Cobalt allotropes

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. 

Table 3 gives the important properties of elemental cobalt. The metal itself is lustrous and silvery-blue at room temperature, and exists in two allofropic forms (see Allotrope), a and The a-form has a hexagonal closest-packed stmcture... 

Arsenic is a metallic element (symbol As atomic no. 33), which exists in several allotropic forms. Various ores contain crystalline forms of arsenic salts cobaltite contains cobalt arsenic sulfide mispickel (arsenopyrite) iron arsenic sulfide orpiment arsenic trisulfide proustite (ruby silver ore) silver arsenic sulfide realgar arsenic sulfide and tennantite copper arsenic sulfide. 

Gray, hard, magnetic, ductile, somewhat malleable metal. Exists in two allotropic forms. At room temp the hexagonal form is more stable than the cubic form both forms can exist at room temperature. Stable in air or toward water at ordinary temp, d 8.92. mp ]493. bp about 3100. Brinell hardness 123. Latent heat of fusion 62 cal/g, Latent heat of vaporization 1500 cal/g. Specific heat (l 5-100 ) 0.1056 cal/g/ C. Readily sol in dil HNO, very slowly attacked by HCI or cold HjS04. The hydrated salts of cobalt are red, and the sol salts form red solus which become blue on adding coned HCI. 

PHYSICAL PROPERTIES steel-gray, shiny, hard metal ductile somewhat malleable hydrated salts of cobalt are red soluble salts form red solutions which become blue on adding concentrated hydrochloric acid exists in two allotropic forms hexagonal form is more stable than the cubic form at room temperature readily soluble in dilute nitric acid insoluble in water magnetic ferromagnetic permeability two-thirds that of iron MP (1493°C, 2719°F) BP (3100°C, 5612°F) DN (8.92 g/cm at 20°C) SG (8.92) CP (0.1056 cal/g/°C at 15-100°C) LHV (1500cal/g) VP (0 mmHg at 68°F BHN (1.25). 

Note also that all the structures discussed above, are determined for bulk materials. If normal thermodynamic conditions are close to the limit of stability of a given phase, then even small external influences (e.g. grinding of the crystals) can change its structure. The effect of particle size on stability of crystal structure of pure cobalt has been studied [36], showing that while common hep allotrope of Co is stable in the bulk, particles with diameters of 100-200 A have a/cc structure and those of 20-50 A have a bcc lattice. In these structures, the particles the minimum of surface free energy, which yields a stable equilibrium configuration of atoms with minimal internal energy. 

The two allotropes of cobalt were designated in the order of their discovery. Therefore the room temperature, hep, was originally called alpha and the medium temperature, fee, was named beta. In modern nomenclature, the hep phase was named epsilon, but the reader must always keep in mind that alpha may refer either to hexagonal or cubic cobalt. 

Isolating the less thermodynamically stable of two interconvertible forms of the same composition (whether it be coordination compounds, allotropes of elements, or any other chemical composition) can be done only if the rate of reaching equilibrium is so slow as to render the conversion of the metastable isomer to the stable one very protracted. This is a form of Ostwald s law of metastable intermediates. Such rates are slow (minutes < ti/2 < years) for the equilibrations of coordinated cobalt(III), chromium(III), a few (spin-paired) d ferrous compounds, such as salts of the ferroin 22a, tris-l,10-phenanthrolineiron(II) cation, and a few octahedral nickel(II) species with strong field ligands (3d, e.g., the tris-l,10-phenanthrolinenickel(II) ion 22b). 

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