Antimony allotropes

Arsenic and antimony resemble phosphorus in having several allotropic modifications. Both have an unstable yellow allotrope. These allotropes can be obtained by rapid condensation of the vapours which presumably, like phosphorus vapour, contain AS4 and Sb4 molecules respectively. No such yellow allotrope is known for bismuth. The ordinary form of arsenic, stable at room temperature, is a grey metallic-looking brittle solid which has some power to conduct. Under ordinary conditions antimony and bismuth are silvery white and reddish white metallic elements respectively. 

Time-weighted average (TWA), 74 215 concentration, 25 372 exposure limit, for tantalum, 24 334 Time-Zero SX-70 film, 79 303, 305-307 Tin (Sn). See Lead-antimony-tin alloys Lead- calcium-tin alloys Lead-lithium-tin alloys Lead-tin alloys, 24 782-800. See also Tin alloys Tin compounds allotropes of, 24 786 analytical methods for, 24 790-792 in antimony alloys, 3 52t atomic structure of, 22 232 in barium alloys, 3 344, 4 12t bismuth recovery from concentrates, 4 5-6... 

There are two allotropes of antimony. The native metallic form is one allotrope, and the other allotrope is an amorphous grayish form. Antimony is a true metalloid that is brittle with a low melting point. And similar to nonmetals, it is a poor conductor of heat and electricity. 

AUatropes. Some or the elements exist in two or more modifications distinct in physical properties, and usually in some chemical properties. Allotropy in solid elements is attributed to differences in the bonding of the atoms in the solid. Various types of allotropy are known. In ertuntiomorphic allotropy, the transition from one form to another is reversible and takes place at a definite temperature, above or below which only one form is stable, e.g., the alpha and beta forms of sulfur. In dynamic alloimpy. the transition from one form to another is reversible, but with no definite transition temperature. The proportions of the allotropcs depend upon the temperature. In monotropic allotropy, the transition is irreversible. One allotrope is mctastable at all temperatures, e.g.. explosive antimony. 

Tin exists in two allotropic forms white tin (p) and gray tin (a). White tin, the form which is most familiar, crystallizes in the body-centered tetragonal system. Gray tin has a diamond cubic structure and may be formed when very high purity tin is exposed to temperatures well below zero. The allotropic transformation is retarded if the tin contains small amounts of bismuth, antimony, or lead. The spontaneous appearance of gray tin is a rare occurrence because the initiation of transformation requires, in some cases, years of exposure at —40°C. Inoculation with Ot-tin particles accelerates the... 

Yellow forms of arsenic and antimony (the latter very unstable) have been described. These are presumably the nonmetallic modifications of these elements, analogous to white phosphorus, and also consisting of discrete molecules (tetrahedral quartets) in the solid state. The grey or metallic forms of arsenic and antimony are the most stable. They are far denser than the yellow forms, are insoluble in organic solvents, and have appreciable electrical conductivities. Black amorphous forms of arsenic and antimony are also known, and an additional allotrope of antimony, explosive (but always impure), has been described. 

Arsenic, antimony, and bismuth also exhibit a variety of allotropes. The most stable allotrope of arsenic is the gray (a) form, which is similar to the rhombohedral form of phosphorus. In the vapor phase, arsenic, like phosphorus, exists as tetrahedral AS4. Antimony and bismuth also have similar a forms. These three elements have a somewhat metallic appearance but are brittle and are only moderately good conductors. Arsenic, for example, is the best conductor in this group but has an electrical resistivity nearly 20 times as great as copper. 

Let us assume for a moment, however, that we had reliable (and direct) thermochemical information about trimethylarsine and tetramethyldiarsine. How valid is it to assume all As—As bonds have comparable dissociation enthalpies Clearly we know enough not to assume the As—As bond in AS4 is the same as in tetramethyldiarsine. The former is immediately identified as strained and so we are to use the more stable allotrope with an infinite network of nonplanar hexagons. What about Sb—Sb bonds If we assume constancy of the dissociation enthalpies of these bonds (excepting the strained Sb4), how about using the bond enthalpy from elemental solid antimony The enthalpy of dimerization of antimonin. CsHjSb, to form the tricyclic Diels-Alder or [4-1-2] cycloaddition product has been determined to be — 30.5 +1.3 kJ mol by a careful study of the monomer-dimer equilibrium (equation 19). 

