Alloys of Antimony

Antimony enters into the composition of a large number of commercial alloys, including antimonial lead (lead containing up to 4 per cent, of antimony) which is used for the framework of accumulator plates, lead shot (in which antimony replaces the more usual alloying element arsenic), lead anodes for chromium plating and other purposes, type metal (consisting of alloys of lead, antimony, tin and sometimes copper), Britannia metal and pewter (alloys of tin, antimony, lead and sometimes copper and bismuth), and antifriction metals, such as Babbitt metal (a wide range of alloys, a number of which contain tin, lead, antimony and copper). In general, antimony acts as a hardening metal, and excess is liable to induce brittleness. It has been stated that alloys of antimony with iron and certain other metals are resistant to acid.

A number of alloy systems with antimony as one of the components has been examined thermally and microscopically, by means of X-rays, and by the correlation of physical properties with composition. The bibliographies which accompany these brief accounts deal mainly with the physico-chemical constitution of the alloys; references to method of manufacture, treatment, working or uses of the alloys have not, in general, been included.

Two compounds are formed, Na₃Sb (M.pt. 823° C.) and NaSb (M.pt. 503° C.), which enter into the formation of three eutectics, at 0.5 per cent, antimony (M.pt. 95° C.), 80 per cent, antimony (M.pt. 430° C.), and 90.6 per cent, antimony (M.pt. 404° C.). There is no range of solid solution. The e.m.fs. of these alloys have also been investigated. Substances of the composition Na₃Sb₇, Na₃Sb₇.NH₃ and Na₃Sb₇.6NH₃ have been obtained by extracting an alloy of sodium and antimony with liquid ammonia.

Two compounds are formed, K₃Sb (M.pt. 812° C.) and KSb (M.pt. 605° C.), which enter into the formation of three eutectics melting at 63° C., 400° C. and 485° C. respectively.

Two definite compounds are formed: Cu₅Sb₂ (M.pt. 680° C.) and Cu₂Sb (decomposing at 580° C.); it is possible that a third compound, Cu₃Sb, may exist below 430° C. There are two eutectics, namely at 23 per cent, copper (M.pt. 535° C.) and 72 per cent, copper (M.pt. 634° C.). Copper forms a solid solution in antimony up to 1.2 per cent., and antimony dissolves in copper up to 7 per cent. Transformations in the solid state occur at 430° C. (when the compound Cu₅Sb₂ undergoes decomposition), and in copper-rich alloys at 450° C. Other physical properties that have been examined are the heats of mixing, which at 1,200° C. reach a maximum of +903 gram-calories at 57.4 atomic per cent, copper, and variations in e.m.f. X-ray examinations of this system have been carried out.

One compound, Ag₃Sb, is formed, decomposing at 560° C., and one eutectic containing 55 per cent, silver (M.pt. 482° C.). The solid solubility of antimony in silver is 6 per cent.; silver appears to be insoluble in antimony in the solid state. The heat of mixing (at 1,050° C.) rises to a maximum of +1,192 gram-calories at 71.6 atomic per cent, silver.

One compound, AuSb₂, is formed, which exists in three modifications with transition points at 355.2° C. and 405° C.; it probably decomposes at 460° C. There is a maximum on the liquidus curve at 55 per cent, gold (492° C.) and two eutectics occur, namely at 46 per cent, gold (M.pt. 480° C.) and at 75 per cent, gold (M.pt. 370° C.). There appears to be no range of solid solution. Electrical conductivity curves agree with the results of thermal analysis.

One compound, Mg₃Sb₂ (M.pt. 1,228° C.) is known, which forms two eutectics, at 86 atomic per cent, antimony (M.pt. 579° C.) and 10 atomic per cent, antimony (M.pt. 629° C.). The compound Mg₃Sb₂ undergoes a transformation at 930° C. and enters into solid solution with magnesium. The temperature of the transformation falls slightly throughout the range of solid solubility.

Only the antimony-rich alloys appear to have been studied. There is a eutectic at 8 per cent, calcium (M.pt. 585° C.).

Two compounds are formed, Zn₃Sb₂ (M.pt. 568° C.) and ZnSb (decomposing at 534° C.), forming two eutectics at 1.7 per cent, antimony (M.pt. 412° C.) and at 80 per cent, antimony (M.pt. 505° C.). At room temperature the compound Zn₃Sb₂ decomposes into metallic zinc and the compound ZnSb. A discontinuity in the curve for the magnetic susceptibility indicates the formation of the compound ZnSb, which has also been examined by X-rays.

