Physical Properties of Antimony
|Numerous varieties of antimony have been described by various workers, but the existence of definite allotropic forms, and their relationship to one another, have not yet been fully established. According to Cohen and his co-workers, two forms of antimony, α and β, can exist, α, or rhombohedral antimony, is more stable than β, amorphous or explosive antimony, at all temperatures below the melting point. A very unstable yellow modification has also been described. |
Irregularities in the dilatometric curve at 96° C. and 101° C. were formerly taken to indicate allotropic transformations, but it is probable that they are due to mechanical deformation. Similar irregularities in the electrical resistance curve disappeared after tempering at 600° C., and X-ray observations taken at room temperature, 150° C. and 200° C., indicated no change in structure.
Rhombohedral Antimony or α-Antimony
|Rhombohedral Antimony or α-Antimony is the ordinary form of the element. It is a white, lustrous metal, crystallising in the holohedral class of the rhombohedral division of the hexagonal system |
a:c = 1:1.3236, α=86°58'
From X-ray analysis it is deduced that the structure consists of two interpenetrating face-centred lattices, the symmetry being that of a rhombohedron. The unit rhomb contains eight atoms; the length of the edge of a unit rhomb is 6.20 A., and the shortest distance between two atoms 2.92 A. The angle between any two edges of the rhombohedron is 86°58',and the angle between the (100)and (010) faces is 92°53'. There is very perfect cleavage at right-angles to the trigonal axis and parallel to the (111) planes, and a less perfect cleavage parallel to the (110) planes. The crystal forms of electrolytic antimony, and of native antimony, have also been investigated.
The density is 6.684 at 25° C.; that calculated from X-ray data is 6.73.
The average compressibility at 20° C. between 100 and 500 megabars is 2.4×10-6.
The linear thermal expansion between the temperatures -190° C. and 17° C. is given by the expression
lt = l0(l + 0.0000102t)
and between the temperatures 17° C. and 100° C. by
lt = l0(l + 0.00001088t)
The cubical coefficient of expansion lies between 0.0000316 and 0.000033. The thermal expansion of single crystals has also been determined.
The hardness of antimony on Mohs' scale lies between 3.0 and 3.5. The Brinell hardness number is 58.0, but higher values have been obtained. The ultimate tensile strength is 1.1 kilograms per square millimetre; Young's modulus is 7,950 kilograms per square millimetre, and the modulus of elasticity in shear 2,020 kilograms per square millimetre.
The specific heat is 0.05. Within the temperature range 412° C. to 460° C. the heat capacity is given by
H= 0.0534496 – 0.4522×10-2t + 0.7944×10-5t2 gram-calories per gram
The heat capacity at low temperatures has also been determined and is as follows:
|Heat capacity (Gram-calories per gram-atom)||6.073||6.043||5.940||5.923||5.943||5.893||5.878||5.834||5.789||5.726||5.691||5.431||5.224||4.700||4.337||3.917|
From this the entropies have been calculated.
The melting point is 630.5° C., the metal being very susceptible to supercooling.
When molten antimony is cooled its colour diminishes in brightness in a normal manner until the solidification point is reached; at this point a sudden increase in brightness occurs, accompanied by an increase in temperature. As this phenomenon occurs in an atmosphere of hydrogen as well as in a vacuum, it cannot be ascribed to oxidation, and is considered to be an instance of crystal luminescence. The ratio of the spontaneous crystallising power to the linear velocity of crystallisation increases with the velocity of cooling in the temperature range between 600° C. and -70° C. As the spontaneous crystallising power diminishes at low temperatures, it may be possible to obtain amorphous antimony by very drastic supercooling.
The latent heat of fusion is 38.84 calories per gram.
The values obtained by earlier workers for the volume change on solidification are confusing, the general conclusion being that the change is very small, antimony probably resembling bismuth in expanding on solidification. Toepler concluded that there was a shrinkage on solidification to the extent of 1.4 per cent, or 0.0022 c.c. per gram. A more recent investigation, however, has revealed an expansion on solidification of 0.95 per cent., a result which appears to have been confirmed.
The density of molten antimony is shown in the following table, though different investigators have obtained slightly different results:
|Temperature, ° C||640||700||800||970|
The variation of surface tension, σ, of molten antimony with temperature is as follows:
|Temperature, ° C.||640||675||700||800||974|
|σ (dynes per cm.)||348||350||350||348||342|
From a consideration of these values it is deduced that antimony in the molten state is probably highly associated. The observed parachor varies from 76.8 to 83.9, the calculated value being 66.0.
The mean specific heat of the liquid is 0.16.
