28
S. J. YOSIM,L. D. RANSOM, R. A. SALLACH AND L. E. TOPOL
Table V for the stepwise formation constants appear large but they only reflect the extreme sensitivity of polvnomials such as equation 25 to small changes in the experimental ligand concentration. In terms of the formation function the range of constants listed describes the complex system over a quite narrow band indicated by the pair of broken lines in Fig. 1. Acknowledgments.-This work was made possible through a fellowship authorized in a cooperative agreement between the U.S. Bureau of
Vol. 66
Mines, Albany, Oregon and Oregon State University. Assisting in some of the analyses were Howard F. Griffin and personnel of the spectrographic laboratory at the Bureau of Mines. Assistance in the statistical treatment of data was given by Dr. R. G. Petersen of Oregon State University and computer programing assistance was given by Robert N. Brenne. Research paper No. 412, Oregon State University, School of Science, Department of Chemistry.
THE BISMUTH-BISMUTH TRIBROMIDE AND BISMUTH-BISMUTH TRIIODIDE SYSTEMS BY S.J. YOSIM, L. D. RANSOM, R. A. SALLACH AND L. E.TOPOL Atomics 3nternationa1, A Division of North American Aviation, Inc., Canoga Park, California Recebed June 3, 1961
The phase diagrams of the Bi-BiBs and Bi-Bi18 systems have been determined. The experimental techniques included sampling a t temperature, visual observations, conventional thermal analyses and differential thermal analyses. The consolute temperatures (538' at 62 mole yoBi and 458' a t 78 mole YoBj for the BiBrs and Bi18 systems, respectively) are considerably lower than that of the BiCl, system (780' at 51 mole % Si). The freezing point depressions of the metal-rich and salt-rich regions were analyzed. The salts dissolved in molten bismuth were found to have a cryoscopic number of 3. As in the BiC18 case, this effect can be explained by dissociation of BiXa solute or by reaction of BiXa with Bi to form the monohalide. I n the case of the salt-rich regions the data did not fit a curve corresponding to one single mechanism over the entire liquidus.
Introduction In a previous report2 the phase diagram of the Bi-BiCl, system was described. A retrograde solubility was found, and the two components became completely miscible at 780O. In order to see the effect of varying the anion on the miscibility gap, the liquid-liquid regions of the Bi-BiBr3 and the Bi-BiIs systems were determined. Since there are considerable discrepancies in the liquid-solid portions of the phase diagram of the Bi-BiB1.s system3-6 and the Bi-BiIa system,6-8 these regions were investigated also. Finally, the freezing point depressions of the bismuth trihalide by bismuth and those of bismuth metal by the salts were examined in order to see what could be learned about the species in these solutions. Experimental Materials.-The purification of bismuth is described elsewhere.2 Bismuth tribromide and bismuth triiodide were synthesized by direct combination of the elements. I n the BiBrs case, the molten bismuth was exposed to bromine vapor supplied by a bromine reservoir. The starting materials were contained in a sealed, evacuated Vycor system. (1) This work was supported by the Research Division of the U. 8. Atomic Energy Commission. I t has been presented in part before the Division of Physical Chemistry at the National Meeting of the A.C.S. in New York, September, 1960. (2) S. J. Yosim, A. J. Darnell, W. G. Gehman and 5. W. Mayer, J . Phys. Chem., 6 3 , 230 (1959). (3) B. G. Eggink, 2. physil. Chsm., 6 4 , 449 (1908). (4) L Marino and R. Becarelli, Atti accad. naz. Lincei. 2 4 , 625 (1915);26, io5 (1916);as, 171 (1916). (5) G. G. Urazov and M. A. Sokolova, Akad. Nauk; X.S.X.R., Inst. Gen. Inorg. Chsm., 24, 151 (1964). (6) L. Rlarino and R. Becarelli, Atti accad. naz. Lincei, 21, 695 (1912). (7) H. S. van Klooster, 2.uanorg. aEZgsm. Chem., 80, 104 (1913). (8) G. G. Urazov and M. A. Sokolova, Akad. Nauk S.S.S.R., I n a l . Cfen.Inorg. Chem., 26, 117 (1964).
I n the case of the iodide, finely ground bismuth metal was intimately mixed with a slight excess of iodine and the mixture was heated in a sealed, evacuated Pyrex tube at 175"for 24 hr. Both salts were sublimed under reduced pressure after the excess halogen was removed from them. The melting points of the bromide and iodide were 218.5 and 407.7", respectively. Chemical analysis of the bromide showed a 46.5 wt. % bismuth as compared to 46.57% theoretical, while that of the iodide showed a 35.9 wt. % ' bismuth as compared to 35.44% theoretical. Experimental Methods and Procedure.-The techniques used in this work have been described p r e v i o u ~ l y . ~The ~~ miscibility gaps were studied by decanting the salt-rich hase at tern erature, by differential thermal analyses and gy the visuafmethod. The solid-liquid equilibrium curve between the salt-rich eutectic and the base of the miscibility gap in the Bi-BiBr3 case was determined by decantation. All other transitions involving the solid phases were determined by conventional thermal analysis or by differential thermal analysis.
