Crystal structures Hexagonal close-packed

Black-gray powder, d 7.66. Unaffected by moisture and O3. Dissolved slowly by dil. mineral acids in the cold, rapidly by cone. HCl and cone. HNO3. Dissolved rapidly by all hot acids. Not attacked by aqueous NaOH. Crystal structure hexagonal close packing of Ni atoms, oriented incorporation of N. 

Gray-black powder, di 7.97. Heat of formation —9.2kcal. per mole. Decomposed at room temperature by cone, and dil. HCl precipitation of C does not occur (see FegC). Soluble in dil. HNO3 dil. H3SO4 causes separation of C. Stable at temperatures up to 380-400°C. Crystal structure hexagonal close packing of Ni atoms. 

Crystal structure, hexagonal close-packed (hep) A crystal sfructure where in alternate layers of atoms the atoms in one layer lie at the vertices of a series of equilateral triangles in the atomic plane, and the atoms in the layer lie directly above the center of the triangles in the atomic plane of the next layer. Example Beryllium. 

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

Silvery-gray metal slowly tarnishes in moist air crystallizes in hexagonal close-packed structure density 11.49 g/cm (calculated) melts at 2,172°C vaporizes at 4,265°C Young s (elastic) modulus 3.76 x 10 kg/cm Poisson s ratio 0.293 thermal neutron absorption cross-section 22 barns superconductor below 11°K insoluble in water and hydrochloric acid dissolves in nitric acid, concentrated sulfuric acid and aqua regia. 

In our study, we utilize a pure vibrational excited state t = 1 as the target of coherent control. The Hamiltonian describing the vibrational subspace of a para-hydrogen crystal with hexagonal close packed structure is given as... 

The density of beryllium is 1.847 g/cm3 based upon average values of lattice parameters at 255C (a — 22.856 nm and c — 35.832 nm). Beryllium products generally have a density around 1.850 g/cm3 or higher because of impurities, such as aluminum and other metals, and beryllium oxide. The crystal structure is close-packed hexagonal. The alpha-form of beryllium transforms to a body-centered cubic structure at a temperature very close to the melung point. 

The Reactions of Ethylene with Deuterium These two metals are grouped together (1) because this crystal structure is close-packed hexagonal and not as in the case of the other Group VIII metals (excepting iron) face-centered cubic, and (2) because they have a number of common features in their catalytic properties. 

Most pure metals adopt one of three crystal structures, Al, copper structure, (cubic close-packed), A2, tungsten structure, (body-centred cubic) or A3, magnesium structure, (hexagonal close-packed), (Chapter 1). If it is assumed that the structures of metals are made up of touching spherical atoms, (the model described in the previous section), it is quite easy, knowing the structure type and the size of the unit cell, to work out their radii, which are called metallic radii. The relationships between the lattice parameters, a, for cubic crystals, a, c, for hexagonal crystals, and the radius of the component atoms, r, for the three common metallic structures, are given below. 

Iron-gray, lustrous powder darkens on exposure to Light. Forms crystals with hexagonal, close-packed structure, d 4,472, mp 1509. bp about 3000. E (aq) Y + /Y -2.37 V (calc). Oxidizes on heating in air or oxygen dec cold water slowly, boiling water rapidly. 

The special restriction caused by tying low molecular mass liquid crystalline substances to a polymer chain was also illustrated with amphiphilic liquid crystals. A hexagonally close-packed structure of rod-like micelle cylinders of sodium 10-undecenoate with about 50% water lost during polyma-ization at 60 °C its structure and became isotropic. On cooling, a lamellar liquid crystalline structure, more suitable to accommodate the macromolecular backbone was found. Bas l on the discussions of Sect. 5.3.4 it is likely that with longer side-chain amphiphiles condis crystals could be grown in analogy to the soaps described in Sect. 5.2.3... 

As mentioned, the rare earths exhibit five different crystal structures at room temperature. Three of these are common metallic structures, hexagonal close packed (hep), cubic close packed (cep), also called face-centered cubic fee, and body-centered cubic. The first two are pictured in Fig. 4. The bcc structure is not pictured but consists of cubes of atoms surrounding another atom inserted into the geometric center or body-center of the cube. The two structures, unique to the rare earths are the double-hexagonal close packed structure (dhep) found for La, Pr, Nd and Pm and the complex structure found for Sm. Both of these are variants of the hep structure. Their occurrence for the early members of the series can be explained by postulating that the 4/ electrons, which have relatively large radial extensions for the early elements, participate in the metallic bonding. 

Metals atoms (cations) pack closely together in a regular structure to form crystals. Arrangements in which the gaps are kept to a minimum are known as close-packed structures. X-ray diffraction studies have revealed that there are three main types of metallic structure hexagonal close packed, face-centred cubic close packed and body-centred cubic. 

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235oG, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure. 

Zinc crystallizes in the hexagonal close-packed system its electronic structure is 4s2 and the melting point is 693 K. Since the zinc dissolution takes place at potentials very close to ffa0 the differential capacitance curves in the region of Ea=c in pure surface-inactive electrolyte solutions (KC1, pH = 3.7) can be determined directly for the Zn(llJO) face only... 

The differing malleabilities of metals can be traced to their crystal structures. The crystal structure of a metal typically has slip planes, which are planes of atoms that under stress may slip or slide relative to one another. The slip planes of a ccp structure are the close-packed planes, and careful inspection of a unit cell shows that there are eight sets of slip planes in different directions. As a result, metals with cubic close-packed structures, such as copper, are malleable they can be easily bent, flattened, or pounded into shape. In contrast, a hexagonal close-packed structure has only one set of slip planes, and metals with hexagonal close packing, such as zinc or cadmium, tend to be relatively brittle. 

Side and expanded views of hexagonal and cubic close-packed crystal types. In the hexagonal close-packed structure, spheres on both sides of any plane are in the same positions, and the third layer is directly above the first. In the cubic close-packed structure, layers take up three different positions, and the fourth layer is directly above the first. 

The term crystal structure in essence covers all of the descriptive information, such as the crystal system, the space lattice, the symmetry class, the space group and the lattice parameters pertaining to the crystal under reference. Most metals are found to have relatively simple crystal structures body centered cubic (bcc), face centered cubic (fee) and hexagonal close packed (eph) structures. The majority of the metals exhibit one of these three crystal structures at room temperature. However, some metals do exhibit more complex crystal structures. 

The chemical bonding and the possible existence of non-nuclear maxima (NNM) in the EDDs of simple metals has recently been much debated [13,27-31]. The question of NNM in simple metals is a diverse topic, and the research on the topic has basically addressed three issues. First, what are the topological features of simple metals This question is interesting from a purely mathematical point of view because the number and types of critical points in the EDD have to satisfy the constraints of the crystal symmetry [32], In the case of the hexagonal-close-packed (hep) structure, a critical point network has not yet been theoretically established [28]. The second topic of interest is that if NNM exist in metals what do they mean, and are they important for the physical properties of the material The third and most heavily debated issue is about numerical methods used in the experimental determination of EDDs from Bragg X-ray diffraction data. It is in this respect that the presence of NNM in metals has been intimately tied to the reliability of MEM densities. 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

Mono- or single-crystal materials are undoubtedly the most straightforward to handle conceptually, however, and we start our consideration of electrochemistry by examining some simple substances to show how the surface structure follows immediately from the bulk structure we will need this information in chapter 2, since modern single-crystal studies have shed considerable light on the mechanism of many prototypical electrochemical reactions. The great majority of electrode materials are either elemental metals or metal alloys, most of which have a face-centred or body-centred cubic structure, or one based on a hexagonal close-packed array of atoms. 

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