Polymorphism in metals

Standard enthalpy of atomization (kJ moP ) Metallic radius for 12-coordinate atom (pm) 

338 pm. In group 14, Pb adopts a cep structure, but in white Sn (the stable allotrope at 298 K), each atom possesses a coordination number of only 6 (for the structure of grey Sn, see Section 6.4). Metals with coordination numbers of less than 8 are among those that are the most volatile. 

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POLYMORPHISM IN METAL-CONTAINING COMPOUNDS VERSUS ORGANIC COMPOUNDS... 

Polymorphism in metals is common, often occurring as the temperature of the metal changes, and the different forms are normally (though not exclusively) labelled a, P, y... with increasing temperature. In some cases, metals can revert to a lower temperature form at high temperature. For example a-Fe is stable up to 906 °C and then alters to y-Fe at 1401 C before reverting to a-Fe at 1530 C. In this case, P-Fe is not stable under normal conditions and is only found at high pressure. 

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

Metallothionein (MT) is a polymorphic nonenzymatic metal-binding protein (6-7 kDa), involved in the detoxification of trace metals like Cd " and Zn. The various isoforms can be separated by RPLC and detected by ESI-MS [50-51]. The chromatogram of horse kidney MT-a and MT-P and mass spectra taken at the apex of the peaks are shown in Figure 16.5. In the spectrum, the occurrence of mixed Cd-Zn-MT complexes is observed. 

Christian JW (1965) The Theory of Transformations in Metals and Alloys. New York, Pergamon Press Chudinovskikh L, Boehler R (2001) High-pressnre polymorphs of olivine and the 660-km seismic discontinuity. Natirre 411 574-577... 

When subjected to high pressure certain phosphides and arsenides of metals of Groups IIIB and IVB adopt either the cubic NaCl structure (InP, InAs) or a tetragonal variant of this structure (GeP, GeAs) shown in Fig. 6.1(c) in which there are bonds of three different lengths and effectively 5-coordination. The compounds of Ge and Sn, with a total of nine valence electrons, are metallic conductors. This property is not a characteristic only of the distorted NaCl structure, for SnP forms both the cubic and tetragonal structures, and both polymorphs exhibit metallic conduction. (IC 1970 9 335 JSSC 1970 1 143.)... 

Univariant Systems.—Equilibrium between liquid and vapour. Vaporisation curve. Upper limit of vaporisation curve. Theorems of van t Hoff and of Le Chatelier. The Clausius-Clapeyron equation. Presence of complex molecules. Equilibrium between solid and vapour. Sublimation curve. Equilibrium between solid and liquid. Curve of fusion. Equilibrium between solid, liquid, and vapour. The triple point. Complexity of the solid state. Theory of allotropy. Bivariant systems. Changes at the triple point. Polymorphism. Triple point Sj—Sg— V. Transition point. Transition curve. Enantiotropy and monotropy. Enantiotropy combined with monotropy. Suspended transformation. Metastable equilibria. Pressure-temperature relations between stable and metastable forms. Velocity of transformation of metastable systems. Metastability in metals produced by mechanical stress. Law of successive reactions. 

Polymorphism is the property or ability of a metal to exist in two or more crystalline forms depending upon temperature and composition. Most metals and metal alloys exhibit this property. Uranium is a good example of a metal that exhibits polymorphism. Uranium metal can exist in three different crystalline structures. Each structure exists at a specific phase, as illustrated in Figure 5. 

The clinical association of Pb poisoning and ALS with an increase of Pb in blood and bone has been reported (Kamel et al., 2006). It has been suggested that patients with polymorphism in the gene of the ALA-D enzyme might be more at risk of presenting ALS in exposure to Pb (Kamel et al., 2003). Other studies have found no association between exposure to metals and ALS (McGuire et al., 1997 Bergomi et al., 2002). 

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