Crystal structure axial ratio

Zinc crystallizes in a deformed A3 structure with large axial ratio, causing the six equatorial neighbors (at 2.660 A.) to be nearer than the six neighbors in adjacent planes (at 2.907 A.). The bond numbers are 0.54 and 0.21, respectively, leading to 72(1) = 1.249 A. 

For example, tin, with v = 2-5, crystallizes with a unique atomic arrangement, in which each atom has six ligates, four at 3-016 A and two at 3-175 A. These distances have been used (1947) in assigning the bond numbers 0-48 and 0-26 to these bonds. It is clear that these bond numbers can be taken as and and that the choice of the structure and the value of its axial ratio (which determines the relative lengths of the two kinds of bonds) are the result of the effort of the tin atom to use its valency 2-5 in the formation of stable bonds with simple fractional bond numbers. 

Three forms of titanium dioxide, Ti02, are known. Of these the crystal structures of the two tetragonal forms, rutile and anatase, have been thoroughly investigated2) in each case only one parameter is involved, and the atomic arrangement has been accurately determined. The third form, brookite, is orthorhombic, with axial ratios... 

To calculate the space-filling value for a specific compound, one has to know the radii of the atoms and the lattice constant. Neither of these is needed for the construction of a space-filling curve of a crystal structure type it is sufficient to know the point positions of the atoms and the axial ratios. The curve is based on a hard sphere... 

The close approximation of metal atoms in these crystals to mutually attracting spheres is further shown by the values observed for the axial ratio c/aot the hexagonal closest-packed structures, as tabulated below. 

Close study of the angles, indices, and axial ratios long since made it clear that every crystalline substance has a structure built upon a space -lattice" characteristic rtf the substance. It lias been established that this is due to the regular arrangement of the alums, molecules, or ions composing the substance. As shown by Table I. the lattice structures of crystals may be classified into 32 symmetry classes (point groups), which are further divided into seven systems. This topic also is discussed under Mineralogy. 

In the crystal structure of nickel asenide, NiAs, the As atoms are in hep with all octahedral interstices occupied by the Ni atoms, as shown in Fig. 10.2.3(a). An important feature of this structure is that the Ni and As atoms are in different coordination environments. Each As atom is surrounded by six equidistant Ni atoms situated at the corners of a regular trigonal prism. Each Ni atom, on the other hand, has eight close neighbors, six of which are As atoms arranged octahedrally about it, while the other two are Ni atoms immediately above and belowitatz = c/2. The Ni-Ni distance is c/2 = 503.4/2 = 251.7 pm, which corresponds to the interatomic distance in metallic nickel. Compound NiAs is semi-metallic, and its metallic property results from the bonding between Ni atoms. In the NiAs structure, the axial ratio da = 503.4/361.9 = 1.39 is much... 

We pass next to the Ln(III) bis-dipicolinate complexes, shown in Fig. 6. There is no crystal structure for the complexes. Flowever detailed examination of the proton NMR spectra at room temperature shows that the shift ratios are again constant throughout the series and that absolute shifts follow Bleaney s predicted values, Table 3. The complexes must be isostructural and must have axial symmetry. Again the use of relaxation data gives an independent assessment of the relative distances of meta and para protons. We can put all the data together and give a structure for the complex ion as in Fig. 6 leaving three water molecules in the inner sphere. To prove that this is so we must analyse the proportions of the water, both bound and outer sphere. 

The crystal structure of the purple Fe(III) complex of 3-hydroxyflavone (45) has an octahedral coordination sphere with a 1 2 Fe(III)-to-fiavone ratio and a Cl ion at an axial position". Similar to the Cu(II) complexes, the Fe(III)-0(enolate) bond distances (1.935 and 1.981 A) are significantly shorter than the Fe(III)-0(keto) bond distances (2.136 and 2.119 A). The Fe(III) center has a rhombic magnetic environment based on EPR studies. Complete cleavage of pUC18 plasmid DNA by this Fe(III)-flavonoid complex (30 p,M) took place in 15 minutes in the presence 1 equivalent of ascorbate/H202 and was still observable without ascorbate, which is attributed to the reduction capability of the bound flavonoid to yield Fe(II) and initiate the oxidative DNA cleavage" . 

At vol, cm /g-atom Crystal structure Lattice parameters, A Axial ratio, cja... 

The elements have not the close-packing of atoms typical of true metals. Zinc and cadmium have distorted h.c.p. structure with axial ratios 1.87 instead of the ideal 1.63 (Fig. 275). These are contributory factors to their low tensile strengths the ultimate tensile stress of zinc is 7.5 tons per sq. in., of copper 15 tons. Mercury gives rhombohedral crystals with 6 6 co-ordination... 

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