Physical Properties of Tellurium

Tellurium is known in amorphous and crystalline forms, and it is important to investigate Physical Properties of Tellurium but it appears to differ from sulphur and selenium in that it yields only one variety of each form. It may also be obtained in colloidal solution.

Amorphous Tellurium is a brownish-black powder usually obtained by precipitation methods, for example by reducing a solution of tellurium dioxide with sulphurous acid. On heating it is transformed into crystalline tellurium with development of heat:

Te_amorph. = Te_cryst. + 2630 calories.

It resembles the crystalline variety in behaviour except in such properties as are influenced by its fineness of division. The electrometric properties of the two forms are identical. The density of the amorphous form ranges from 5.85 to 5.87; the specific heat is 0.052.

Physical Properties of Tellurium in crystalline form are different from those in amorphous tellurium. Molten tellurium solidifies to a brittle, silvery, crystalline mass, which is easily powdered. The crystalline modification can also be obtained by sublimation of the element or by its slow formation, for example in the gradual decomposition of hydrogen telluride or in the slow atmospheric oxidation of an aqueous solution of an alkali telluride. When obtained of appreciable size the crystals are generally found to be prismatic, of the trigonal system, and isomorphous with "metallic" selenium (a:c = 1:1.3298; a = 86.8°).

The density of crystalline tellurium is curiously variable, the mass obtained after fusion having a density about 6.24, whilst distilled or precipitated tellurium generally gives a lower value, usually 6.15 to 6.23. According to Cohen and Kroner the density alters under the influence of heat, and this, together with other inconstancies, is attributed to the presence of two dynamic allotropes in ordinary tellurium, the density as well as the other physical properties depending on the relative proportions. This view is not supported, however, by the more recent work of Damiens, in which the tellurium used had been carefully purified by successive distillation in hydrogen and in vacuo, followed by crystallisation either by vaporisation or by slow cooling of the molten material. The density of the tellurium obtained by vaporisation was found to be 6.310, and this value did not alter on heating. Specimens prepared in other ways were slightly porous and consequently had a slightly lower density.

The compressibility of tellurium at 25° C. between 100 and 500 megabars is 5.00×10^-6 per megabar, a value which falls on the smooth compressibility - atomic volume curve for the elements.

Tellurium melts at 452° C. and boils near 1390° C. under ordinary pressure, but volatilises at as low a temperature as 430° C. in a cathode- ray vacuum; the vapour is yellow in colour. Like the density, the specific heat of the solid is inconstant, ranging from 0.0475 for the distilled element to 0.0524 for the precipitated amorphous substance. It has been observed that exposure to X-rays increases the specific heat of tellurium by about 8 per cent., possibly owing to a change in the structure of the element.

Solid tellurium is a bad conductor of heat and electricity; a maximum value for the electrical resistance has been observed and placed at 50° to 140° C. by different investigators. The occurrence of this maximum has also been attributed to the presence of two dynamic allotropic forms. At higher temperatures, above 360° C., the specific resistance decreases exponentially with rise in temperature.

Liquid tellurium is comparatively a good conductor of electricity, the specific conductance at the freezing-point being about 15 times that of the solid, whilst at 500° C. it is equal to one-sixth of that of mercury at the ordinary temperature.

Tellurium shows a higher resistance towards an alternating current of high frequency than towards a direct current. Exposure to light causes a very slight increase in the electrical conductivity, the effect being very much less than that produced with selenium.

The "Hall effect," i.e. the displacement of the equipotential lines when a current flows along a thin strip of metal placed between the poles of a magnetic field, is greater in the case of tellurium than for any other metal, the constant being + 530. Tellurium is diamagnetic, its susceptibility being -0.32×10^-6, a value which varies only slightly with the temperature but falls suddenly at the melting- point.

The linear coefficient of expansion of tellurium at the ordinary temperature is 0.0000344. The latent heat of fusion at 446° C. is 33.50 gm. cals.

Molten tellurium dissolves many metals and its cryoscopic constant has been found to have a value between 510 and 535. Tellurium itself dissolves in pyrosulphuric acid, and freezing-point determinations show that it is present in this solution as monatomic molecules.

