Interstitial octahedral site

In tire transition-metal monocarbides, such as TiCi j , the metal-rich compound has a large fraction of vacairt octahedral interstitial sites and the diffusion jump for carbon atoms is tlrerefore similar to tlrat for the dilute solution of carbon in the metal. The diffusion coefficient of carbon in the monocarbide shows a relatively constairt activation energy but a decreasing value of the pre-exponential... 

Although the face-centred cubic structure of metals is close packed, it is still possible for atoms which are much smaller than the host metal atoms to fit into interstitial sites inside the structure, while maintaining the essential properties of metals such as electrical conductivity and heat transport. These interstitial sites are of two kinds. The octahedral interstitial sites have six metal atoms at equal distances from the site, and therefore at the apices of a regular octahedron. The tetrahedral interstitial sites have four nearest neighbour metal atoms at the apices of a regular tetrahedron. A smaller atom can just fit into the octahedral site if the radius ratio is... 

The same atom-centered polyhedra can be used to describe interstitial diffusion in all the many metal structures derived from both face-centered cubic and hexagonal closest packing of atoms. In these cases the polyhedra are centered upon a metal atom and all the tetrahedral and octahedral interstitial sites are empty. The hardening of metals by incorporation of nitrogen or carbon into the surface layers of the material via interstitial diffusion will use these pathways. 

The crystal structures of the end-members with x = 0 are so-called y-tetrahedral structures , with distorted hexagonal close packed oxide arrays and cations distributed over various tetrahedral sites. In the solid solutions, Li ions are found, by powder neutron diffraction, to occupy partially various tetrahedral and octahedral interstitial sites, which link up to form an essentially three-dimensional conduction pathway. 

Figure 18 Schematic representation of the alkali metal diffusion paths. Octahedral interstitial sites are represented by black ions. The transition states are situated at the midpoints connecting the interstitial sites.

The spatial localization of H atoms in molecular H2 and HD crystals has been inferred by Miyazaki et al. [1991] from analysis of the hyperfine structure in the EPR spectrum caused by the interaction of the unpaired electron with the matrix protons. The mean distance between an H atom and protons of the nearest molecules, inferred from the ratio of line intensities for the allowed (Amj = 0) and forbidden (Amj = l) transitions, is 3.6-4.0 A and 2.3A for H2 and HD, respectively. It follows from the comparison of these distances with parameters of the hep lattice of H2 that the H atoms in the H2 crystal occupy substitutional sites, whereas in the HD crystal they occupy octahedral interstitial sites. 

The ordering of atoms on the sites of AB3 alloys of DOi9 structure increases the solubility of interstitial impurity, if C atoms are distributed over the octahedral interstitial sites and ae parameter is negative (P < a). In the rest cases the atomic order prevents the dissolution of interstitial impurity. At the boundary values of order parameter ae = -1/3 and +1 the concentration of interstitial atoms in T positions reduces to zero. 

Sm2Fei7(iso-structural to S1112C017) there are three octahedral interstitial sites which may be filled by nitrogen atoms to form Sm2Fei7N3. The effect of interstitial atoms on the intrinsic magnetic properties of iron-based intermetallics can be further exploited in materials design. 

The diamond structure represents the archetype of a periodic covalent molecule and should resolve any paradox between the two models. For this purpose it is rewarding to re-examine the rocksalt structure as described in Figure 5.11. It is seen to derive from a CCP (Al) structure defined by the red spheres, with anions in the set of octahedral intersticial positions - green spheres. The crystallographic unit cell contains four metal-ion positions8 and four octahedral intersticies. In addition there are two sets of four tetrahedral interstitial sites, as shown in Figure 5.17. In the zincblende structure the octahedral interstitial sites are vacant and anions are located in four equivalent tetrahedral sites.9... 

Free energy of fulleride is calculated with regard to the interaction of nearest pairs cf> 1 cf> 1, d>2ct>2, < i 2, dfiPt, d>2Pt, taking that fullerenes are distributed in lattice sites and platinum atoms occupy the octahedral interstitial sites of lattice. 

The elementary cell has four sites (three of the first type and one of the second one) and four octahedral interstitial sites (three of the Oi type and one of the 02 type). 

