Metal allotropic transformation temperatures

Thin films (qv) of lithium metal are opaque to visible light but are transparent to uv radiation. Lithium is the hardest of all the alkaH metals and has a Mohs scale hardness of 0.6. Its ductiHty is about the same as that of lead. Lithium has a bcc crystalline stmcture which is stable from about —195 to — 180°C. Two allotropic transformations exist at low temperatures bcc to fee at — 133°C and bcc to hexagonal close-packed at — 199°C (36). Physical properties of lithium are Hsted ia Table 3. 

Colorless or light yellow metal at ordinary temperatures it occurs in hexagonal close-packed crystalline form, known as alpha-gadolinium alpha form transforms to a body-centered cubic allotropic form, beta-gadolinium upon heating at 1,262°C density 7.90 g/cm melting point 1,313°C vaporizes at 3,266°C vapor pressure 9.0 torr at 1,800°C (calculated) electrical resistivity 134.0 microhm-cm at 25°C Poisson ratio 0.259 modulus of elasticity 8.15x106 psi thermal neutron absorption cross section 46,000 barns insoluble in water dissolves in acid (reacts). 

Engel (1883) and Linck 1 (1899) stated that amorphous arsenic is transformed at 360° C., irreversibly and with considerable development of heat, into metallic arsenic Erdmann and Reppert gave 303° C. as the transformation temperature, while Jolibois 2 and Gaubeau 3 determined the point of irreversible transformation both of the brown and grey varieties to be 270° to 280° C. Erdmann gave the transition point between the brown form and the grey form as 180° C., but such a critical point has not been substantiated. Jolibois asserted that his thermal observations admitted only two allotropes, the ordinary grey... 

In the solid state, uranium metal exists in three allotropic modifications. The transformation temperatures and the enthalpies of transformation are given in Table 5. The thermodynamic properties of uranium metal have been determined with great accuracy and have been discussed (50). 

No allotropic transformations, accompanied by marked changes in expansion rate, should occur in the metal over the range of temperature to which it may be subjected, either in making the seal or during its subsequent use. This range may be as extensive as -50°C to 2000°C. 

Three allotrope modifications are known. The stable form is the grey metallic modification with a sublimation temperature of 613°C. Melting is possible in closed tubes at 817°C and a pressure of 28 bar. The metallic allotrope exists in a crystalline (specific weight 5.73 g/cm ) and an amorphous form (specific weight 4.7-5.1 g/cm. The yellow crystalline and the black amorphous modifications are metastable and transform into the grey arsenic under the influence of light or heat. Arsenic is nontoxic in its elementary form [1-5]. 

The properties of tin is shown in Table 10.1 [1]. The valence number is 2 or 4. Valence 2 is always positive however, valence 4 has amphoteric properties showing +4 or —4 according to its reaction partner. The metal has at least two allotropic modifications, i.e., the a- and /5-forms. White tin, which is usually seen, is the /5-tin. The transformation temperature from white to gray a-tin is 13.2°C. The transformation rate increases as the temperature decreases, and reaches a maximum at — 48°C. The small amounts of Bi, Sb, Pb, Ag and Au in the tin retard the transformation. Thus, commercial grade tin resists the transformation because of the inhibiting effect of these elements which are present as impurities. Tin is inert and does not react with air and water at ordinary temperatures. But at high temperatures it forms a very thin oxide layer on the surface. In oxygen, tin hardly shows any reactivity at ordinary temperatures. However, when tin is heated in... 

An allotropic transformation results in a change in the crystal structure of a solid material. Allotropic transformations between crystal structures in the metallic class are often not detrimental and are, in fact, used to great advantage [31]. Steel, which is an alloy of iron with a small amount of carbon, is a familiar example. At low temperature, steel has a body-centered cubic crystal structure. As the temperature is increased, the structure changes to face-centered cubic... 

Gobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two thirds that of iron. Gobalt tends to exist as a mixture of two allotropes over a wide temperature range. The transformation is sluggish and accounts in part for the wide variation in reported data on physical properties of cobalt. 

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... 

Silvery-white metal exhibits three crystalline modifications an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm when heated at 280°C the beta form converts to a body-centered cubic crystaUine gamma modification at 577°C, having a density 18.0 g/cm . 

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). 

III.18 TIN, Sn (Ar 118-69) - Tin(II) Tin is a silver-white metal which is malleable and ductile at ordinary temperatures, but at low temperatures it becomes brittle due to transformation into a different allotropic modification. It melts at 231-8°C. The metal dissolves slowly in dilute hydrochloric and sulphuric acid with the formation of tin(II) (stannous) salts ... 

The occurrence of polymorphic forms and the persistence of the metastable state are facts of the highest practical and theoretical importance. In the case not only of tin, but also a number of other metals, e.g. bismuth, cadmium, copper, silver, and zinc, allotropic modifications exist with transition points at temperatures above the ordinary and, owing to the slowness of transformation, these metals exist, at the ordinary temperature, in a metastable state. On this fact depends the practical, everyday use of these metals. ... 

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. 

Uranium metal displays three allotropic forms between room temperature and its melting point at 1,135°C. The orthorhombic a phase exists at temperatures up to 668°C. Uranium metal is rolled only in this easily worked a phase, otherwise cracking occurs. At temperatures over 500° C the metal starts to soften, but heat generated in working the metal can cause transformation into the complex tetragonal p phase, which is hard and brittle P-phase uranium exists at temperatures between 668°C and 776°C. Above 776°C, uranium metal displays the body-centered-cubic y phase the metal is quite soft at these temperatures, deforming under its own weight. Consequently, the y phase is easiest to extrude. Hot uranium metal reacts with steel dies and rollers therefore other materials are used in its manipulation. 

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