B2 compounds

Theoretical calculations were performed with the linear muffin tin orbital (LMTO) method and the local density approximation for exchange and correlation. This method was used in combination with supercell models containing up to 16 atoms to calculate the DOS. The LMTO calculations are run self consistently and the DOS obtained are combined with the matrix elements for the transitions from initial to final states as described in detail elsewhere (Botton et al., 1996a) according to the method described by Vvedensky (1992). A comparison is also made between spectra calculated for some of the B2 compounds using the Korringa-Kohn-Rostoker (KKR) method. 

Summing up, our investigation indicates that the process of formation of excess-compound solid solutions of the investigated systems does not correspond to a simple model of statistical substitution. This may possibly be connected with the formation of a ternary Af B2 compound (valence electron concentration 3.42) of the group of valence compounds. The existence of such a compound was predicted by one of the authors. 

In addition to the research on LI2 and B2 compounds described above, ductility and fracture mode have been reported on compounds with other crystal structures. We have chosen not to discuss these alloys in depth because the research on them has not been as broad and because most of the important features of brittle fracture in these compounds have already been described by considering the other systems. Table 7 summarizes these studies. (Note that in the table a crystal structure that is given in parentheses is the structure of the base compound but alloying additions have caused a change in the structure to the one that is listed first.) The main point to note is that, just as with the LI2 and B2 compounds, these compounds tend to be brittle at room temperature and the fracture is either cleavage or intergranular. Attempts to alloy these compounds have in some cases produced significant... 

An explanation for the oocurroice of structural vacancies has been given from the Hume-Rothery rules. One knows that the ordered B2 compounds exist only if the number of conduction electrons per atom is near to and does not exceed 1.5. This is the case for sttndiiometric NiAI, if one admits that Ni, a transition metal, contributes zero electrons. Then, for an aluminum-rich alloy Ni, if non-stoichiometry occurs via Al,., antisites,... 

Kim (1984, 1986, 1987) applied his model to NiGa, FeAl, CoGa, CU3AU, and CuAu. Reasonable input parameters c a. bb. ind e allowed the calculations to reproduce very satisfactorily the total vacancy concentration measured in these alloys for several compositions and temperatures. In the case of the B2 compounds, the vacancies were shown to be entirely on the transition-metal sublattice, and it was suggested that they are thermal vacancies with low formation energies rather than structural vacancies (see discussion in Section 3.2). 

Compound A (C4H10) gives two different monochlondes on photochemical chlorination Treatment of either of these monochlondes with potassium tert butoxide in dimethyl sulfoxide gives the same alkene B (CaHg) as the only product What are the structures of compound A the two monochlondes and alkene B2... 

Ga2Se3 [12024-24-7] 1005-1010 monoclinic, B2/m (superstmcture of deformed blend type) cubic by quenching of Hquid compound (disordered sphalerite type) 4.95... 

Eight variants of the DD reaction mechanism, described by Eqs. (21-25) have been simulated. The simplest approach is to neglect B2 desorption in Eq. (22) and the reaction between AB species (Eq. (25)). For this case, an IPT is observed at the critical point Tib, = 2/3. Thus this variant of the model has a zero-width reaction window and the trivial critical point is given by the stoichiometry of the reaction. For Tb2 < T1B2 the surface becomes poisoned by a binary compound of (A -I- AB) species and the lattice cannot be completely covered because of the dimer adsorption requirement of a... 

Another route is via the more stable B2(NMe2)4 (see reaction scheme). Thermal stabilities of these compounds parallel the expected sequence of Pjr-Pjr bonding between the substituent and B ... 

The reaction mixture was removed from the vessel and distilled at a pressure of 30-60 mm, and a bath temperature of 30°C to 50°C until the methanol had all been removed. The extremely viscous tarry residue remaining in the still pot was given a very crude distillation, the distillate boiling at B2°C to 1 32°C/2 mm. In an attempt to purify this distillate by a more careful distillation, 5.3 g of a liquid distilling from 53°C to 150°C/5 mm was collected. At this point, much solid sublimate was noted not only in this distillate but in the condenser of the still. 7 g of the solid sublimate was scraped out of the condenser of the still. Recrystallization of the sublimate from ethyl acetate containing a small amount of petroleum ether gave beautiful crystals melting at 175°C to 177°C (5 g). Infrared analysis confirmed that this compound was hydroquinone (9% conversion). 

First, the stability of the fitted Llo structure relative to other crystal structure with the same composition can be studied. In the present case we calculated the cohesive energies of fully relaxed B2 and structure 40 compounds and found 4.41eV and 4.50 eV, respectively. These are both lower than the cohesive energy of the Llo structure. Structure B19 was also investigated but relaxation always transformed this structure into Llo. 

Electrode materials with a spinel-type structure, A[B2]X4, have become the subject of intensive research over the past 10 years. This interest is partly due to the fact that many spinel compounds exist in nature, and are therefore intrinsically stable materials. Furthermore, the family of spinels encompasses a vast number of compositions because it is possible to vary not only the type and valency of the A and B cations within the A[B2]X4 structure, but also the type and valency of the X ani-... 

Electrodes that are prepared from acid-leached LT-LiCo, xNix02 compounds (0< x<0.2) show significantly enhanced electrochemical behavior over the parent LT-LiCo1 xNix02 structure. The improved performance has been attributed to the formation of compounds with a composition and cation arrangement close to the ideal Li[B2]04 spinel structure (B = Co, Ni) [62]. These spinel-type structures have cubic symmetry, which is maintained on lithiation the unit cells expand and contract by only 0.2 percent during lithium insertion and extraction. 

Consider now a series of compounds A, that react with two reagents, Bi and B2. A good example is the reactions of alkyl radicals (the A, s are R") with BrCCl3 (B ) and CCI4 (B2). The radicals considered are planar, tt radicals that are primary, secondary, and tertiary, but not bridgehead.21 The scheme is... 

The inequality indicates that if a concerted mechanism (where b4 and b2 change simultaneously) gives a Ag which is much lower than our stepwise estimate, we will have smaller Ag< age. This possibility, however, is not supported by detailed calculations (Ref. 6). Direct information about Ag age can be obtained from studies of model compounds where the general acid is covalently linked to the R-O-R molecules. However, the analysis of such experiments is complicated due to the competing catalysis by HaO+ and steric constraints in the model compound. Thus, it is recommended to use the rough estimate of Fig. 6.8. If a better estimate is needed, one should simulate the reaction in different model compounds and adjust the a parameters until the observed rates are reproduced. 

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