance of boron compared with other elements was vividly presented (Figure 2) by Lipscomb [3]. In the final analysis, understanding of the diversity of crystalline lattices containing boron atoms reduces to their electronic structure in isolated state. Configuration of valence electrons peculiar to the free boron atom is 2s22p. It is an energetically unstable configuration, which in lattice tends to more stable ones, first to 2s22p2 and then to 2s22p3. Thus, boron is strongly distinct acceptor and then elemental boron structures should be electron-deficient. It a reason, why all-boron crystalline forms, as well as amorphous boron, exhibit very complex, clustered structures. They are considered [4] as a structures stabilized by the intrinsic point defects and/or certain impurities in highconcentrations precisely compensating electron-deficiency inherent in corresponding ideal structures. For instance, unit cell of the β-rhombohedral boron, which is believed to be a ground-state modification, consists of 105 regular atomic sites (Figure 3), but in real crystals with partially occupied regular
Figure 1. Cosmic abundances of chemical elements (dashed and solid curves are for interstellar medium and galactic cosmic rays, respectively) [2].
Figure 2. Relationship of boron to other chemical elements by Lipscomb [3].
sites and interstitials there are about 106.5 boron atoms per cell. Figure 4 present an icosahedron B12 with boron atoms at vertexes, which serves as a main structural unit of the β-rhombohedral boron crystals. Because of electrondeficiency and high concentration of intrinsic point defects, at the moderatetemperatures, it is a p-type semiconductor revealing specific mechanism of hopping conduction [5]. In addition to crystalline modifications, elemental boron forms diatomic molecule B2, small molecular clusters Bn, and some nanophases which also are constructed from the interconnected icosahedra. Thus, as a rule most of boron atoms are members of the almost regular atomic triangles and surrounded by 6 nearest neighbors (5 intra- plus 1 inter-icosahedral bonds). This circumstance leads to the possibility to synthesize a nanotubular material in form of cylindrically rolled boron surfaces with triangular two-dimensional lattices. Boron nanotubes at the first time theoretically predicted by Boustani et al [6] afterwards have been obtained experimentally as well [7]. Large clustered unit cells of boron crystals contain a few different types of interstitial voids sufficiently large to accommodate foreign atoms in highconcentrations and without noticeable distortions in lattice. Consequently, doping makes it possible to form the series of boron-based alloys with well-tuned properties. Acceptor behavior of boron atoms and their clusters favors formation of the huge number of borides, i.e. compounds of boron B with metals Me which usually are characterized by the donor behavior. Only binary crystalline compounds B/Me with chemical formulas from Me5B to MeB66 can be counted of ~ 250 with ~ 40 different crystal sublattices [8]. Like the pure boron, higher borides are characterized by the clustered structures based on icosahedron and/or other (smaller or bigger) boron-cages, while among the lower borides one can find wide vi
Figure 3. Unit cell of β-rhombohedral boron.
Figure 4. Boron icosahedron B12.
variety of one- and two-dimensional structural motifs constructed from the constituent boron atoms (Figure 5). For instance, hexaboriedes MeB6 usually consist of two simple cubic sublattices with Me atoms and boron octahedra B6 in sites, respectively (Figure 6); while diborides MeB2 can be imagined as a layered structures alternating B and Me plane atomic networks (Figure 7). Great variety of the possible combinations of B−B, B−Me, and Me−Me bonds allows tuning of the boride properties controlling their composition and structure. Among higher borides, one can find all kinds of electronic structures: they can be dielectric, semiconductor, semimetal or metal. As for the lower borides, combining strength and hardness with deal of plasticity usually they are metallic what according to Samsonov means specific kind of electronic structure revealing in high conductivity, in some cases even exceeding that for the corresponding metal. Boron compounds with non-metals which are characterized by the higher electron affinity, i.e. boron hydrides, carbides, nitrides, oxides, silicides, and arsenides show lesser complicated, common structures. For example, boron nitride BN can be found in layered hexagonal or denser cubic and wurtzite-like structures (Figure 8), as well as in form of nanotubes and fullerenes. They also greatly differ from metal borides by properties being semiconductors or wide-gap insulators. vii
Figure 5. Structural units from boron atoms in borides: isolated atoms (1), coupled atoms (2), zigzag chain (3), straight chain (4), branched chain (5), paired chains (6), three-throw chains (7), goffered plane network (8), plane stacked network (9).