The optical constants—refractive indices, reflecting powers and absorption constants—of isolated crystals of tellurium placed in various positions relative to the plane of incidence have been determined for wave-lengths of 3000 to 5000 Å. The absorption of light by tellurium vapour reaches a maximum at 1200° C.

Vapour density determinations show that tellurium is diatomic at high temperatures (about 1500° C.), whilst at 2100° C. considerable dissociation into single atoms has taken place. According to Jouniaux the element is monatomic at 357° C. At lower temperatures, as in the case of sulphur and selenium, the molecule is more complex. The vapour pressure of tellurium has been determined and is as follows:

t(°C.)	488	578	671
p (atm.)	6.10×10-4	4.40×10-3	1.86×10-2

and the mean value for the heat of volatilisation has been calculated to be 26.5 cals. per gm. mol.

On reduction of dilute solutions of tellurium dioxide or telluric acid by means of hydrazine, hydroxylamine, hypo- phosphorous acid, sulphurous acid or salts of these compounds, brown or blue colloidal solutions can be obtained, the stability of which is greatly increased by the presence of an organic colloid such as gum arabic, sodium lysalbate or sodium protalbate. Similarly the addition of an extract of the seeds of Plantago psyllium, in amount not exceeding 0.3 per cent., renders extremely stable the sols obtained by the reduction of telluric acid with hydrazine hydrate. Stable sols may also be obtained by the reduction of telluric acid with dextrose in the presence of ammonia. Dialysis does not completely remove the adsorbed dextrose and this undoubtedly contributes to the remarkable stability. The particles of colloidal tellurium in these sols are negatively charged.

Tellurium in a very finely divided condition may be obtained by mixing aqueous solutions of dextrose (30 per cent.) and sodium tellurite (5 per cent.) and heating to boiling for half an hour.

Various other methods of preparing colloidal tellurium have been described. If a small quantity of the element is dissolved in a boiling solution of potassium hydroxide and the product poured into a large bulk of cold water with vigorous stirring, tellurium remains in colloidal solution. Colloidal aqueous solutions have also been produced by the cathodie pulverisation of tellurium under water, and by the electrolysis of an aqueous solution of telluric acid containing either potassium cyanide or ammonium oxalate.

The presence of colloidal tellurium in glass produces a colour tint which varies from blue to brown according to the size of the colloid particles, the blue glass containing larger particles than the brown. Polytellurides are present in some coloured glasses and impart a colour which is red or violet-red.

The coagulating powers of various electrolytes for tellurium hydrosols have been determined; 4 the results obtained do not agree with Whetham's law.

The arc and spark spectra of tellurium have been investigated, the arc being produced in an atmosphere of carbon dioxide between tellurium electrodes or between carbon electrodes one of which carried pieces of tellurium in a small cavity. Fifteen distinctive lines between 3175 and 2081 Å and forty of wave-length less than 2080 A have been measured. The most prominent lines are: 2142.75, 2259.02, 2383.24, 2385.76, 2769.65 and 3175.13 Å. The lines at 2769.65 and 3175.13 have been shown to be distinct from those of antimony (2769.94) and tin (3175.04) by photographing the spectra of mixtures of these elements with tellurium, when in each case the two separate lines were obtained.

The non-luminous vapour of tellurium absorbs the lines 2081, 2143, 2147, 2209, 2259, 2383 and 2386 Å. At 1600° C. the vapour, presumed to contain some monatomic tellurium, absorbs the lines 2143 and 2259 Å, but according to Kimura not 2383 and 2386 Å, so that the latter lines probably originate in transitions involving metastable states. In the ultra-violet region, tellurium vapour shows eleven wavelengths absorbed between 2000 and 1650 Å.

When illuminated by an incandescent gas lamp, tellurium vapour exhibits an intense bluish-green fluorescence. Under the light of a mercury vapour lamp the fluorescence is much less intense. The fluorescence spectrum consists of regularly spaced bands in the visible region.