Allotropic crystal structures, metallic

Another element that exhibits allotropy because of variations in the crystal structure is tin. The common allotrope is tin metal, also known as a alpha) tin, which is stable at ambient temperatures. The other allotrope, which generally occurs as a gray powder and is known as p beta) tin, but also as tin pest, is formed only at very low temperatures when tin cools down to temperatures below -18°C, the ordinary allotrope, a tin, is converted to p tin, and the transformation is irreversible under ordinary temperatures. Tin objects exposed to temperatures below -18°C in very cold regions of the world, for example, are generally severely damaged when part of the tin converts to tin pest. In extreme cases, when exposure to low temperatures extends for long periods of time, the allotropic conversion may result in the transformation of tin objects into heaps of gray p-tin powder. 

At atmospheric pressure, pure solid tin adopts two structures or allotropes, depending on temperature. At room temperature white metallic tin is stable but, at temperatures below 13°C, white tin undergoes a phase transformation into gray tin. White tin (also known as / -tin) adopts a body-centered tetragonal crystal structure (Fig. 8.5.1). Allotropic gray tin (a-tin) crystallizes in a cubic diamond... 

In the same chapter (Chapter 5), as an introduction to the paragraphs dedicated to the various groups of metals, the values relevant to a number of elementary properties have been collected. These are atomic properties (such as metallic and ionic radii, ionization energies, electronegativities, Mendeleev number, chemical scale, Miedema parameters, etc.), crystal structure and lattice parameters data of the allotropes of the elements, and selected thermodynamic data (melting and boiling temperatures and enthalpies, etc.). All these data indeed represent reference values in the discussion of the alloying behaviour of the elements. 

Table 5.4. Alkali metals crystal structures, lattice parameters of their allotropes, calculated...

Greyish lustrous metal malleable exhibits four allotropic modificatins the common y-form that occurs at ordinary temperatures and atmospheric pressure, P-form at -16°C, a-form below -172°C, and 5-form at elevated temperatures above 725°C crystal structure— face-centered cubic type (y-Ce) density 6.77 g/cm3 melts at 799°C vaporizes at 3,434°C electrical resistivity 130 microohm.cm (at the melting point) reacts with water. 

Silvery-white, soft maUeable metal exists in two aUotropic forms an alpha hexagonal from and a beta form that has body-centered cubic crystal structure the alpha allotrope converts to beta modification at 868°C paramagnetic density 7.004 g/cm compressibility 3.0x10 cm /kg melts at 1024°C vaporizes at 3027°C vapor pressure 400 torr at 2870°C electrical resistivity 65x10 ohm-cm (as measured on polycrystalline wire at 25°C) Young s modulus 3.79xl0 ii dynes/cm2 Poisson s ratio 0.306 thermal neutron cross section 46 barns. 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

URANIUM. [CAS 7440-61-1], Chemical element symbol. U, at. no. 92, at. wt, 238,03, periodic table group (Actinides), mp 1,131 to i. 33°C, bp 3,818°C, density 18.9 g/cm3 (20UC). Uranium metal is found in three allotropic forms (1) alpha phase, stable below 668°C, orthorhombic (2) beta phase, existing between 668 and 774°C. tetragonal and (3) gamma phase, above 774°C, body-centered cubic crystal structure. The gamma phase behaves most nearly that of a true metal. The alpha phase has several nonmetallic features in its crystallography. The beta phase is brittle. See also Chemical Elements. 

Arrangements of atoms which are unfamiliar to chemistry (because they are unstable) may be found in intrazeolitic space (because they are stable within the zeolite). The cationic clusters of the alkali metals (Section 6.2.1.2.) are an example of this. Within the zeolite, such new species may be studied and put to use. Allotropes of elements and compounds (continua), some charged, will continue to be found within zeolites substances must adopt new structures if they choose to conform to the geometrical requirements of intrazeolitic space, with different structures for different zeolites. In all cases, the properties of these allotropes must be different from those of the native substances in their original crystal structures. 

The crystal structures of the chemical elements provide a simple starting point for illustrating what can be learned from listed atomic coordinates. Tin has both a nonmetallic and a metallic allotrope (polymorph). 

Silicon is a metalloid, an element with properties of both metals and non-metals. Silicon exists in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope is in the form of shiny, grayish-black, needle-like crystals, or flat plates. The second allotrope has no crystal structure and usually occurs as a brown powder. 

Arsenic, Antimony and Bismuth. These elements are obtained by reduction of their oxides with hydrogen or carbon. For As and Sb unstable yellow allotropes, presumably containing tetrahedral As4 and Sb4 molecules, can be obtained by rapid condensation of vapors. They are easily transformed into the stable forms, and yellow Sb is stable only at very low temperatures. Bismuth does not occur in a yellow form. The normal forms of As, Sb and Bi are bright and metallic in appearance and have crystal structures similar to that of black P. When heated, the metals burn in air to form the oxides, and they react directly and readily with halogens and some other non-metals. They form alloys with various other metals. Dilute non-oxidizing acids are without effect on them. With nitric acid, As gives arsenic acid, Sb gives the trioxide and Bi dissolves to form the nitrate. 

Crystal structures, lattice parameters, metallic radii and atomic volumes of five cerium allotropes... 

Table 18.13 gives the crystal structures and phase transformations of the transuranium elements (Katz et al. 1986). The lighter actinide elements like Np and Pu have a bcc structure at the melting point and Pu has six distinct allotropic forms. The heavier actinides from Am to Es show face-centered cubic (fee) structures at the melting point and double hexagonal close-packed (dhep) structures below the melting point. These heavier actinides behave like lanthanide metals. 

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