Figure 6. Unit cell of metal hexaboride MeB6 (B and Me atoms are shown by black and white circles, respectively).
Figure 7. Unit cell of metal diboride MeB2 (B and Me atoms are shown by black and white circles, respectively). viii
(2)
(1)
(3)
Figure 8. Unit cells of hexagonal (1), cubic (2), and wurtzite-like (3) boron nitrides BN. (B and N atoms are shown by black and white circles, respectively).
Strong B−B bonds makes boron based solids refractory and aggressiveenvironment resistive, while diversity of space and electronic structures and, consequently, the diversity of their properties yields extremely large sphere of technical and technological applications. At the beginning of Century, boron compounds altogether more than ~ 200 were used in ~ 250 fields [9]: from glass to detergent industry, in metallurgical, nuclear, tooling, vacuum and solid-state electronic applications etc. Importance of boron and boron compounds is testified by the fact that since 1959 the developments in boron-studies systematically are summarized at the regular International Symposia on Boron, Borides & Related Materials (ISBB). Here is the list of these Symposia: 1st – Asbury Park (USA) 1959, 2nd – Paris (France) 1964, 3rd – Warsaw (Poland) 1968, 4th – Tbilisi (Georgia) 1972, 5th – Bordeaux (France) 1975, 6th – Varna (Bulgaria) 1978, 7th – Uppsala (Sweden) 1981, 8th – Tbilisi (Georgia) 1984, 9th – Duisburg (Germany) 1987, 10th – Albuquerque (USA) 1990, 11th – Tsukuba (Japan) 1993, 12th – Vienna (Austria) 1996, 13th – Dinard (France) 1999, 14th – Saint Petersburg (Russia) 2002, 15th – Hamburg (Germany) 2005, and 16th – Matsue (Japan) 2008. The next, 17th ISBB will hold in Istanbul, Turkey which is a country possessing 72 % of the World’s boron reserves [10]. Obviously, within the limits of one Volume it is impossible to perform comprehensive consideration for such wide set of problems. Luckily, we succeeded in collecting of papers, which concern all main aspects of the present-day studies ix
on boron based solids. On the on hand, here are considered topics, like the crystalline structure, nonstoichiometry, doping, and nanostructures; synthesizing and thermodynamics; mechanical properties, chemical bonding, and electronic structure. On the other hand, presented investigations deal with materials, like the crystalline boron, metal borides, and III-V compounds of boron. Opening paper of the Volume by T. Oku et al emphasizes that to understand electronic structures and properties of boron-based materials, it is mandatory to investigate their crystal structures and, especially, the local atomic structures. Giant clusters B12@B12@B60 and B12@(B12)12 consisting of B12-based icosahedral clusters in doped β-rhombohedral boron (of composition Al2.6Cu1.8B105) and YB56 crystals, respectively, were directly observed by highresolution electron microscopy (HREM), image calculation, and crystallographic image processing. In addition, positions of all doping atoms were detected. The accurate positions in boron clusters for Y-holes, which consist of a single yttrium vacancy and a single yttrium atom, were determined combining digital HREM with electron diffraction. The local atomic disordering also was detected from differential images between HREM images observed and simulated on the basis of X-ray data. It is believed that the disordering would be caused by the local Y-hole arrangements. The carried out investigation clearly indicates that structure analysis combined with digital HREM, electron diffraction, and differential images is useful for the evaluation of atomic positions and disordering in boron-based crystals, especially in their nanoscale regions. Next paper by O. Tsagareishvili et al keeps on the β-rhombohedral boron based alloys. Influence of the alloying with transition metals Ti, V, Fe, Co, Ni, Zr, and Re on β-rhombohedral boron mechanical properties and internal friction spectra is shown to reveal changes in the dislocations’ mobility and, consequently, in its elasticity. Obtained results may be of a practical interest for raising elasticity of the boron-based materials. Synthesis of diborides and their low-temperature thermodynamic properties are considered in paper by V.V. Novikov. It has been found that samples of better quality (with small inclusions of the excess phases) of the magnetic rare earth (RE) metals diborides TbB2, DyB2, HoB2, and ErB2 can be synthesized from elements combining short-term high pressure and high temperatures effect with the subsequent homogenizing heating. In the low-temperature region, their heat capacity temperature-dependences revealed anomalies in form of sharp maxima caused by the ferromagnetic transitions. There is suggested model of the interacting metal and boron sublattices well-describing lattice heat capacity temperature-dependences. Anomalies of temperature changes in crystal lattice parameters of the magnetic RE diborides were obtained and explained in model of ellipsoidal form of RE ions. x
