Octahedral site occupancy

Substitution of iron by nickel can be expected to order the Ni atoms at the corner positions since placement of a nickel atom (tigers1) at a face requires the eg orbital perpendicular to the N—Nif bond to be filled, thus preventing N atoms from occupying adjacent octahedral sites. (Occupancy would require antibonding Nif states to be populated.) Such an effect hinders N diffusion. In fact it was found... 

Figure 5 Pattern of octahedral site occupancies between pairs of close-packed layers in (a) rutile and (b) wolframite. Small shaded circles are octahedral sites above the plane of paper. Small open circles are sites below the plane...

The definition of the verdine facies is largely due to the work of Odin (1988). Bailey (1988) defined the mineral odinite. Verdines and odinite span much of the range of octahedral site occupation from 2.5 to 2.0 ions, containing much ferric iron and alumina. Assuming that all of the minerals are 7 A structures, the difference in observed composition of the different phases can be illustrated by their octahedral cation occupancy AI2 kaolinite (dioctahedral) (Mg, Al)3 2.5 7 A trioctahedral chlorite and (Mg, Al)2 2.5 odinite (di-, trioctahedral). 

Subsequent to an extensive review of brittle micas (Guggenheim 1984), additional crystal-chemical details of clintonite-IM (space group C2/m) were reported by MacKinney et al. (1988) and Alietti et al. (1997). These studies confirmed that natural clintonite crystals do not vary extensively in composition (i) the octahedral sites contain predominant Mg and Al with Fe to <7% of the octahedral-site occupancy (ii) the extent of the substitution Mg.2 " Si (A1,D), which involves the solid solution of... 

In the case of inverse ferrites (or in any case where more than one type of cation occupies the same set of crystallographic sites), ordering phenomena can be expected (Gorter, 1954). In inverse ferrites, octahedral site occupancy by two kinds of cations (divalent and trivalent) can lead to long-range order, where successive (001) layers of octahedral sites are occupied alternately by D and T cations. In this case, there are two ionic sublattices (Fuentes, Aburto Valenzuela, 1987) on octahedral sites. Fig. 2.4. 

The relative increase in intensity of the 1.6 eV peak at higher Ep is therefore evidence of a hydride phase increasing in H concentration with depth. The system is disordered, consistent with random occupation d tetrahedral sites, and a CeH (x < 2) stoichiometry. Heating to 550°C leads to partial H desorption, but at lower temperatures a different hydride phase may become established with possible octahedral sites occupation near the surface. 

In general less quantitative insight has been achieved thus far in dilute metal-hydrogen systems. Because of its simplicity (cubic symmetry, regular octahedral site occupancy) and its importance as a prototype metal-hydrogen system we will first consider recent results on a-PdH. Let us consider a hydrogen atom on the cubic octahedral site, as... 

It is difficult to determine the 5 parameters A, B, C, N and E in the Eqs. 5-8 in a unique i/ay because all contribute to -In K°° at high temperatures. To reduce the number of free parameters we assume N = 1 for octahedral site occupancy and select a value of C similar to those obtained by inelastic neutron scattering spectroscopy. In the case of D and T the Einstein temperature Cq and Cj were obtained by scaling C with the inverse of the square root of the hydrogen mass. The results are listed in Table II and plotted as solid lines in the Figs. 2 and 3. One sees that Eqs. 5-7 give a very good description of the -In C values. 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

The electronic structure of the spinel type compound NiCo204 has been investigated by XANES, EXAFS, and Ni Mdssbauer studies. On the basis of the derived cation valencies, the octahedral and tetrahedral site occupancies as well as the formula in standard notation for spinel compounds could be delineated [25]. 

Finally, Al (/= 5/2) and Co NMR spectroscopy have been used to probe AP+ in Al-doped lithium cobalt oxides and lithium nickel oxides. A Al chemical shift of 62.5 ppm was observed for the environment Al(OCo)e for an AP+ ion in the transition-metal layers, surrounded by six Co + ions. Somewhat surprisingly, this is in the typical chemical shift range expected for tetrahedral environments (ca. 60—80 ppm), but no evidence for occupancy of the tetrahedral site was obtained from X-ray diffraction and IR studies on the same materials. Substitution of the Co + by AF+ in the first cation coordination shell leads to an additive chemical shift decrease of ca. 7 ppm, and the shift of the environment A1(0A1)6 (20 ppm) seen in spectra of materials with higher A1 content is closer to that expected for octahedral Al. The spectra are consistent with a continuous solid solution involving octahedral sites randomly occupied by Al and Co. It is possible that the unusual Al shifts seen for this compound are related to the Van-Vleck susceptibility of this compound. 

Table 2 shows a good correlation between the relative energetics of octahedral and tetrahedral site occupancy by a 3d metal ion and the projected change in LFSE for moving a 3d metal ion from octahedral to tetrahedral coordination. From rows 4 (Ti) to 8 (Co) in Table 2 the correlation is perfect the value calculated for Ai oct—tet increases along with the projected change in LFSE (from 0 for Ti to (14/5)Aq - (6/5)At for Co). 

Agreement is also poor concerning entropy and volume excess terms. Because divalent cations (Mg, Ca, Fe, Mn) occupy only dodecahedral sites whereas octahedral sites are reserved for trivalent cations (Cr, Fe, Al), each cation has only one site at its disposal and permutability is fixed by stoichiometry (cf. section 3.8.1). As regards the occupancy on tetrahedral positions, we have already seen that analyses of natural specimens show silicon deficiencies, compensated by AF ... 

The intermediate octahedral sheet is normally made up of cations of charge 2 or 3 (Mg, Al, Fe, Fe, or, more rarely, V, Cr, Mn, Co, Ni, Cu, Zn), but in some cases cations of charge 1 (Li) and 4 (Ti) are also found. In the infinite octahedral sheet, formed by the sharing of six corners of each octahedron, there may be full occupancy of all octahedral sites ( trioctahedral micas ) alternatively, one site out of three may be vacant ( dioctahedral micas ). Nevertheless, the primary classification of micas is based on the net charge of the mixed 2 1 layer. In common micas this charge is close to 1, whereas in brittle micas it... 

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

The single hydrogen of the trianion was found to occupy only one of the two octahedral sites rather than being randomly disordered in the crystalline state in both octahedral sites, with an occupancy factor of 0.5 for each position. This observed crystal ordering of the polar C v trianions, which are identically oriented... 

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