The coefficient of viscosity, measured in grams per cm. per sec., is as follows:
|Temperature, ° C.||702||801||902||1002|
The boiling point under atmospheric pressure is 1635° C. ±8° C. This value is much higher than that given by previous investigators, as will be seen from the following data for the vapour pressure of antimony:
|Temperature, ° C||1075||1135||1175||1225||1265||1325||1330|
Under reduced pressure volatilisation is said to take place at temperatures as low as 292° C.
The vapour density of antimony indicates that at high temperatures the vapour is probably monatomic, association taking place at lower temperatures.
The thermal conductivity of solid antimony lies between 0.038 and 0.050 gram-calories per cm. per sec.; the probable mean value is 0.044. The effects of pressure and of temperature upon the thermal conductivity have been investigated.
The electrical resistance at various temperatures is shown in the following table:
|Temperature, ° C||Resistance (ohm-cm.)||R/R0|
The influence of strong magnetic fields upon the electrical resistance has also been investigated; while the electrical resistance of molten antimony at the melting point is 115.0×10-6 ohm-cm. Both the temperature coefficient and the pressure coefficient have been determined.
For an antimony-platinum couple, if the cold junction is maintained at 0° C., the thermo-electromotive force in microvolts may be calculated for any temperature between 0° C. and 630° C. from the equation:
SbEPt = 46.24t + 0.0313t2 – 0.0000477t3
and the thermo-electric power, in microvolts per degree centigrade, from the equation:
SbQPt = 46.24 + 0.0636t – 0.00001433t2
The magnetic susceptibility, using a field lying between 1029 and 13,680 gauss, is -0.8138×10-6. This value appears to be independent of the field strength. For a single crystal the mean magnetic susceptibility is -0.80×10-6, the susceptibility depending upon the position of the crystallographic axis with respect to the direction in which the susceptibility is measured. There is little variation in the susceptibility between room temperature and that of liquid air. The magnetic anisotropy is ascribed to the unequal valency group in this element. The influence of particle size upon the diamagnetic susceptibility of antimony has been examined. According to some investigators the diamagnetic susceptibility falls as the size of particle decreases, tending to become constant at a diameter less than 0.5μ. It has also been stated, however, that the size of particle is practically without effect.
The Hall effect at 20° C. in a magnetic field lying between 4 and 16 kilogauss is +19.2×10-9 volt-cm. per ampere-gauss. The variation of Hall effect with temperature, and with variation of the magnetic field, has been investigated. The Corbino effect, Ettingshausen effect, Nernst effect and Righi-Leduc effect have also been examined.
The refractive index varies from 3.04 to 3.17, according to wavelength, the corresponding absorption coefficients varying from 1.63 to 1.56.
Explosive, Amorphous or β-Antimony
|Explosive, Amorphous or β-Antimony is usually obtained by the electrolysis of solutions of antimony trichloride. It was first prepared by Gore in October 1854 from solutions of anti mony trichloride, tribromide and triiodide. The product obtained in each case was different, and in each case was contaminated with the corresponding halide, that from the solution of antimony trichloride containing about 6 per cent, of halide, and that from the solution of antimony triiodide about 22.2 per cent. Two specimens obtained from the electrolysis of antimony trichloride gave the following analysis: |
Gore concluded that this variety of antimony was either capable of forming an unstable compound of indefinite composition with antimony halides, or that it was an amorphous variety in which the halide was mechanically entangled.
Both crystalline and amorphous antimony may be obtained by electrodeposition from solutions of antimony trichloride in hydrochloric acid the nature of the deposit depending upon the temperature, concentration and current density, increase in temperature and decrease in current density favouring the formation of the crystalline modification. Cathodes of platinum, copper, manganin, graphite, zinc and mercury have been employed. X-ray examination of the deposit obtained from solutions of antimony trichloride in glacial acetic acid, using a copper cathode, indicates that the nature of the deposit is not affected by a change in current density from 0.1 to 0.7 ampere per square centimetre; that for a given concentration amorphous antimony is deposited at a higher temperature than from aqueous solutions (particularly at lower concentration), while at a given temperature amorphous antimony is deposited at lower concentrations than from aqueous solutions. Amorphous antimony is deposited from solutions in glacial acetic acid containing 10 grams SbCl3 in 100 grams solution below 40° C., and from solutions containing 50 grams SbCl3 in 100 grams solution below 55° C. Within this range a mixed deposit is obtained.
Thin layers of antimony deposited on cellulose nitrate films show, from electron diffraction patterns, an amorphous structure if the deposit is not too thick, while thick deposits show only a crystalline structure. With very thin layers the amorphous structure persists indefinitely, while with deposits of medium thickness crystalline spots appear after a time and gradually spread throughout the deposit.
