Octahedral cavity

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

The main source of spontaneous polarization in crystals is the relative freedom of cations that fit loosely into the crystal s octahedral cavities. The number of degrees of freedom of the octahedrons affects the spontaneous polarization value and hence influences the crystal s ferroelectric properties. Abrahams and Keve [389] classified ferroelectric materials into three structural categories according to their atomic displacement mechanisms onedimensional, two-dimensional and three-dimensional. 

The natural way to seek improvement on the plain FeS2—m, hep X2 model would be to take into account the T atoms in the form of rigid spheres whose size exceeds those of the original octahedral cavities. (The clearly less realistic FeS2—m, hep X model need not be considered separatly in this connection, but it should be noted that all conclusions for this model would be virtually identical with those drawn for FeS2—m, hep X2.) However, if the only modifying influence of T was due to its attributed spherical shape, this would produce a uniform overall expansion of all cell dimensions without appreciable alterations in the axial ratios. The thus modified atomic arrangement would fit the experimental... 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

The diffusion of Li+ in the octahedral cavities of the Na+montmorillonite allows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 300°C, have larger surface area basal distancy than the pure Zir montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. 

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

Compounds of the type M7M6VF31 (Table 44) are isostructural with Na7Zr6F3i, in which the basic coordination geometry about the Zr atom is approximately square antiprismatic, and six antiprisms share corners to form an octahedral cavity which encloses the additional F atom.155... 

Fig. 41. Rh15C2(CO)jg, 33, as in its H30+ salt (75). Bonds to the carbon atoms (shaded circles) and the interstitial rhodium atom are omitted. The cluster is most easily visualized as a tetracapped pentagonal bipyramid. Two octahedral cavities with a common vertex are provided by the capped square faces and the interstitial rhodium atom. The carbon atoms, which are remote from one another, occupy these octahedral cavities, with Rh-C distances ranging from 1.93 to 2.12 A (mean 2.04 A). Rh-Rh distances range from 2.738 to 3.332 A (mean 2.87 A).

Fig. 42. [Re CO),]1-, 34, as in its Ph4P salt (76). The Re, core comprises a monocapped octahedron of rhenium atoms, with the carbide carbon in the octahedral cavity (mean Re-C = 2.13 0.02 A). The metal-metal bonds fall into several categories. Bonds from the capping atom to the capped face of the octahedron average 2.929 A those on the capped face, 2.9S5 A those between the capped face and the opposite uncapped face alternate longer (3.017 A) and shorter (2.977 A), and those in that uncapped face, average 3.080 A. There are three.terminal carbonyls on each metal atom.

Fig. 43. [Re,C(CO)24p, 35, as in its Et4N+ salt (77). The Re, polyhedron comprises a trans-bicapped octahedron, with the carbide carbon at the center of the octahedral cavity (mean Re-C = 2.12 A). Re- Re bond lengths average 2.993 A within the octahedron and 2.970 A for bonds to the capping atoms. There are three terminal carbonyls per metal atom, and the anion has overall D d symmetry.

The vibrational frequencies of the encapsulated carbon atom in [Osl0C(CO)24]2-, 21 (Fig. 26), and its protonated derivative H2OS 0C(CO)24 have been identified by Oxton et al. (57) and the assignments confirmed by isotopic enrichment. The carbon atom in the tetrahedral Os,0 dianion resides in an octahedral cavity, and at room temperature a band at 753 cm-1 is observed and assigned to Vos.c- On protonation three absorptions of approximately equal intensity are observed in this region and 3C enrichment of the central carbon atom identified these absorptions, at 772.8,... 

When a sphere lies on top of three other spheres in a close-packed structure, there is a cavity between those spheres, the so-called tetrahedral cavity (figure 4.2). Those same close-packed structures also contain octahedral cavities formed by six spheres, as shown in figure 4.4. 

Figure 4.4 represents two layers of spheres in a close-packed structure. The three spheres with a little dot in their centres form the bottom layer (A layer), the three crossed ones the top layer (B layer). Together the six centres of the spheres form the vertices of an octahedron with an octahedral cavity between the spheres. 

Many crystal lattices can be described by filling the tetrahedral and /or octahedral cavities in close-packed structures with other particles. In many cases the particle will be too big to fill a certain cavity. In those cases the particles of the close-packed structure will shift a little and in this way the perfect close-packed structure is lost. Small particles sooner fit in a tetrahedral cavity and larger ones in an octahedral one. Thus we speak of a tetrahedral and an octahedral coordination of a particle in the cavity and the number of nearest neighbours is called the coordination number. 

This structure can be described as a cubic close-packed structure of chloride ions. For every chloride ion there is one octahedral cavity which is occupied by a sodium ion. However, you might also describe the structure as a cubic close-packed structure of sodium ions with chloride ions in the octahedral cavities. The following compounds have similar structures Li20, MgO, CaO, AgF and NH4C1. 

This form of magnetism can be demonstrated by means of lodestone or magnetite (Fe304) which freely occurs in nature. A unit of this contains one Fe2+ ion, two Fe3+ions and four O2 ions. Its crystal structure is a cubic close packing of oxygen ions with an Fe 3+ ion in 1/8 of the tetrahedral cavities, an Fe 3+ ion in 1/4 of the octahedral cavities and an Fe2+ions in 1/4 of the octahedral cavities. Magnetic dipoles at tetrahedral sites line up antiparallel to the external field and dipoles in the octahedral cavities line up parallel to the field. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

Luminescence spectra give evidence that the concentration of dimers in C60 with He in octahedral cavities is notably lower than in pristine material. This inference is corroborated by our structure findings. 

Interstitial H atoms have also been found in larger metal clusters. In a neutron diffraction study of [HNii2(CO)2i]3 and [H2Ni12(CO)21]2, Williams, Dahl, Chini and co-workers found H atoms lodged in octahedral cavities of these multi-hole... 

Fig. 59. a An ORTEP plot of [HCo6(CO)15l, in which a six-coordinate H atom was discovered using single-crystal neutron diffraction techniques (Ref. 246). b A close-up view of the core of [HCo6(CO)i5]showing the near-perfect centering of the interstitial H atom within the octahedral cavity... 

Fig. 60. The structure of the lH2Nij2(CO)2i ]2-ion, showing the two interstitial H atoms in octahedral cavities. Note that the H atoms are slightly displaced towards the central Ni(5,7,9) triangle of the cluster (Ref. 247)...

Fig. 61. A blow-up of the filled octahedral cavity of the HNii2(CO)2iJ3- ion, showing the highly distorted positioning of the interstitial H atom (Ref. 247). This H atom appears to be approaching a triply-bridging condition, capping a triangular face of an octahedron from within...

All H atoms in this table have been located in octahedral cavities, except those of the Rh13 clusters, which are believed to exist in square pyramidal cavities b In this table we have adopted the convention (for ionic compounds) of listing the hydrido species first... 

The M—C carbide distances found in some HNCC are summarized in Table IV. It seems probable that in the smaller octahedral cavities (compare Tables III and IV) the positive charge on the carbide atom will become higher to allow the necessary contraction. 

---