It is probable that two compounds are formed, Cd₃Sb₂ (decomposing at 410° C.) and CdSb (M.pt. 455° C.), although the existence of the former has been queried. Three other compounds have been indicated for which the following formulae have been proposed: Cd₅Sb₃, Cd₄Sb₅ and Cd₃Sb₅. There are two eutectics, at 40 per cent, cadmium (M.pt. 445° C.) and 93 per cent, cadmium (M.pt. 290° C.). The magnetic susceptibility has been studied, while the heat of mixture (at 800° C.) shows a maximum of +829 gram-calories at 46.7 atomic per cent, antimony.

It is difficult to obtain equilibrium with these alloys. The solid solubility of antimony in aluminium is less than 0.10 per cent, at 645° C.; a eutectic is formed at 11 per cent, antimony (657° C.) and a maximum on the liquidus curve at 1,080° C. corresponds to the compound AlSb. The liquidus curve shows another maximum at 32 per cent, antimony (984° C.) and a minimum at 35 per cent, antimony (942° C.) There appear to be two eutectics. The compound AlSb decomposes in moist air, aluminium hydroxide being formed.

One compound is formed, Tl₃Sb (decomposing at 187° C.), which gives solid solutions with thallium but not with antimony. A eutectic is formed at 19 per cent, antimony (M.pt. 196° C.), while the thallium-rich alloys undergo a transformation (probably connected with the allotropy of thallium) at 226° C. Another compound, ₇Sb₂, has also been reported.

Antimony and silicon show only slight solid solubility; the eutectic melts at 630° C. No compounds are formed.

This system is complex, a number of solid solutions being formed. There are two compounds, SnSb (decomposing at 427° C.) and Sn₃Sb₂ (decomposing at 319° C.). A transformation occurs in the range 30 to 70 per cent, antimony, which is connected with a polymorphic change in the β-solid solution. An X-ray examination of these alloys has been made, and it is suggested that the compound SnSb has the structure of a simple cube of the KCl type - a very unusual structure for an intermetallic compound. The unit cell contains 4 molecules, with a =6.120 A. The more usual structure of a body-centred cube, with a = 6.13 A. has, however, also been proposed for this compound, and further, as the result of a more recent investigation, the structure has been described as of the KCl type, but deformed. This compound is also unusual in dissolving both antimony and tin. The existence of the compound SnSb is also indicated on the curve of magnetic susceptibility.

These metals form a eutectiferous series of alloys with a eutectic at 87 per cent, lead (M.pt. 247° C.). A more recent examination of these alloys after very slow cooling suggests that the eutectic contains 11.4 to 11.5 per cent, antimony. It is suggested that a compound Pb2Sb is formed which is soluble in liquid antimony, and that it forms a solid solution in monatomic molecules of lead at all temperatures between 25° C. and the melting point of pure lead. A maximum is found on the boiling point curve. The hardness and specific heats have been determined, the specific heat between 0° and 100° C. being given by the expression

s = 0.04965 – 0.0001884p

where p is the percentage of lead in the alloy. These alloys have been examined by X-rays, and from the results it is deduced that the solid solubility of antimony in lead cannot be more than 0.5 per cent. From electrical conductivity experiments it is concluded, however, that at 249° C. the solid solubility of lead in antimony is 5.8 atomic per cent., and that of antimony in lead is 1.5 atomic per cent.

These elements form a continuous series of solid solutions up to 40 per cent, arsenic. There is a minimum on the liquidus curve at 17.5 per cent, arsenic (612° C.).

These metals form a continuous series of solid solutions, the liquidus curve lying wholly between the melting points of the two metals, and the solidus being practically horizontal between 0 and 60 per cent, antimony. Some evidence for the existence of Bi₃ molecules has been obtained, and the anomalous form of the solidus curve has been ascribed to this. The hardness has been determined. The boiling point curve shows a maximum. These alloys have been examined by X-rays, homogeneity being obtained by prolonged annealing at 280° C. The lattice edge of the rhombohedral crystals varies almost linearly with composition.

Two compounds are formed, CrSb (M.pt. 1,110° C.) and CrSb₂ (decomposing at 675° C.), and two eutectics, at 2 per cent, chromium (M.pt. 620° C.) and 38 per cent, chromium (M.pt. 1,100° C.). Antimony is soluble in chromium to the extent of 12 per cent.

One compound, Se₃Sb₂ (M.pt. 570° C.) is formed, and two eutectics, at 47 per cent, selenium (M.pt. 493° C.) and at 99.7 per cent, selenium (M.pt. 210° C.).

