Borides electronic structure

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

Borides Sohd-state Chemistry Carbides Transition Metal Solid-state Chemistry Electronic Structure of Sohds Quasicrystals Structure Property Maps for Inorganic Solids Superconductivity Zintl Compounds. 

The metal borides are one of the five major classes of boron compounds (1). In the following we review the geometric and electronic structural data with an emphasis on the transition metal borides. Because the structures of transition metals and elemental boron provide end points, we begin by reviewing the solid state structures of these elements. A brief survey of the range of metal boride structures in general is followed by some more detailed consideration of the problems of electronic structure raised by the geometries of the transition metal borides. 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

It is interesting that the silicides of U provide examples of all the main types of silicide structure/ for in addition to those listed there is U3Si which has a distorted version of the CusAu structure (p. 842), in which there are discrete Si atoms entirely surrounded by U atoms. They also show that some silicides adopt the same structures as certain borides which contain extended systems of linked B atoms, though because of the greater size of Si and its different electronic structure borides and silicides are not generally isostructural. 

Binary and Ternary Compounds with Elements of Groups II, III, and V. 2f-ray electron spectroscopy has been used to probe the electronic structures of tungsten borides, nitrides and also some carbides and oxides. Enthalpies of formation AH°298.i5 the tungsten borides WB and W2B have been determined by fluorine calorimetry and are — 66.5 and — 68.2 kJ mol respectively. The theoretical possibilities of preparing M02B, a-MoB, MO2B5, WjB, a-WB, and W2B5 by chemical vapour deposition is discussed and MoXj (X = B or Be) have been prepared and examined by X-ray methods. ... 

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... 

The electronic structure of quaternary borides of the type RETBC was presented and discussed recently in a review-type paper. Theoretical calculations, predictions, and syntheses of were also presented for BN analogues of fullerenes (fulborenes) and BN nanotubes. ... 

We have performed ab initio calculations on the diatomic molecule scandium boride cation, ScB", determining the electronic structure both of its ground state and of a series of low-lying excited states. 

Electronic Structure of Elemental Boron and Boron-Rich Metal Borides 139 5.9... 

Materials based on boron compounds have been explored for many decades because of their exceptional properties in respect to chemical bonding, crystal structure, and phonon and electron conduction. Especially in the field of energy conversion, electron emission, and neutron absorption, borides occupy many niches of application for which no other material can be employed. Until approximately 1980, the main interest in borides always came, however, from basic research aimed at the understanding of their electronic structure, being either responsible for the unique transport properties or the peculiarities in chemical bonding. It is, therefore, no wonder that the most information about borides was at that time created from the viewpoint of physicists and chemists. 

Burdett JK, Candell E, Miller GJ (1986) Electronic structure of transition-metal borides with the AIB2 structure. J Am Chem Soc 108 6561-6568... 

H Ihara. Electronic structures of the transition metal carbides and borides studied by X-ray photoelectron spectroscopy and band calculation. Researches of Electrotechnical Laboratory No. 725, 1977. 

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