A black, amorphous modification of antimony, probably identical with that described by Cohen, has been prepared by the action of oxygen or air on liquid antimony trihydride cooled to about -40° C.; and also .by the rapid cooling of antimony vapour. A similar product has been obtained by the reduction of antimony compounds in the presence of antimony trichloride. In the absence of the trichloride, however, only the crystalline modification is obtained.
By heating antimony in a stream of nitrogen a substance resembling amorphous antimony has been obtained; it is stated, however, to be a mixture of antimony with antimony trioxide, and was not obtained when pure antimony and pure nitrogen were used.
A vitreous amorphous form of antimony has also been formed by the rapid quenching of small drops of antimony which has been fused with antimony triselenide. The presence of antimony triselenide retards the crystallisation of antimony considerably. These amorphous pellets are moderately stable even when heated to 500° C., but crystallise much more rapidly at 520° C.
Amorphous or β-antimony is metastable at all temperatures below the melting point. The transformation to the stable rhombohedral or α-form takes place very slowly at ordinary temperatures, particularly in the absence of any external stimulus. If the β-allotrope is scratched, however, the transformation takes place much more rapidly, and may even approach explosive violence. The system is probably monotropic; the conditions of equilibrium are represented diagramatically in fig. It will be seen that the vapour pressure of the β-allotrope is always higher than that of the more stable a-form, and that the two vapour pressure curves intersect at a point (the "transition point") above the melting points of both allotropes.
| Curves of Antimony. |
Amorphous antimony obtained by electrolysis is always contaminated by antimony halides, and it has been shown that the halides are not held mechanically; it is assumed that these preparations are solid solutions of antimony halide in the metastable β-allotrope, and on this assumption it is possible to formulate an explanation of the explosive nature of the transformation from to a. When amorphous antimony is scratched, the heat developed by the scratch is sufficient to accelerate appreciably the rate of transformation. As the transformation itself is exothermic, the surrounding material becomes heated, and thus the transformation progresses almost instantaneously throughout the mass. Allowing for heat losses by radiation, etc., the rise in temperature is sufficient to volatilise the contaminating antimony halide, and this rapid volatilisation is responsible for the explosive effect. It should be stated, however, that a transition temperature of 96° to 100° C. for the transformation from to α-antimony has recently been reported.
Photomicrographs of explosive antimony (deposited electrolytically) have been examined before and after the "explosion." (The "explosion" was effected by a spark from an induction coil or from a Leyden jar, or by touching the film with a hot needle). The polished surface before the explosion resembles that of any other soft bright metal. After the explosion, a very large number of fine lines is, however, developed, parallel in that part of the film remote from the origin of the explosion, but arranged in concentric circles around that origin. These lines apparently are not a surface effect.
The heat of transformation is 20 gram-calories per gram.
The heat capacity between 150° C. and 411° C. is given by
H =0.0535656 – 0.46635×10-4t + 0.15497×10-6t2 gram-calories per gram.
The specific electrical resistance is 50,000 to 90,000 times greater than that of rhombohedral antimony.
|Yellow Antimony, an unstable allotrope, has been prepared by the action of oxygen on antimony trihydride at -90° C., and by the action of antimony trihydride on chlorine dissolved in ethane at -100° C. in red light. It has only been obtained in very minute quantities, reverting (above -90° C.) to the black (probably amorphous) modification. It is believed to be isomorphous with yellow phosphorus and yellow arsenic. |
By condensing the vapour of antimony obtained by cathode spluttering, the metal can be obtained in a form which has an extremely high electrical resistance. In this condition the characteristic X-ray patterns are no longer obtained, thus suggesting that the metal is now amorphous. By heating to 173° C. the normal form of the metal is again obtained.
|Antimony compounds impart no characteristic coloration to the Bunsen flame. The wavelengths of the principal lines of the arc spectrum, measured in Angstrom units [1 A. = 10-8 cm.], are given below. The numbers in parenthesis indicate the relative intensities of the lines, the lowest numbers indicating the weakest intensities. Wavelengths marked (a) are those of lines emitted by the neutral atom, those marked R are of lines that are easily reversed. |
11864 (4), 11268 (4), 10840 (5), 10743 (5), 10678 (10), 10587 (5), 10263 (4), 10080 (4), 7924.6 (6), 7844.4 (4), 6806.3 (6), 6778.4 (6), 6129.9 (6), 6079.6 (6), 6005.0 (6), 5730.4 (4), 5632.0 (4), 4033.5 (6 a), 3722.8 (8 a), 3637.8 (9 a), 3383.2 (5 a), 3267.48 (8 a R), 3232.52 (8 a R), 3029.8 (8 a R), 2877.920 (10 a R), 2851.1 (4), 2769.95 (10 a R), 2727.22 (5 R), 2682.77 (4 R), 2670.67 (5 a R), 2598.076 (10 a R), 2528.53 (6 a R), 2373.7 (4 R), 2311.5 (6 a R), 2306.5 (5 R), 2179.25 (4 R), 2175.88 (5 a R), 2068.38 (4 a R).