Paper by I.R. Shein et al develops diborides subject, considering nonstoichiometry in s, p, and d metals diborides. The problem of nonstochiometry in diborides with AlB2-type structure attracts the great attention due to the recent discovery of superconductivity in MgB2. There are presented the synthesis methods of nonstoichiometric diborides together with the experimental and theoretical results on the effect of cation vacancies on their properties. It is shown that the homogeneity range for Мe1−xВ2 can reach up to х = 0.5 and the presence of Me metal vacancies leads to the essential changes of physical and chemical properties. This investigation carried out on nonstoichiometry in borides has a practical importance for the elaboration of multi-component ceramics. Paper by M. Ferhat et al provides with deep insight into the unusual electronic structure of zinc-blende III-V boron compounds BP, BAs, BSb, and BBi (without BN, because it behaves more like other nitrides). Their electronic, structural, bonding, pressure, alloying and dynamical properties were discussed based on the fundamental electronic structures review in the light of available experimental and theoretical results. It was surveyed in what bulk III-V boron compounds and their alloys electronic structures are different from those of conventional III-V semiconductors: a great dial of band extrema positions and electron energy intervals of band-structure in such boron compounds are distinguished from those in most III-V semiconductors; they are characterized by a strong p-p mixing in the valence-band maximum in contrast with most III-V compounds and show strong covalency more reminiscent to the VI family in bonding properties; boron compounds have distinguished band gap volume deformation potential compared to other III-V; all III-V boron alloys show strong band gap bowing; and finally, the effective charges calculated for boron in these compounds are negative, while the III group element in other III-V compounds usually have a positive effective charge. Closing paper by L. Chkhartishvili devoted to the boron nitride BN nanosystems continues discussion on III-V boron compounds. Here is given an overview of the present state of studies in synthesizing methods, atomic geometry, binding, stability, electron structure, and applications of boron nitride nanosystems, which constitute the base for one of the mostly perspective class of nanoscale materials. In particular, the explicit expressions in term of B−N bond length are obtained for atomic sites coordinates and intersite distances in regular boron nitride nanotubes and fullerenes. Description made for the boron nitride nanosystems geometries may serve as basis for further detailed investigations on their stabilities and electronic structures.
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Kuznetsov, N.T. 1988, Chemical Encyclopedia, 1, Knunyants, I.L. (Ed.), Soviet Encyclopedia, Moscow, 299. Viola, V.E. 1991, AIP Conf. Proc., 231, 1. xi
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Lipscomb, W.N. 1981, J. Less-Comm. Met., 82, 1. Jemmis, E.D., and Prasad, D.L.V.K. 2006, J. Solid State Chem., 179, 2708. Chkhartishvili, L.S. 1989, Nature of Hopping Conduction in β-Rhombohedral Boron, TSU, Tbilisi. 6. Boustani, I., Quandt, A., Hernández, E., and Rubio, A. 1999, J. Chem. Phys., 110, 3176. 7. Ciuparu, D., Klie, R.F., Zhu, Y., and Pfefferle, L. 2004, J. Phys. Chem. B, 108, 3967. 8. Peshev, P. 1994, Jpn. J. Appl. Phys. Ser., 10, 118. 9. Acarkan, N. 2002, Proc. 1st Int. Boron Symp., DU, Kütahya, 1. 10. Çelen, M.E., and Özmen, L. 2008, 16th Internat. Symp. Boron, Borides & Rel. Mat., KM, Matsue, 36.
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