Results
(A) The Bi-BiBra System.-The
phase diagram of the Bi-BiBr3 system under its own pressure is shown in Fig. 1. The consolute temperature was found to be 538', considerably lower than that of the Bi-BiC13 system (780O). A plot of the mean values of the bismuth metal compositions of the two conjugate solutions vs. temperature was linear and the composition corresponding to the consolute temperature was 62 mole yoBi. Just as in the BiBiC13 case,2 a retrograde solubility was observed in the salt-rich region (from 57 mole % at 294' to 45% at 430') while the solubility of the salt in the metal continues to increase with increasing temperature. The results of the liqiud-solid portion of the system are compared in Table I with the results re(9) L. E. Topol and A. L. Landis, J . Am. Chem. SOC.,82, 6291 (1960).
BISMUTH BROMIDE AND BISMUTH IODIDE SYSTEMS
Jan., 1962
29
TABLEI COMPARISON OF RESULTS FOR Bi-BiBi-8 SYSTEM -This workYEgginkaMole % Bi Temp., O C . Mole % Bi Temp., OC.
1. Melting point (a) BiBrr (b) Bi 2. Eutectics (a) Salt-rich (b) Metal-rich 3. Base of miscibility gap
0.0 100.0 21.0 98.7 57.4-98.0
-Urarov
and--Marino and-Sokolovab Becarelli* Mole % Bi Temp., O C . Mole % BI Temp.,
OC.
218.5 272.0
0.0 100.0
217.5 271.5
0.0 100.0
217 272
0.0 100.0
210 272
205.0 263.0 294.0
22.2 99 46-96
204.0 262.0 287.0
33.0 99 38-99
200 255 262
29.2 95
200 250 305
...
TABLHI1 COMPARISON OF RESULTS FOR Bi-BiIs SYSTEM -This workMole % Bi
1. Melting point (a) Bi4 (b) Bi 2. Eutectics (a) Metal-rich (b) Salt-rich 3. Base of miscibility gap 4. Disproportionation of solid BiI Phase transformation of BiI
0 100
Temp.,
OC.
407.7 272
99.5
270
47.5-99.0
336 298(?) 285(?)
..
...
ported by Eggink,a Marino and Be~arelli,~ and Urazov and Sokolova.g In agreement with Eggink and with Uraaov and Sokolova, it is concluded that a solid subhalide exists. Since the general features of the phase diagram of this system are so similar to those of the Bi-BiCla system, it is assumed that the composition of subbromide is the same as that of bismuth uubchloride, Le., of the form B X . This assumption is consistent with the observation that the duration of the syntectic halt was longest with the sample containing 70 mole % Bi. The solidliquid results reported in this work are in relatively good agreement with those of Eggink but are in poor agreement with those of Urazov and Sokolova and those of Marino and Becarelli. One of the main reasons for this disagreement is the fact that both Urazov and Sokolova and Marino and Becarelli used only thermal analysis to determine the phase diagram. It has been shown, at least in the chloride case,Tthat for the salt-rich subhalide "liquidus" thermal analysis does not yield accurate results-presumably because of the metastable nature of the system. The fact that the bromide system tends to be metastable is shown by the eutectic halts beyond the composition corresponding to BiBr. (B) The Bi-Bile System.-The phase diagram of the Bi-BiIa system under its own pressure is shown in Fig. 2. The consolute temperature was found to be 458", the lowest of any metal-salt system to date. A, plot of the mean values of the bismuth metal compositions us. temperature for the liquid-liquid region again was linear, and the composition corresponding to the consolute temperature was 78 mole % Bi. I n contrast to the chloride and bromide systems, there does not appear to be any retrograde solubility in this system. The results of the liquid-solid portion of this system are compared in Table I1 with the data reported by Marino and Becarelli,6van Klooster' and
Klooster7Marino and Beoarellis Urazov and Sokolovaa -van Temp., Temp., Temp., Mole 9% Bi "C. Mole % Bi "C. Mole % Bi OC.
0 100 99.9 42.2 46-99
405 272
0 100
408 272
0 100
412 285
266 321 327
99
270
99
284
46-98
339 281
59-97
340
..
....
..
....
..
..
..
281
500
I i
1
8
THERMAL ANALYSIS-0 DECANTATION 0 VISUAL-A
-
a - B i B r 3 + El Br
~
500
r
-
I
500
400
300
F
L
loo0
10
:
(BlII+ BI
o;-
BIO
B I I H811) ~
200c
;"o
30
40
io
$0
I
200
90IW100
MOLE % BISMUTH.