The principal lines of the spark spectrum are:
5639.7 (5), 4693.0 (10), 4591.8 (5), 4352.2 (10), 4265.0 (10), 4195.1 (8), 4033.5 (4 a), 3722.8 (5 a), 3637.8 (6 a), 3504.5 (10 a), 3498.5 (10), 3473.9 (10), 3267.5 (10 a), 3241.2 (10), 3232.5 (10 a), 3040.7 (10), 3029.8 (10 a), 2913.3 (5), 2877.920 (10 a R), 2851.1 (4), 2790.4 (10), 2769.95 (10 a R), 2727.22 (8), 2718.90 (10), 2682.77 (5), 2670.67 (5 a), 2652.60 (8), 2612.32 (8), 2598.076 (10 a R), 2590.29 (10), 2528.54 (10 a R), 2478.34 (6), 2445.55 (6 a), 2383.64 (4), 2311.5 (10 a R), 2306.5 (4), 2054.0 (6), 2039.7 (5), 2023.9 (4), 1926.6 (5), 1870.6 (10), 1867 (8), 1810 (5), 1783 (10), 1762 (10), 1731 (5), 1725 (6), 1712 (6), 1585 (8), 1566.3 (8), 1514 (10), 1438 (10), 1307 (10), 1225 (10), 1211 (10), 1205 (10), 1193 (10), 1171 (10), 1168 (10), 1162 (10), 1048 (10), 1042 (10), 1012 (10), 981 (10), 976 (10), 861 (6), 805 (5).
The most persistent lines emitted by the neutral atom, together with the combinations of spectral terms (energy states) from which they arise are: 2068.38(4S2-4P3), 2175.88(4S2-4P2), 2311.50(4S2-4P1), 2528.53 (2D3-2P2), 2598.08(2D2-2P1), 3232.52(2P'2-2P2), 3267.48(2P'1-2P1).
The absorption spectrum of antimony vapour shows a banded structure extending from λ 2305 to λ 2250 A. with a constant interval of nearly 15 A. At higher temperatures another banded structure occurs in the region λ 2830 to λ 3000 A. Fine lines have also been observed at λ 2312 and λ 2306 A. and at higher temperatures at λ 2770 A., but subsequent investigation failed to reveal these lines.
From a study of the absorption spectrum it has been suggested that three types of diatomic molecules can exist in the vapour of antimony and that they occur in the proportions
(Sb121)2:Sb121.Sb123:(Sb123)2 = 5:8:3
In the accompanying bibliography references are given to the more important researches dealing with the arc spectrum, and deductions from it regarding sub-atomic structure, the spark spectrum, the flame spectrum, the ultra-violet spectrum, series spectra, the ultimate rays, the Zeemann effect, and the critical potential, resonance potential and thermionic discharge spectra.
From the spark spectra of antimony in various degrees of ionisation it is calculated that the ionisation potential for singly ionised antimony is 18 volts (or 18.8 volts), and that for doubly ionised antimony 24.7 volts.
By illuminating the vapour of diatomic antimony by a mercury arc of high luminosity, a rather strong fluorescence has been obtained, excited by four mercury lines and possibly by two others.
The wavelengths of the most persistent lines in the spark spectrum of solutions of antimony chloride, together with the dilution at which the lines persist, are as follows:
|1 per cent, to 0.1 per cent. Sb+++||0.1 per cent, to 0.01 per cent. Sb+++||0.01 per cent, to 0.001 per cent. Sb+++|
|3739.95||. . .||. . .|
|3597.51||. . .||. . .|
|3504.51||. . .||. . .|
|3337.15||. . .||. . .|
|3267.53||. . .||. . .|
|3232.54||. . .||. . .|
|3029.86||3029.86||. . .|
|2790.39||2790.39||. . .|
|2528.54||2528.54||. . .|
|2311.48||. . .||. . .|
The X-ray spectrum has also been examined, the lines in the K series and the L series having been recorded.
|Single crystals of antimony have been prepared, the methods adopted being, in general, similar to those employed for bismuth. Among the properties of such single crystals that have been investigated are the mechanical properties, the electrical resistance, the thermo-electric properties, and the magnetic susceptibility.|