Reconstruction transition metals

Kooli, F., Rives, V. and Ulibarri, M. A. (1995). Preparation and study of decavanadate-pillared hydrotalcite-like anionic clays containing transition metal cations in the layers. 1. Samples containing nickel-aluminium prepared by anionic exchange and reconstruction. Inorg. Chem. 34, 5114. 

The replacement of main-group atoms in clusters by transition-metal atoms generates a richer structural chemistry superimposed on the cluster basics illustrated by the p-block systems. A logical question arises here. What would a one-dimensional material containing a transition metal look like Well, the d AOs will generate bands in a similar manner as the s and p orbitals. The major novelty will be the introduction of orbitals of 8 symmetry. Let s look at a hypothetical chain composed of equidistant Ni atoms (d = 2.5 A). The computed band structure, DOS and COOP are illustrated in Figure 6.14. The COs at k = 0 and ir/d arc drawn below. As a review of the previous section, we will reconstruct it starting from the Bloch functions associated with the nine Ni AOs. 

The oxides containing cations in octahedral coordination, such as MgO and TiOz, seem to suffer little or no reconstruction of the crystal structure at the surface, but there are major changes in electronic structure. These effects have important implications for surface reactivity, especially when oxygen vacancy defects are considered and particularly in transition-metal oxides. The example of TiOj was discussed above more complex behavior is shown by species such as TijOj and NiO (Henrich, 1987). The latter, like other transition metals, shows increasing [Pg.415]

It is clear that much work remains to be done to extend our understanding to polax surfaces of transition metal oxides in which the cations have partially filled d orbitals. An especially challenging issue is related to mixed valence metal oxides, such as Fe304, in which the cations exist under two oxidation states. In addition, considering the rapid development of ultra-thin film synthesis and characterization, a simultaneous effort should be performed on the theoretical side to settle the conditions of stability of polar films. More generally, on the experimental side, it seems that one of the present bottlenecks is in a quantitative determination of the surface stoichiometry, an information of prominent interest to interpret the presence or absence of reconstruction. 

Fig. 1 shows the rocksalt lattice [15]. We will discuss MgO and NiO as limiting cases of oxides, one containing a simple metal ion and the other one a transition metal ion. The (100) surface of such a material represents a non-polar surface, the (111) surface represents a polar oxide surface. Since the lattice constants are very similar for both oxides (MgO 4.21 A, NiO 4.17 A) [15], we expect the surface structures to be similar. The non-polar surface exhibits a nearly bulk terminated surface as shown in Fig. 2a and it is very similar for both materials. We have put together information from FEED [16-21] and STM [22-25] analysis. There is very small interlayer relaxation and only a small rumpling of the surface atoms, whereby the larger anions move outwards and the small cations very slightly inward. A completely different situation is encountered for the polar (111) surfaces. Due to the divergent surface potential [13] on an ideally, bulk terminated polar surface, the surfaee reconstructs and exhibits a so... 

Now we will overview some experiments that reveal the specificities of the Jahn-Teller effect in diluted crystals. First of all, we will discuss a justification of their relaxation origin. We have mentioned before that the first experiments were done on the crystals of aluminum oxide (corundum), yttrium aluminum garnet, yttrium iron garnet, and lithium gallium spinel doped with a number of 3d ions [10,11]. The main result was the discovery of attenuation maximum which was considered to be observed at cot 1 and reconstruction of the relaxation time temperature dependence. In some experiments reported later both the velocity and attenuation of ultrasound were measured as functions of the temperature. They were done on ZnSe and ZnTe crystals doped with transition metals. These crystals have the zinc-blende structure with the Jahn-Teller ion in tetrahedral coordination. The following... 

In this section we will discuss main features of atoms and molecules chemisorbed to transition-metal SLufaces. Adsorption induced reconstruction will be included. 

Our discussion of dislocations has thus far made little reference to the significant role that can be played by the dislocation core. Whether one thinks of the important effects that arise from the presence of stacking faults (and their analogs in the alloy setting) or of the full scale core reconstructions that occur in materials such as covalent semiconductors or the bcc transition metals, the dislocation core can manifest itself in macroscopic plastic response. Our aim in this section is to take stock of how structural insights into the dislocation core can be obtained. 

Surface Reconstructions in W. In earlier sections, we have noted that the pair functional formalism is not always appropriate as the basis for an energetic description of metals. One case where this is evident is in the context of the surface reconstructions seen on certain bcc transition metals. In particular, the (001) surface of both W and Mo exhibits fascinating reconstructions, and as in the case of the Au(OOl) surface, the symmetry breaking distortions leave few vestiges of their crystallographic ancestry. 

The (110) surfaces of the transition metals Au, Ir. Pt display both a stable (1x2) and a metastable (1x3) reconstruction. The structural details of these reconstructions are listed in table 2 and the (1x2) reconstructed surface is illustrated in fig. 2. Both the (1x2) and (1x3) reconstructions are of the missing-row type which involve the removal of every second (1x2) or third (1 x3) row of atoms from the top atomic plane of the bulk termination. The removal of this row is accompanied by significant atomic relaxations of at least the first three atomic planes perpendicular to the surface, see table 2. Both Pt(110)(lx2), Pt(110)(lx3) and Au(110)(lx2) have relaxations of a similar magnitude but the relaxations of Ir(110)(lx2) are significantly smaller. In addition to the planar relaxations, all of these surfaces exhibit lateral motions of the atoms within the second atomic plane out towards the valleys left by the missing rows. In addition, the removal of the atomic row causes a buckling of the third atomic layer which conforms with the hill and valley structure of the missing-row surface. 

The adsorption of 0.4 ML of the transition metal Co on Si(100) results in the removal of the (2x1) reconstruction with the adatom occupying the coplanar four-fold hollow site formed by four top layer Si atoms (Meyerheim et al., 1991). The Si Co bond length is found to be 2.35 A. 

The synthesis of hydrocarbons from CO hydrogenation over transition metals is a major source of organic synthetic chemicals and fuels. The Fischer-Ttopsch (FT) reaction, which is directed to the production of hydrocarbons from syngas, implies the polymerisation of-CHx entities and carbon-carbon bond formation is required. The historical achievements have been revised on several occasions see for instance, the reviews by Vannice,2 Schulz3 and the special issue of Catalysis TodayA devoted to FT. Also, a synopsis of the main recent industrial developments has been presented by Adesina.5 Furthermore, the recent edition of Topics in Catalysis6 should be mentioned, where different aspects of the reaction mechanism, surface reconstruction of active surfaces, improved reactors and optimisation of catalyst preparations have been treated by various specialists, scientists and engineers. 

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