NOTES
June, 19612
1205
THE BISiMUTF[-SULFUR PHASE DIAGRA 11’ BY DANIEL CUBICCIOTTI Stan/ford Research Institute,
Menlo Park, Calz/ornia Received December 21, 1961
The phase diagram of the Bi-BizSa system from pure Bi to mixtures containing as much as 0.55 atom fraction S has been investigated by several w0rkers.2--~ For mixtures containing more S 1 he pressure became too great for the glass bulbs used by l’6labon and Aten. It is not clear what containers were usedl by Urazov, et al. We have found that small silica-glass bulbs were strong enough to resist the pressures, so that we could contain even pure sulfur under its own vapor pressure up to the temperatures required for this work. In, attempting standard thermal analyses of the mixtures we encountered two problems: (a) the melts showed decided supercooling; and (b) bulbs made with internal thermocouple wells almost invariably broke. Therefore, we decided to determine the liquidus curve by visually observing the point a t which the last solid disappeared as the mixture was heated. Experimental Weighed amounts of 99.999+ % Bi and S, from American Smelting and Refining Com any, were sealed into evacuated silica-glass Ixdbs. The bufbs were about 7 cm. long and made of 25-mm. o.dl., 1-mm. wall tubing for low-S, and of 12-mm. o.d., 2-mm. wall tubing for high-S samples. A Pt-lOyo Rh thermocouple, calibrated a t the Al, Pb, Sn, and Zn points, was tied t o the outside. The bulb was heated in a furnace having a 5 mm. longitudinal window-slot with a Vycor tube inside to act as a window. The sample bulb could be manipulated by a handle. As a sample of Bi and S was heated, the S was observed to melt fink, then the Bi. Shortly thereafter a rapid, exothermic reaction occurred between them to form a gray, spongy mass of BizSa. For all mixtures having less than 0.67 atom fraction S, the sample melted to a silver metallic liquid, from which needles (presumably Bi&) separated with sufficient cooling. The dissolution of these needles in the metallic liquid was observed as thesample was shaken and heated dowly (2 to 3 degrees per minute near the disappearance temperature). The temperature a t which the last solid disappeared was recorded as the liquidus temperature for the mixtyre. Repeated trials on a sample agreed to better than 3 . Pure Bi was foundOto liquefy over a Stoichiometric short temperature range ending a t 271 BizSa also mlelted over a short temperature range, as a congruently melting substance, a t 775’. These substances would be expected to have sharp melting points. Their melting over a short range of temperature probably was due to temperature gradients in the bulb. Samples with atom fraction S of 0.15 and 0.40 were cooled after solution temperature determinations and examined under a microscope after being cut and polished. Two phases were observed, a crystalline precipitate and a matrix material. The relative amounts of these two phases agreed with the amounts expected if the precipitate were Bi& and the matrix Bi. X-Ray powder photographs were made of samples that had the composition and (60-hr.) equilibration temperature marked by X’s in Fig. 1. All the lines of these samples could be fit to either those of Bi or Biz&. These examinations of the cooled samples indicated, therefore, that only two phases were present-Bi and Bi&. This point m,zy seem belabored; however, it seemed possible that an incongruently-melting compound of stoichiometry
.
(1) This work was made possible b y the financial support of the Research Division of the United States Atomio Energy Commission. (2) H. PBlabon, J . chzm. phys., 9 , 320 (1904). ( 3 ) A. H.W. Aten, 2. anorg. Chem., 47,386 (1906). (4) G. G . Urazov, K. A. Bol’shakov, P. I. Fedoro.i, a n d I. 1. Vaeilevakaya, Russ. J . Inorg. Chem., 6 , 303 (1960).
0.6 0.8 1.o BizSa s Atom fraction sulfur. Fig. 1.-The Bi-S phase diagram: full curve, liquidus for the present work; dotted curve indicates region of two immiscible liquids; dashed curve, data of Aten (ref. 2); broken curve, data of lJrazov, et al. (ref. 4).