The jump vector. A, wUl obviously depend on the mechanism and the structure. For example, an atom diffusing through the octahedral interstitial sublattice in an FCC metal, with lattice spacing a (Fig. 6.6), must jump the distance between interstitial sites, A = fl/V2. This is, of course, the same distance an atom diffusing by the vacancy mechanism must jump. It will be recalled that for every atom in a close-packed stmcture, there are two tetrahedral interstitial sites and one octahedral interstitial site. The reader might ask if the distances between the tetrahedral sites ate the same. 

Figure 6.6. Simple geometry shows that the jump distance between octahedral interstitial sites and between lattice sites in an FCC metal, with unit cell edge a, is a/V2.

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

Figure 2.1. Ionic model for sodium chloride. This is a face-centered arrangement of chloride ions (white), with sodium ions occupying the octahedral interstitial sites (red). The attractive electrostatic forces, a, between adjacent Na+ and Cl ions, and repulsive forces, r, between Na" " ions are indicated.

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. 

The FCC lattice of Pd accommodates hydrogen in the octahedral interstitial sites. The main part of the phase diagram (Fig. 20) looks like that of a fluid with a critical point at a temperature of approximately 565 K, a pressure of 20 bar, and a density x = H/Pd of 0.25. At temperatures below 50 K, ordered hydrides exist, which are not considered here (59).] Above the critical temperature, single-phase samples can be obtained for all x by loading Pd under suitable pressures of hydrogen. At RT, the gas (low x) a phase is stable for 0 < x < amax = 0.015, and the liquid (high x) (3 phase is stable for 0.61 Bmin x < 1. The (3 phase is stable to low temperatures for x > 0.65. The only structural difference between the a... 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

The LiMn204 spinel structure, shown in Figure 19, can be described as a cubic close-packed oxygen array with the manganese cations occupying one-half of the octahedral interstitial sites and the lithium cations one-eighth of the tetrahedral sites. The interstitial tetrahedral and octahedral spaces in the [Mn2]04 framework are interconnected to form three-dimensional pathways for Li+ ion diffusion (lO -lO " m s ). 

Usually, the formation of nanoparticles proceeds under non-equilibrium conditions, and the free energy of nanoparticles may exceed the energetic state of the perfect crystalline structure. The higher energy can lead to an atomic disordering in the PbS structure [4]. One possible disordering mechanism could be the occupation of octahedral interstitial sites in the fee subblattice with an additional occupation of tetrahedral interstitial places. 

Many of the papers appearing about uranium hydride in the last few years are concerned with technical problems arising from the possibility of its use for storing tritium as UT3 in connection with nuclear fusion devices. Several papers on these topics can be found in ref (336). From the chemical point of view, perhaps the most interesting study was the demonstration, using He NMR, that the He formed by tritium decay soon forms microscopic gas bubbles rather than being trapped in octahedral interstitial sites. The two situations can be expected to lead to very different relaxation times (337-340). 

The ways in which the empty volume is distributed in a crystal are both interesting and important. For the close-packed fee structure, two types of interstitial sites, upon which the free volume in the unit cell is centered, are identifiable. An octahedral site is surrounded at equal distances by six nearest neighbor atoms. Figure 21.15 shows that such sites lie at the midpoints of the edges of the fee unit cell. A cell has 12 edges, each of which is shared by four unit cells, so the edges contribute three octahedral interstitial sites per cell. In addition, the site at the center of the unit cell is also octahedral, so the total number of octahedral sites per fee unit cell is four, the same as the number of atoms in the unit cell. 

Chromium(III) oxide has a structure in which chromium ions occupy two thirds of the octahedral interstitial sites in a hexagonal close-packed lattice of oxygen ions. What is the if-electron configuration on the chromium ion ... 

Figure 14.9 Unit cells of fullerite with fee (a) and bee (b) structures with lattice hydrogen only. (Shaded circle) Sites of crystalline lattices, in which fullerenes molecules are distributed ( ) octahedral interstitial sites (o) tetrahedral interstitial sites, in which atoms of lattice hydrogen are located. (Reprinted from Ref [80] with permission from Elsevier.)...

---