Interstitial structures table

Most inorganic chemistry texts list cut-off values for ther+/r ratios corresponding to the various geometries of interstitial sites (Table 2.3). However, it should also be pointed out that deviations in these predictions are found for many crystals due to covalent bonding character. An example for such a deviation is observed for zinc sulfide (ZnS). The ionic radius ratio for this structure is 0.52, which indicates that the cations should occupy octahedral interstitial sites. However, due to partial covalent bonding character, the anions are closer together than would occur from purely electrostatic attraction. This results in an effective radius ratio that is decreased, and a cation preference for tetrahedral sites rather than octahedral. 

This completes our discussion of the three principal classes into which we have divided the alloy systems. As a summary, a condensed survey of the chief structural characteristics of these classes is given in table 13.10. The remaining class of alloys to be considered is that embracing the interstitial structures. These systems, however, differ materially from the alloy systems so far discussed, and a description of them is accordingly deferred until later in the chapter ( 13 37). 

Most inorganic chemistry texts fist cut-off values for the r r ratios corresponding to the various geometries of interstitial sites (Table 2.4). For instance, the halite or rocksalt stmcture exhibited by MX (M = Grp I, Mg, Pb, Ag X = F, Cl, Br, I) are predicted to have occupation of octahedral interstitial sites. Indeed, these structures are described as a fee array of the halide ion (except for v. small F ), with the cation occupying all of the octahedral interstitial sites i.e., 4 MX units per unit cell). 

The heats of formation of the more common nitrides are shown in Table 9.1. All of the transition metal nitrides are close-packed interstitial structures and in some cases a wide variety of nitrides are formed depending on the number of octahedral holes that are filled. Manganese, for instance, forms... 

Another example for an investigation of multinuclear transition metal clusters is the SOS-DFPT-IGLO study of the C shift tensors for interstitial carbides enclosed in carbonyl clusters. The interstitial shifts are important, both as a proof for the existence of an interstitial atom, and as a potential probe of electronic structure. Table 2 compares computed and experimental shifts. For the two rhodium clusters, it has been possible to compare not only isotropic shifts but the entire shift tensors, as an independent solid-state NMR study given the first tensor data for a number of interstitial carbides and nitrides. The overall agreement between computation and experiment is good, both for the isotropic shifts and for the available tensors. The largest deviation (43 ppm) was found... 

Tantalum carbide (TaC) is arefractory interstitial carbide with a high melting point. It is structurally and chemically similar to niobium carbide. It has two phases Ta and the monocarbide TaC. Thelatteris the only phase of industrial importance and the only one described here. The characteristics and properties of TaC are summarized in Table 9.7. 

In theory, the III-V compound semiconductors and their alloys are made from a one to one proportion of elements of the III and V columns of the periodic table. Most of them crystallize in the sphalerite (zinc-blende ZnS) structure. This structure is very similar to that of diamond but in the III-V compounds, the two cfc sublattices are different the anion sublattice contains the group V atoms and the cation sublattice the group III atoms. An excess of one of the constituents in the melt or in the growing atmosphere can induce excess atoms of one type (group V for instance) to occupy sites of the opposite sublattice (cation sublattice). Such atoms are said to be in an antisite configuration. Other possibilities related with deviations from stoichiometry are the existence of vacancies (absence of atoms on atomic sites) on the sublattice of the less abundant constituent and/or of interstitial atoms of the most abundant one. 

The method can be illustrated by reference to a classical 1933 study of the defects present in wilstite, iron monoxide. Wustite adopts the sodium chloride (NaCl) structure, and the unit cell should contain 4 Fe and 4 O atoms in the unit cell, with an ideal composition FeOi.o, but in reality the composition is oxygen rich and the unit cell dimensions also vary with composition (Table 1.1). Because there is more oxygen present than iron, the real composition can be obtained by assuming either that there are extra oxygen atoms in the unit cell (interstitial defects) to give a composition FcO 1 +v, or that there are iron vacancies present, to give a formula Fci-JD. It is... 

There are large numbers of anion excess fluorite-related structures known, a small number of which are listed in Table 4.4. The defect chemistry of these phases is enormously complex, deserving of far more space than can be allocated here. The defect structures can be roughly divided into three categories random interstitials, which in... 

A few general remarks about a group of metal-hydrogen phases have been included in 3.8.4.1 where interstitial hydrogen solutions in metallic structures have been described. However, as previously observed, a number of intermediate phases are also formed in several systems. A short summary of these is shown in Table 5.2 where their formulae very often have only an indicative character and several structure types correspond to more or less extended solid solution ranges. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

The NH3 treatment for 120 min completed the clusterization of the Re species as shown in Figure 10.9. The CN of the Re-Re bonds was 5.210.3 (0.276 0.002 nm) (Table 10.7). A desorphon peak of N2 in TPD of the NHs-treated Re/HZSM-5 catalyst appeared at around 673 K, which indicates that the Re cluster possesses N atoms supplied by the NH3 treatment [73]. The amount of N2 evolved was 1.2 N2 per Reio. DFT modeling of the structure of the Re cluster based on the structural parameters obtained by FXAFS analysis revealed the formation of an N-interstitial Reio cluster in the HZSM-5 pore (structure is illustrated in Scheme 10.4) [73]. N atoms at the edge and hollow sites of the Re cluster never stabilized the Re cluster framework with the Re-Re bonds at 0.276 nm. Adsorption of nitrogen atoms on the exterior surface of the Re cluster also did not reproduce the Re-Re bond distances. 

EXAFS analysis for the sample after the fifth pulse reaction of benzene and O2 revealed the formation of Re monomers with Re=0 bonds (CN = 3.7 0.2) at 0.173 O.OOlnm and Re-O bond (CN = 1.3 0.6) at 0.211 0.002nm (Table 10.7). The monomeric structure (vi) in Table 10.7 was similar to that after the steady state reaction (i). The Re monomers (vi) were transformed into the Rejo clusters again by NH3 treatment for 2h. NH3 has two roles, N supplier and reduc-tant, in producing the catalytically active N-interstitial Re cluster, which converts benzene and O2 into phenol with a selectivity of 93.9%, accompanied with oxidation of the cluster to the inactive Re monomer (Scheme 10.4). Thus, the formation of the N-interstitial Rejo clusters and the decomposition of the Rejo clusters to the Re monomers are balanced under the steady-state reaction conditions [73]. 

Combine your information. According to Table 1.16, what is the fraction of the total type of interstitial site occupied by each cation, and what is the structure name of this compound ... 

Table III. Structural Features of Representative Interstitial Hydrides (27, 28)°...

It should be noted that for / Ni2B2C the counterpart without carbon does not exist. Co is, so far, the only transition metal for which both the filled (with C) and the nonfilled structures could be prepared (see table 4). The examples of ferromagnetic GdCo2B2 and antiferromagnetic GdCo2B2C show that the introduction of interstitial carbon has a remarkable effect on the magnetic and, consequently, electronic properties of these compounds. 

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