Fig. 2.-Bismuth-bismuth triiodide system: 0, thermal analysis; e, decantation; A, visual.
Urazov and Sokolova.* The results of this work, in general, are in better agreement with those of van Klooster than with those of Marino and Becarelli or
30
S.J. YOSIM,L. D. RANSOM, R. A. SALLACH AND L. E. TOPOL
MOLE %BISMUTH.
Fig. 3.-Comparison of experimental freezing point depressions of BiBr, with those calculated for various bismuth species.
Vol. 66
The remaining halts at 285 and 298' cannot be unambiguously assigned to definite transitions. Since pure Bi13 undergoes no phase transition a t these temperatures, it is probable that these halts are due to transitions involviiip an intermediate compound, Le., a subhalide. At 285', the durations of the thermal halts were greatest near the composition corresponding to BiI. At 298', however, the magnitudes of all the halts were too small to permit a corresponding evaluation. I n an attempt to confirm the presence of the subhalide, a sample similar to one described in the preceding paragraph was quenched from 219', and a chemical and X-ray analysis was carried out on the upper layer. The I/Bi ratio was 1.70, which suggested that a subhalide was present; however, the X-ray analysis revealed Bib lines only. Nevertheless, on the basis of the thermal analysis results, it is assumed that a solid subhalide exists. The subhalide is designated in Fig. 2 as BiI although the composition of the subhalide has not been established. If one assumes the existence of the solid subhalide, then the two thermal halts a t 285 and 298' suggest that a phase change of the subhalide takes place at the lower temperature and that disproportionation occurs a t the higher temperature. More conclusive evidence such as X-ray diffraction measurements of the subhalide taken a t temperature would be desirable to test these conclusions further.
Discussion of Results
MOLE % B I S M U T H
Fig. 4.-Comparison of experimental freezing point depressions of BiIa with those calculated for various bismuth species.
Urazov and Sokolova. I n addition to the thermal arrests corresponding to the melting points of the components, four thermal halts were found in the liquidsolid region of the phase diagram. These were a t 270, 285, 298 and 336'. The halt at 270' corresponds to the metal-rich eutectic. Since the temperature of the base of the miscibility gap is, within experimental error, the same as that of the first halt below the Bi13 liquidus (33G0), it is concluded in agreement with van Klooster that this halt is a monotectic. Thus, in the zone directly below this halt no solid subhalide exists, in contrast to the results of Urazov and Sokolova vho found two halts in this temperature region (at 321 and 327'). They concluded, therefore, that a salt-rich eutectic was present and that the subhalide was stable at temperatures up to that of the base of the miscibility gap. Additional evidence for the absence of the subhalide just below the miscibility gap is that a chemical analysis of the upper portion of a 66% bismuth metal sample, quenched from 312', yielded an I/Bi ratio of 2.95. Also, the duration of the thermal halt a t 336' was greatest for samples in the region of the monotectic composition. Thus, this portion of the phase diagram is in contrast to the bromide and chloride cases, where the base of the miscibility gap is a syntectic and the halt below the trihalide liquidus is a eutectic.
Considerable doubt as to both the species and the nature of the chemical bonding, particularly a t higher temperatures, exists for bromide and iodide as well as chloride solutions containing dissolved bismuth. As in the chloride case, the mechanisms which can be considered for the solution of Bi in BiBr3 and in Bi13fall into two classes. The first is solution as Bi atoms, dimers or higher polymers, and the second is solution by reaction of Bi with the trihalide to form a lower-valent compound. In the case of metal-rich solutions, less is known about the species and bonding. The salt-rich and metal-rich liquidus curves of both the bromide and iodide systems were analyzed as described in the Bi-BiCls studylo in order t o see whether information on the mechaniFm of solution could be obtained. Thc cryoscopic number n (the number of foreign particles formed in molten bismuth per molecide of solute) was calculated with the Raoult-van't Hoff equation. In both the bromide and iodide systems. as in the BiC13case, a cryoscopic number n of 3 was obtained. Thus, the salts do not dissolve in a molecular form. As in the BiC13 case there are a t least 3 mechanisms which would correspond to an n of 3. (1) The BiX3 dissociates into 4 particles upon dissolving and the bismuth from BiX, becomes indistinguishable from the bismuth particles of the molten metal. (2) The BiX3 reacts with the Bi solvent to form undissociated RiX molecules and (3) the BiX dissociates to yield Ri entities indistinguishable from the bismuth metal particles. Actually the first and third mechanisms cannot be differentiated cryoscopically since the final compositions are ident'ical. (10) 8. n7.Mayer, S,J. Yosim and L. E. Topol, J. Phgs. Chem., 64, 238 (1960).
