Beryllium structure

Only a limited number of structural studies have been reported on beryllium compounds. The simple alkyls appear to be polymeric with chain structures as shown in XVI (109). For comparison, the structure of di-(t-butyl)beryllium (XVII) is shown as determined from electron diffraction studies (6). In this case, the compound is a linear monomeric species with a Be—C bond length of 1.699 A. Similarly, dimethylberyl-lium has a Be—C bond distance of 1.70 A in the gas phase (5). Comparison of these beryllium structures with the polymer shows that the Be—C distance in the bridge is considerably greater than that in a normal Be—C single bond, a result similar to that observed for the aluminum derivatives. 

Four-coordinate beryllium structures related to amides are exemplified by [Beidad), ... 

Be40(02CCH3)e. The acetate is typical of the basic beryllium carboxylates (Be(OH)2 plus ethanoic acid). The structures have O at the centre of a tetrahedron of Be with carb-oxylate spanning each edge of the tetrahedron. Be(02CCH3)2 is formed from BeCl2 and glacial ethanoic acid. 

The hydrides of beryllium and magnesium are both largely covalent, magnesium hydride having a rutile (p. 36) structure, while beryllium hydride forms an electron-deficient chain structure. The bonding in these metal hydrides is not simple and requires an explanation which goes beyond the scope of this book. 

Beryllium is used as an alloying agent in producing beryllium copper, which is extensively used for springs, electrical contacts, spot-welding electrodes, and non-sparking tools. It is applied as a structural material for high-speed aircraft, missiles, spacecraft, and communication satellites. Other uses include windshield frame, brake discs, support beams, and other structural components of the space shuttle. 

UNS C 81500 (chromium—copper alloy) is used structurally where strength and hardness are required and UNS C 81700 (beryllium—copper alloy) is used stmcturaEy where high strength and hardness are required. 

No fewer than 14 pure metals have densities se4.5 Mg (see Table 10.1). Of these, titanium, aluminium and magnesium are in common use as structural materials. Beryllium is difficult to work and is toxic, but it is used in moderate quantities for heat shields and structural members in rockets. Lithium is used as an alloying element in aluminium to lower its density and save weight on airframes. Yttrium has an excellent set of properties and, although scarce, may eventually find applications in the nuclear-powered aircraft project. But the majority are unsuitable for structural use because they are chemically reactive or have low melting points." ... 

Staeking faults and sometimes proper polytypism are found in many inorganic compounds - to pick out just a few, zinc sulphide, zinc oxide, beryllium oxide. Interest in these faults arises from the present-day focus on electron theory of phase stability, and on eomputer simulation of lattice faults of all kinds investigators are attempting to relate staeking-fault concentration on various measurable character-isties of the compounds in question, such as ionicity , and thereby to cast light on the eleetronic strueture and phase stability of the two rival structures that give rise to the faults. 

The hydrides of the later main-group elements present few problems of classification and are best discussed during the detailed treatment of the individual elements. Many of these hydrides are covalent, molecular species, though association via H bonding sometimes occurs, as already noted (p. 53). Catenation flourishes in Group 14 and the complexities of the boron hydrides merit special attention (p. 151). The hydrides of aluminium, gallium, zinc (and beryllium) tend to be more extensively associated via M-H-M bonds, but their characterization and detailed structural elucidation has proved extremely difficult. 

The discovery of beryllium in 1798 followed an unusual train of events. The mineralogist R.-J. Haiiy had observed the remarkable similarity in external crystalline structure, hardness and density of a beryl from Limoges and an emerald from Peru, and suggested to L.-N. Vauquelin that he should analyse them to see if they were chemically identical. As a result, Vauquelin showed... 

Beryllium salts rapidly hydrolyse in water to give a series of hydroxo complexes of undetermined structure the equilibria depend... 

Figure 5.4 The molecular structure of basic beryllium acetate showing (a) the regular tetrahedral arrangement of 4 Be about the central oxygen and the octahedral arrangement of the 6 bridging acetate groups, and (b) the detailed dimensions of one of the six non-planar 6-membeted heterocycles. (The Be atoms are 24 pm above and below the plane of the acetate group.) The 2 oxygen atoms in each acetate group are equivalent. The central Be-O distances (166.6 pm) are very close to that in BeO itself (165 pm).

Figure 5.10 Cyclopentadienyl derivatives of beryllium showing (a) the slruclure of fBe(ij -C5H5)Y] and (b) (he structure of crystalline IBe(/j -C5H5)(ij -C5H5)] at —120 (see text).

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

There are a few species in which the central atom violates the octet rule in the sense that it is surrounded by two or three electron pairs rather than four. Examples include the fluorides of beryllium and boron, BeF2 and BF3. Although one could write multiple bonded structures for these molecules in accordance with the octet rule (liable 7.2), experimental evidence suggests the structures... 

Table 21-IV shows some properties of the metals and their crystal forms. Since different crystal forms are involved in the series, trends in the properties are obscured. Figure 21-2 shows scale representations of the crystal structures of metallic beryllium, calcium, and barium.

Beryl. 385 Beryllium atomic size, 379 boiling point, 374 bonding capacity, 285 chemistry of, 382 electron configuration. 378 heat of vaporization, 374 ionization energies, 379 occurrence, 384 preparation, 385 properties, 381 structure, 381... 

Beryllium compounds have a pronounced covalent character, and the structural... 

The Transition to the Sphalerite Structure.—The oxide, sulfide and selenide of beryllium have neither the sodium chloride nor the cesium chloride structure, but instead the sphalerite or the wurzite structure. The Coulomb energy for the sphalerite arrangement is... 

In Table XVIII are given values of the radius ratio for the salts of beryllium, magnesium and calcium (those of barium and strontium, with the sodium chloride structure, also obviously satisfy the radius ratio criterion). It is seen that all of the sodium chloride type crystals containing eight-shell cations have radius ratios greater than the limit 0.33, and the beryl-... 

It was concluded by Zachariasen [Norsk geol. Tidsskrift, 8, 189 (1925) Z. physik. Chem., 119, 201 (1926)] from the intensities of reflection of x-rays that beryllium oxide does not contain Be++ and O" ions. However, it has since been shown by Claassen [ibid., 124, 139 (1926)] and Zachariasen himself [Z. Physik, 40, 637 (1926)] that if the electron distribution of the ions is taken into account, the x-ray measurements are compatible with an ionic structure. 

It is also shown that theoretically a binary compound should have the sphalerite or wurzite structure instead of the sodium chloride structure if the radius ratio is less than 0.33. The oxide, sulfide, selenide and telluride of beryllium conform to this requirement, and are to be considered as ionic crystals. It is found, however, that such tetrahedral crystals are particularly apt to show deformation, and it is suggested that this is a tendency of the anion to share an electron pair with each cation. 

THE STRUCTURE OF THE CARBOXYL GROUP. II. THE CRYSTAL STRUCTURE OF BASIC BERYLLIUM ACETATE... 

The arrangement of the centers of the molecules in the crystal is that corresponding to the diamond structure. Each molecule is surrounded tetrahedrally by four molecules. If we consider a molecule as roughly tetrahedral in shape with similar orientation to the tetrahedron formed by the four beryllium atoms, then the adjacent molecules are so oriented as to present tetrahedral faces to one another. 

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