These alloys are described as forming a series of "mixed crystals."

Three compounds have been described: MnSb (M.pt. 809° C.), Mn₃Sb₂ (decomposing at 872° C.) and Mn₂Sb (M.pt. 971° C.). Two eutectics are also formed, at 9.5 per cent, manganese (M.pt. 570° C.) and at 55 per cent, manganese (922° C.). Ranges of solid solution exist between 32 to 41 and 45 to 50 per cent, manganese. Some of the alloys show paramagnetism.

Two compounds have been described, Fe₃Sb₂ (M.pt. 1015° C.) and FeSb₂. There are two eutectics, at 50.5 per cent, antimony (M.pt. 1,002° C.) and at 92.5 per cent, antimony (M.pt. 628° C.). The eutectic range in the iron-rich alloys extends from 5 to 52 per cent, antimony, and throughout this range there is a transformation at 798° C. corresponding to the change from γ-iron to α-iron. The solid solubility of antimony in iron is reported to be 6.5 per cent.; X-ray examination, however, suggests that the solid solubility of antimony in iron is higher. There is also a range of solid solution between 55 and 65 per cent, antimony with a maximum on the liquidus curve at 63.5 per cent, antimony (1,018° C.) corresponding with the compound Fe₅Sb₄. It is possible that the so-called compound Fe₃Sb₂ may not be a separate chemical entity. The crystal structure of FeSb₂ is rhombic, with a = 3.189 A., b = 5.819 A. and c = 6.520 A. The unit cell contains two molecules of FeSb₂. It is claimed that some of these alloys are resistant to acids, particularly to hydrochloric acid.

Two compounds are formed, CoSb (M.pt. 1,190° C.) and CoSb₂ (decomposing at 900° C.), with eutectics at 39 per cent, antimony (M.pt. 1,090° C.) and at 99 per cent, antimony (M.pt. 620° C.). Antimony is soluble in cobalt to the extent of 13 per cent., and the alloys, within this range, are magnetic, losing their magnetism at temperatures varying from 1,132° C. for pure cobalt to 927° C. for the 13 per cent, antimony alloy.

Two compounds are formed, NiSb (M.pt. 1,160° C.) and Ni₅Sb₂ (M.pt. 1,170° C.), with eutectics at 3.2 per cent, nickel (M.pt. 612° C.), at 47 per cent, nickel (M.pt. 1,072° C.) and at 65 per cent, nickel (M.pt. 1,100° C.). There are three solid solutions: α, between 33 and 40 per cent, nickel, β, between 92 and 100 per cent, nickel above 330° C., and γ, between 67 and 100 per cent, nickel below 330° C. Magnetic alloys are found in the range 92 to 100 per cent, nickel below 330° C., the magnetic transformation taking place at that temperature. The compound Ni₅Sb₂ is formed from another compound Ni₄Sb at 677° C. Within the range 55 to 67 per cent, nickel, both compounds appear to be stable at ordinary temperatures.

Several compounds of these metals have been described, including PdSb₂ (decomposing at 680° C.), PdSb (M.pt. 799° C.), Pd₅Sb₃, Pd₂Sb (decomposing at 830° C.) and Pd₃Sb (M.pt. 1,182° C.). The existence of the compound Pd₅Sb₃ has not, however, been confirmed. Eutectics are formed at 9.7 per cent, palladium (M.pt. 586° C.), at 55 per cent, palladium (M.pt. 734° C.) and at 77 per cent, palladium (M.pt. 1,070° C.). Solid solutions are formed of antimony in palladium (up to 15 per cent, antimony), and of antimony in the compound Pd₃Sb (between 68.5 and 72.5 per cent, palladium).

A number of intermetallic compounds of these two metals has been reported, but the existence of one only, PtSb₂ (M.pt. 1,225° C.), has been definitely confirmed. It is possible that two others exist, namely, Pt₄Sb and PtSb, with transition points at 670° C. and 660° C., respectively; the compound Pt₅Sb₂ previously reported has not been confirmed. Two eutectics are formed, one containing a trace of platinum (M.pt. 630° C.), and the other containing 77 per cent, platinum (M.pt. 670° C.).

Among them may be mentioned antimony-copper-silver, antimony-copper-cadmium, antimony-copper-tin, antimony-copper-lead, antimony-copper-bismuth, antimony-copper-iron, antimony-silver-cadmium, antimony-silver-zinc, antimony-zinc-lead, antimony-zinc-bismuth, antimony-magnesium-aluminium, and antimony-tin-lead. The last-mentioned system includes the industrial alloys known as type-metals.