0 Bi
0.2
0.4
BizS might have formed by analogy with BiCl, BiBr, and BiI.6 No evidence for such a compound was observed. Compositions of atom fractiof S greater than 0.67 consisted of two liquids above 727 , The denser liquid was silvery (and presumably of composition 0.67 atom fraction S-see Fig. 1). The lighter liquid was dark brown in color and almost pure S. In a sample of 0.992 atom fraction sulfur, the “silver” liquid phase still was observed above 727O, i.e., it had not all dissolved in the S phase. After rapid cooling, the solidified S phase was found to contain small silvery crystals B S though some Bid% had been in solution in the molten S. The vapor over the molten samples of atom fraction S about 0.65 and greater was very dark, almost indistinguishable from the liquid S phase. The intensity of the vapor color diminished with reduction of S content below 0.67 such that in the range 0.58 to 0.60 it was difficult to see whether the vapor was colored (because of the background color of the furnace). Below 0.58 the vapor appeared colorless. A rou h estimate of the amount of S in the va or indicated that onyy a negligible correction was to be appled to the atom fractions of the condensed phases.
Results The liquidus of the Bi-S phase diagram, under its equilibrium gas pressure, derived from these measurements is given in Fig 1. The circles represent the temperatures at which the “silver” phase was completely liquefied. The two circles containing crosses represent the temperature a t which the “silver” phase appeared completely solidified. The diagram between Bi and Bi& apparently is a eutectic type with the eutectic very close to Bi. Our results are in general agreement with those of other workers. The lack of better agreement may be due to impurities in the materials used by the other workers. Aten reports that his Bi froze at 2 7 7 O , while recent determinations6 of the melting point give 271.0’. IJrazov, et u Z . , ~ report using Bi of 99.8% and “flowers of sulfur.” It has been our experience that “flowers of sulfur” when boiled away under nitrogen leaves a substantial amount of charred residue. The part of the diagram from Biasato S has not been reported before this. The results indicate that BizSa is a congruently melting compound. The liquid dissolves S up to an atom fraction of ( 5 ) (a) S. J. Yosim, A. J. Darnell, W. G. Gehman, and S. W. hlayer. J. Phys. Cham., 63, 230 (1959): (b) S. J Yosim, L. D. Ransom,
R. A. Sallaoh, and L. E. Topol, zbzd.. 66,28 (1962). (6) h’ational Bur. Standards. Circ. 500, 1952.
0.67 and additional S gives rise t o a second liquid hlicroscopic examination of the adsorption-desor tion phase of almost pure S. The limits of miscibility of phenomena was carried out with a Bausch and t o m b microscope; photomicrographs (100 X and 450 X ) these two liquids were not investigated and so are polarizing of the various samples were taken with an Exa single lem represented as dotted lines. reflex camera with adapter. Acknowledgments.-Much of the experimental Results work was performed by Mr. William E. Robbinfi. Spectral Reflectance.-Mereuric iodide mixed Dr. J. W. Johnson showed us that silica-glass with alumina was found to show a gradual increase would stand up to these conditions. in absorbance a t 290 mp, a t the same time the absorption peak a t 576 mp decreases steadily and THE COLOR OF MERCURIC IODIDE ON finally becomes unobservable. When adsorbed on sodium fluoride the color of the mercuric iodide ALUMINA remains unchanged, the discrete absorption maxima BY HARRY GOYA,JOHKL. T.WAUGH,AND HARRY ZEITLIN a t 293 and 573 mp increasing only slightly with time, without any shift in the wave length being detectDepartment of Chemistry, University of Hawaiz, Honolulu, Hawaii able. Exposure of the mercuric iodide-alumina Received December WW, 1061 mixtures to the moist atmosphere was observed to The red, tetragonal modification of mercuric result in desorption of the iodide. This process iodide undergoes an enantiotropic transformation also was followed by reflectance measurements and to a yellow, orthorhombic form when heated above the observed spectra indicated that the changes its transition temperature of 127'. When initially occurring were essentially the reverse of the adprecipitated from solution or when formed from sorption process. the vapor phase, the yellow modification reverts Microscopic Studies.