B2 type compounds

Fig. 3. Lattice constants 3332 and 2 as as a function of x for B32 type and B2 type compounds LiCdi, Tl, (Baden et al. ). There is a two-phase region within the range of composition 0.45 < x < 0.625...

Well-ordered intermetallic compounds (alloys), when processed in high-energy ball mills exhibit atomic (chemical) disordering in the early stages of ball milling [153]. Let us take as an example ordered AlRu intermetallic crystals that are of B2 type and P-CuZn structure. This structure consists of two simple cubic interpenetrating... 

Fig. 1.20 The B2-type CsCl structure of FeTi stoichiometric, ordered, compound (left), and room-temperature hydrogen PCT properties for B2-type FeTi hydrogen storage aUoys amorphous - a, nanocrystalline - b, and crystalhne - c (adopted from [155])...

Ordered Perovskite-type Compounds, A2(BB )06 Systems Cubic Fmim A feature of the perovskite structure is that, with the proper substitutions, many types of ordered structures can readily be formed. This can be accomplished by the substitution of two suitable metal ions (with different oxidation states) in the octahedral sites of the structure. In this case the unit ceil is doubled along the three cubic axes to generate an 0.8 A unit ceil (Figure 15). Partial substitution of different transition metal ions in the octahedral sites is also possible the general formulation for these compounds would be A2(B2 xB x)06. The parentheses in this formulation enclose atoms occupying the octahedral sites in the structure. 

AB structures. The five principal structures of AB type compounds are rocksalt (Bl), CsCl (B2), zinc blende (B3), wurtzite (B4) and NiAs (B8) and these are shown in Fig. 1.5. In the first four of the structures, the cation and anion sublattices are entirely equivalent and the coordination geometry around the cation and anion is the same. The rocksalt NaCl) structure is exhibited by a large number of AB type compounds. The structure (Fig. 1.5) may be thought of as consisting of two interpenetrating... 

This could have far-reaching consequences, as mechanically induced disorder can be a factor in activation of well-ordered intermetallic compounds (alloys) for reversible reactions with molecular hydrogen. Figure 1.20b shows the PCT plots, which details hydrogen sorption properties for B2-type FeTi, an important hydrogen storage alloy with capability for reversible storage at room temperature applications. The plateau of equilibrium sorption is lowered in ball milled, disordered FeTi in comparison with the not-milled counterpart. Therefore, the alloy has been... 

In Sect. C, the band structure data based on self-consistent relativistic augmented-plane-wave calculations performed by the author " are presented. Besides the electronic bands and the densities of states, the nature of the chemical bond is discussed. In Sect. D the electronic states in Zintl phases are compared with those having the B2 type of structure. As shown in Sect. B the B2 structure is closely related to the B32 structure. For intermetallic compounds the B2 structure seems to be the more natural because in this lattice all nearest neighbours of an atom A are B atoms. The reason why the compounds mentioned above crystallize in the B32 structure whereas similar compounds like LiTl and KTl form B2 phases has been frequently discussed in the literature 5 ... 

The number of AB compounds which crystallize in the B2 type of structure is much larger than those which form B32 type Zintl phases. As pointed out in Section B the NaTl type is restricted to alloys for which the sizes of the parent atoms are almost equal... 

After having discussed the three approaches, let us now turn to the results of the calculations. The values for AEband were calculated for all binary B32 type Zintl compounds and for the B2 type phase LiTP >. In the perturbation approach ll and the ASA method ) AEband always favours the B32 type of structure (also in case of the B2... 

It follows from the generalized formula AB(C03)2 of carbocernaite that, in principle, two types of compounds characterized by the simplest compositions, A2 + b2 + (c03)2 and A B (003)2, may have the carbocernaite structure. However, Voronkov and Pyatenko (1967a) pointed out that the A + B (C03)2-type compounds, such as CaCa(C03)2 = Ca(C03) (calcite, aragonite or vaterite), CaSr(C03)2 (aragonite type),CaBa(C03)2 (barytocalcite), do not have the carbocernaite-type structure. So far, carbocernaite is the only known phase that has the formula A" " B " "(003)2 nd its structure is difFerenl from any other carbonate structure... 

ThBi, which are of the CsCl (B2) type, and a few compounds of truly intermetaliic character. 

Fig. 21. Logarithm of upstroke-transition pressure versus cation/anion radius ratio for the B1 to B2 transition in lanthanide and actinide compounds. Trivalent ionic radii are used for all cations. Some transitions to other high-pressure structures are included for comparison and have been marked as such in the graphs, (a) Mono-pnictides with As, Sb, and Bi. The isolated data for the phosphides CeP and ThP are not included. No mononitrides have been observed to transform to the B2 type under pressure, (b) Mono-chalcogenides with S, Se, and Te. The EuO transition is outside the graph.

On the other hand, the B2 phase spectrum reveals two minima in the DOSs near the Fermi level, corresponding to valence electron concentrations of 6.3 and 9.6. One may thus suppose that stabilisation of the B2 carbide can be achieved when the electron concentration corresponds to the position of the Fermi level in the second DOS minimum. Then, from the calculated results (Fig. 2.9), one may conclude that the stable phase condition can be fulfilled for the B2 Vo.7Cro.3C alloy, or when part of carbon crystal lattice is replaced by more electronegative elements, e.g., by oxygen, which would result in the formation of VC0.7O0.3 oxycarbide. At present, only VC0.62O0.i6 solid solutions with a valence electron concentration (VEC) of 8.44e and VC0.525O0.441 with a VEC of 9.74c (Zainulin, Dyachkova and Alyamovsky, 1979), have been obtained experimentally. When these compounds were exposed to pressure P = 7.5 GPa at r = 1673 K, some metalloid vacancies were filled, but the transition to the B2-type structure was not observed. For this reason, additional studies of structural transformations in vanadium oxycarbides of similar compositions would be of interest. For CrC, the optimal VEC of 9.6 can probably be achieved by alloying with lower-valence metals (V, Ti, Nb, etc.). 

The same behavior was found for a number of other group VIII-IIIA intermetallic compounds with the B2 (cP2) structure (CsCl type) (Neumann et al., 1976), in particular FeAl (Ho and Dodd, 1978), CoAl (Bradley and Seager, 1939 Wachtel et al., 1973 Fleischer, 1993), CoGa (Berner et al., 1975 Van Ommen et al., 1981), and NiGa (Donaldson and Rawlings, 1976 Seybolt and Westbrook, 1964 Wasilewski et al., 1968), as well as for a few other B2-structure compounds (see review by Chang and Neumann, 1982). 

Few calculations have been done for grain-boundary structures and energies in intermetallic compounds with other than LI2 crystal structures. Only one example is presented the grain-boundary structures of B2-type NiAl were calculated using many-body potentials (Chen et al., 1989b, c). E = 5 (2 10) and (3 10) symmetrical tilt... 

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... 

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. 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

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