Strong face-centered cubic

Notes 1. Preparation All preparations are by equilibration of the oxide with oxygen at temperatures and oxygen pressure predetermined from experimental data. 2. The diffraction patterns all show strong face-center-cubic reflections with commensurate, weak, superstructure reflections by whatever means they are taken. 

We can then meike the determination that since Cd2+ is a strongly diffracting atom (it has high atomic weight, which is one way of stating that it has many electron shells, i.e.-ls 2s 2p6 3s 3p6 3di0 4s 4p6 4d 0), the structure is probably face-centered cubic. Indeed, this turns out to be the case. In the unit cell, Cd atoms are in the special positions of 0,0,0, l/2,l/2,l/2 0,l/2.1/2 172,1/2,0. TTiere are four... 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

Abbreviations BCC. body centered cubic DOS. density of states ESR. electron spin resonance HX.AI S, extended X-ray absorption fine structure F CC. face centered cubic (a crystal structure). FID, free induction decay FT, Fourier transform FWHM, full width at half maximum HCP, hexagonal close packed HOMO, highest occupied molecular orbital IR, Infrared or infrared spectroscopy LDOS, local density of states LUMO, lowest unoccupied molecular orbital MAS. magic angle spinning NMR. nuclear magnetic resonance PVP. poly(vinyl pyrrolidone) RF. Radiofrequency RT, room temperature SEDOR, spin echo double resonance Sf, sedor fraction SMSI, strong metal-support interaction TEM. transmission electron microscopy TOSS, total suppression of sidebands. 

Ensemble effects are useful when adsorption requires a special grouping of surface atoms. To explain this, let us examine the simple example of ethylene adsorption on nickel, which occurs in a dt-adsorbed mode. Two nickel atoms, the right distance apart, are needed to bond a pair of carbon atoms. The bonds must be stable, but not too strong or subsequent reaction is difficult. Figure 4.2 shows symmetry and distances for tow index planes of the face centered cubic nickel surface. 

Solid fullerene displays interesting properties. After condensation, the Cgo molecules forms a face centered cubic (f.c.c.) structure fullerite. This is the only material which consists of quasi-spherical molecules, all atoms of which are of one kind. X-ray dispersion experiments show that fullerite forms a closely packed f.c.c. crystal in which the distance between the nearest molecules is 10.04 A [2]. The least distance between two molecular surfaces is 2.9 A, and the distance between the nearest atoms in a crystal is 1.42 A. Thus, the experiments specify that the molecular structure of Ceo is preserved in the solid. Strong orientational disorder is observed at room temperature [64] and this disorder decreases as the temperatures decreases. 

---