Crystallographic position

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Copper preferentially occupies the holes in /3-rh boron as shown by powder diffraction . In the CuB solid solution the D hole is represented by two crystallographic positions (denoted 2 and 2" in Fig. 1) " situated 40 pm from each other. The accommodation of the Cu atoms in three (or four) different holes in the preferred order shown in Fig 1 demonstrates the complex nature of the solid solution. 

Figure 1. Preferential occupancy of holes in j3-rh boron expressed as the occupancy of the crystallographic positions vs. total Cu content . ...

The first sample has the greatest deviation from stoichiometry. Samples JV°1 - JV°4 have been found to be nonstoichiometric oxide -hydroxide type. They have a mixed conductivity - ionic (o,) and electronic (ae). The ionic one is due to the presence of OH" - groups. Namely, they stabilize the defects of chemical nature in such compounds. These defects are determined by the presence of Mn4+ and Mn3+ in the same crystallographic position. 

The effect of structural defects in various types of MnCh on the electrochemical behavior has been investigated. It was shown that the basic types of defects in Mn3+ are in the same crystallographic position as in Mn4+. Defects were seen to be are stabilized by the OH" - groups. The catalytic and electrochemical activity of Mn02 increases with increase in defect content. 

Figure 2 shows 29Si solid state NMR data of high-silica ZSM-5 with 24 crystallographic Si positions, 15 of which are well enough resolved in the MAS NMR spectrum to be assigned to crystallographic positions. The CSA parameters are determined by simulation of the lineshapes of the powder pattern as obtained from the 2D experiment. 

Many other clusters have been postulated to occur in these systems, together with a large number of ordered structures in which the clusters are arranged on regular crystallographic positions. 

The distinctive feature of a surface atom is that it has fewer neighbors than an atom in the interior. This unsaturated coordination forms the reason why the electronic and vibrational properties and sometimes the crystallographic positions of surface atoms differ from those of the bulk atoms. 

As shown by DFTB and CPMD simulations, the principal features of the transport mechanism are rotational diffusion of the protonic defect and proton transfer toward a neighboring oxide ion. That is, only the proton shows long-range diffusion, whereas the oxygens reside in their crystallographic positions. Both experiments " " and quantum-MD simulations, have revealed that rotational diffu-... 

By applying this technique, it is not only possible to prepare relatively well-defined catalysts that may be alloys of a given composition but also catalysts in which adatoms of main group elements may be located on the surface of transition metal particles or organometallic fragments that are likely adsorbed (coordinated) at some particular crystallographic positions of the metallic particles. Each of these three different types of materials exhibits interesting and unusual selec-tivities in many catalytic reactions [33, 34]. 

Some of the uncertainties concerning site occupancy assignments of iron cations might be alleviated if comparisons could be made between measurable parameters for different cations in the same coordination site. One such correlation is suggested by the isomer shift parameter for coexisting Fe2+ and Fe3+ ions in similar coordination environments in a mineral, particularly when the two cations occupy the same crystallographic position in the crystal structure. 

According to the crystal field model, the crystalline electrostatic field of ligands in a crystal structure influences the 3d orbital energy levels of a central transition metal ion in a coordination site. There is a two-fold influence of temperature on a crystal structure. First, increased thermal motion results in increased amplitudes of atoms vibrating about their crystallographic positions. Second, thermal expansion causes small increases in interatomic distances, so that according to eq. 

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