Bismuth 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... 

Bismuth may be used in place of lead, but it dissolves only one-fifth as much phosphorus, and the crystals obtained are less pure. The metals appear to be held in solid soln. Only very minute quantities of Hittorf s phosphorus are obtained by sublimation. According to L. Troost and P. Hautefeuille, the same variety is formed when red phosphorus is heated under press, to 580°. The work of A. Pedler, J. W. Retgers, and D. L. Chapman shows that this variety differs from ordinary red phosphorus only in the size and development of the crystals. Fine-grained red phosphorus is scarlet phosphorus, while coarse-grained red phosphorus is metallic or violet phosphorus. A number of other allotropes have been reported, but many of them are the result of a misinterpretation of facts, or of an incomplete knowledge of facts. 

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... 

These elements have fewer allotropic forms than phosphorus. For As and Sb, unstable yellow allotropes comparable to white phosphorus are obtainable by rapid condensation of vapors. They readily transform to the bright, metallic a-rhombo-hedral forms similar to rhombohedral black phosphorus. This is also the commonest form for bismuth. Other reported allotropes are not well characterized. 

The metal is soft and has properties similar to those of bismuth. It melts at 254 °C and boils at 962 °C. Two allotropic crystalline forms are recognized, the low-temperature a form and the high-temperature /1-form. The a form crystallizes in the 0l space group and the /i-form in the space group. 

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. 

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. ... 

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. 

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. 

Elemental antimony under normal conditions has a structure of A7 and at 8.5-12 GPa transforms into a modification with a complex atomic lattice (Sb-II) [20], which at 28 GPa converts to a bcc structure. The heavier analogue Bi has the same phase transitions as Sb, at generally lower pressures Bi-I (A7 structure) transforms at 2.5 GPa to a strongly distorted sc structure (Bi-II). Already at 2.8 GPa there occurs a structural reorganization into the Bi-III phase, which has an incommensurate crystal structure with Nc 9. This arrangement is very similar to that found for high-pressure allotropes of As and Sb. Upon further compression, up to P > 8 GPa, bismuth transforms to a bcc solid [20]. 

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