-Red mercuric iodide is spontaneously to the red, normally stable form. soluble in acetone to form a colorless ~olut~ion. Recently, a red to yellow transition has been ob- Crystallization from this solution, and the subseserved to take place, apparently similar to the quent transformation of the red to the yellow moditransformation which takes place on heating above fications on heating and the reverse conversion the transition temperature, when mercuric iodide from the yellow to the red forms on cooling were is adsorbed on alumina a t normal atmospheric carefully observed a t various magnifications. Simitemperatures.l However, in the adsorbed state lar observations were made of the yellow material under anhydrous conditions, the yellow species on desorption from alumina and its subsequent remains as such indefinitely. I n a preliminary reversion to the red variety. This latter effect communication, this phenomenon was attributed could be accomplished either mechanically, by to a polymorphic conversion, rather than to any suitable prodding with a needle, or by moistening change in particle size or to ionization by polariza- the yellow material adsorbed on alumina; in either t i ~ n . ~It, now ~ has been established, by means of case, the yellow material is initially detached from X-ray powder, diffraction studies, reflectance anal- the alumina as yellow crystals, which then transysis, and microscopic examination that the yellow form to the red variety, in a manner very similar to material, which is stable when adsorbed on alumina the initial precipitation of mercuric iodide from a t atmospheric temperatures, is the same species solution as the. yellow form. The homogeneous which results on heating mercuric iodide above nature and the stability of the yellow iodide when 127'. adsorbed on alumina is quite remarkable. X-Ray Diffraction Data.-Powder photographs Experimental Reagents.-Woelm alumina, grade 1 activity, ground t o of the red, tetragonal mercuric iodide showed narless than 200 mesh; C.P. grade sodium fluoride, less than row, well-defined diffraction maxima, while those 140 mesh; reagent grade mercuric iodide, less than 200 mesh; of alumina and of mercuric iodide adsorbed on all mesh sizes refer to U.S. standard screens. The alumina and sodium fluoride were dried for 48 hr. a t 115" prior t o alumina displayed broader, more diffuse maxima. use and desiccated until required. Samples containing 8% The relative intensities and the observed interby weight of mercuric iodide adsorbed on alumina were planar spacing values as obtained from the powder used. photographs are shown in Table I. The spacings Apparatus and Procedure. The powdered samples were packed into lithium borate glass capillaries of 0.5 mm. di- for the mercuric iodide-alumina mixture are those ameter and 0.01 mm. wall thickness and mounted in a 114.59 observed after subtracting the spacings which mm. diameter Straumanis-VCiilson camera, to record their correspond to those due to alumina. The interX-ray powder diffraction patterns, using Cu Ka: radiation. planar spacings corresponding to the strongest arcs The diffuse reflection spectra of the solid mixtures were measured with st Beckman DK-2 automatic recording spec- on the red mercuric iodide photographs are not trophotometer, equipped with a reflectance attachment; observed for the yellow mixture and those corabsorbance as a function of wave length was examined. responding to the medium and weak arcs are disConcentrations used were 200 mg. of adsorbate mixed with similar. It was not possible to obtain a powder 10 g. of adsorbent. The samples were mixed, mechanically agitated for 6 to 9 min., and poured into 6-ml. cylindrical, photograph of the pure, yellow form of mercuric quartz cells of 1-cm. light path, which were fitted with iodide. However, from the unit cell dimensions of ground glass stoppers. The reflection spectra were measured the yellow form reported in the l i t e r a t ~ r e ,inter~,~ a t varying intervals of up t o 45 days, after initial mixing. planar spacings were calculated for a small field of indices hlcl with the results shown in the last column (1) H. Goya and H. Zeitlin. Nature, 18, 1941 (1959). ( 2 ) E. U'eitz and F. Schmidt, BET.Deut. Chem. Ces., l B , 2099 of the table. [
1939). 13) G . Kortunr, J. T'iipel and W. Braun. Angew. Chem., 21, 651
(1936).
(4)
W. S. Gorsky, Ph&k Z . Sowjetmion, 6 , 367 (1934).
( 5 ) €1. .J. \'errreel and J. M. Bijvoet, 2. K r i s t . , 7 7 , 122 (1931).