The STM data therefore indicate that a new allotropic modification of antimony composed of Sb4 clusters exists on the nanometer scale. 

Fig. 14.3 (a) The tetrahedral P4 molecule found in white phosphorus, (b) Part of one of the chain-like arrays of atoms present in the infinite lattice of Hittorf s phosphorus the repeat unit contains 21 atoms, and atoms P and P" are equivalent atoms in adjacent chains, with chains connected through P -P" bonds, (c) Part of one layer of puckered six-membered rings present in black phosphorus and in the rhombohedral allotropes of arsenic, antimony and bismuth. 

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. 

In those days the semi-metals or metalloids were regarded as variations of true metals, probably much as we regard allotropic forms to-day, though of course their ideas were confused. Basil thus termed antimony plumbum antimonii, that is, the antimonial form of lead. He was familiar with the characteristic fern leaf and star appearance on the surface of the solidified metalloid which, he stated, the learned before his time had termed the philosophical signet star. 

The allotropic form of oxygen, ozone, can also be employed for the oxidation of saturated hydrocarbons. The reactivity of ozone without additional reagents is not sufficient for the preparative functionalization of alkanes in solution however, its reactivity is increased substantially by the addition of iron(III) chloride [6] or antimony pentafluo-ride. [7] The dry ozonation variant [8] of Mazur et al. [9] by which alkanes are hydroxy-lated at tertiary C atoms with high selectivities and yields, was shown to be especially useful. According to this method, silica gel is coated with roughly 1 wt% of the substrate, cooled to -78 °C, saturated with ozone, and subsequently allowed to warm to room temperature within 0.5-2 h. Adamantane (1) is converted almost quantitatively into 1-adamantanol (4) in this way (Table 1), and this method of oxy-functionalization has been applied successfully even on certain steroids. [10]... 

The form of elemental antimony that is stable at normal temperatures and pressures is the gray, metallic rhombohedral a-form, mp 630.7 °C, bp 1587 °C, density 6.70 gcm. Crystals are lustrous. They have a relatively high electrical resistivity (41.7 1 2cm at 20°C). The structure of o -8b consists of sheets of covalently bonded antimony stacked in layers, which are formed of puckered slx-membered rings. Each antimony forms three shorter bonds (2.91 A) in the same layer as well as three longer bonds (3.36 A) to antimony atoms in the adjacent layer. In addition to the a-form, other allotropes include a very unstable yellow form and black forms obtained electrolytically or by condensing the vapor on cold surfaces. Two crystalline allotropes are made by high-pressure techniques. At 85 kbar, a modification with a primitive cubic lattice is formed where each antimony atom is in an octahedral environment of six equidistant (2.96 A) neighbors. Further... 

Antimony, like phosphorus and arsenic, has several allotropic modifications. Besides the gray form of antimony, the most common and the best known, there is another form which is an explosive. This latter is very unstable and changes easily to the stable metalhc form. The most unstable modification is yellow antimony, which does not possess any metallic property, corresponding thus to white phosphorus and yellow arsenic. 

The group displays a remarkable number of allotropes of its members showing the trend from non-metallic forms through to metallic forms. Thus nitrogen has only the diatomic form phosphorus has a highly reactive form (brown), and a tetrahedral form P4 (white), forms based on broken tetrahedra P (red and violet), and a hexagonal layer-type lattice (black). Arsenic and antimony have the AS4 and Sb4 forms, which are less stable than the layer type in this case bismuth has the layer-lattice form only. 

Arsenic, antimony, and bismuth also exhibit allotropes. The most stable aUotrope of arsenic is the gray (a) form, which is similar to the rhombohedral form of phosphorus. 

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