Jan., 1962
INTERCRYSTALLINE ENERGIES IN ALKALIHALIDES
31
The method for analysis of the salt-rich liquidus is also the same as that described previously,1owith the excieption that the heats of fusion of the salts were not assumed to be independent of temperature. The heats of fusion of BiBrs and BiIa used in the analysis are 5.1911 and 9.4112 kcal./mole, respectively, and $he ACp values are 11.711 and 16.012cal./moile-degree, respectively. The results of the analyses of the salt-rich liquidus curves are shown in Figs. 3-4. Unlike the results of the BiC4 case, the freezing point depression data do not fit a curve corresponding to one single mechanism over the entire liquidus for either system. This may be due to deviation of the solvent from ideality or to an equilibrium between 2 or more lower-valent bismuth species. Spectrophotometric’“ and e.m.f.14studies on the Bi-BiCla system have shown that indeed more than one species exists in these solutions. The e.m.f. results suggest that at the melting point of BiCla, BiCl is the predominate lower-valent Bi species only in very dilute concentrations of dissolved metal (< 0.1 mole %). (Thus, BiC1 would not be detected cryoscopically at such low concentrations.) From an acid-base standpoint, the monomer subhalide should become more stabilized as the acidity of the system increases. Thus, it would be reasonable that the Bi+ entity becomes increasingly stabilized as one goes from C1- to Br- to I-. E.m.f.16 studies have shown that the monomer is indeed
more important in the Bi-BiBra system than in Bi-BiCla melts. The data of Fig. 3 which fit most closely to the BiBr curve at concentrations up to 2 mole % Bi are in accord with this. The data of Fig. 4 suggeiit that BiI is the most important species up to about, 10 mole % Bi. However, while the activities of BiBra calculated from the Bi-BiBra vapor pressure datal6 are in agreement with those calculated from this work, the activities of BiI3 calculated from the Bi-BiIa vapor pressure results’’ are not, but follow Raoult’s law up to about 30 mole % bismuth. If one plots the ideal vapor pressure vs. Bi13concentration for various species, it can be seen that the experimental pressure data follow much more closely the curves for Bi atoms or for BiJ2 rather than BiI. Further, whereas the solubility of Bi in BiC1, is increased by the addition of the acid A1Cla,’8 the solubility of Bi in Bi13 was found to decrease when A113was added. This would suggest that Bi dissolves in Bi13 by a mechanism other than subhalide formation, since one would expect the relatively basic subhalide to be stabilized by the acid. Thus, it would appear that the BiBiC1, and Bi-BiBr3 systems are rather similar in nature, but the behavior of the Bi-Bi13 system is quite different. Perhaps this is not too surprising in view of the greater similarity in physical and chemical properties of pure BiC13 and BiBra (e.g., melting and boiling points and reactivity with water) as compared to pure BiIa.
(11) L. E. Topol and L. D. Ransom, J . Phys. Chem. 64,1339 (1960). (12) hI. A. Bredrg and A. Dworkin, private communioation. (13) C . R. Boston and G. P. Smith, Annual Progress Report, Metallurgy Division, Oak Ridge National Laboratory, ORNL-2988 p. 9-10 (July 1960). (14) L. E. Topol, S.J. Posim and R. A. Osteryoung, J. Phus. Chem., 66, 1511 (1961).
(15) L. E. Topol and R. A. Osteryoung, “E.M.F., Polarographio & Chronopotentiometric Measurenients in Molten Bi-BiBra Solutions,” to be published. (16) D. Cubicciotti and F. J Keneshea, Jr , J . Phys. Chem., 68,999 (1958). (17) D.Cubicciotti and F. J. Kenesbea, Jr., ibid., 63, 295 (1959). (18) J. D. Corbett and R. X. McMullan, J . Am. Chsm. SOC.,78, 2906 (1956).
INTERCRYSTALLINE ENERGIES I N THE ALKALI HALIDES’ BYDONALD P. SPITZER~ Institute for the Study of Metals, The University of Chicago, Chicago SY, Illinois Received June 6,1961
Because of the basic importance of intercrystalline energies in determining properties of polycrystalline bodies, relative intercrystalline interfacial energies were determined in many alkali halide systems by measurement of the angles a t which intercrystalline b’oundaries meet one another a t equilibrium. The energies of boundaries between two crystals of the same phase were found to vary from 0.2 to 1.3, relative to the KaF intercrystalline energy. The relative energies of boundaries between two dissimilar crystals varied only from 0.7 to 1.0. From the equilibrium angles and the liquid-li uid interfacial energy in the system NaF(solid)-Te( liquid)-CsCl(liquid), the absolute energy of the average NaF grain bounjary was determined to be 306 5 15 ergs/cm.2. Generally, no correlation was found between intercrystalline energies and other basic parameters.
Introduction The recognition t,hat solid interfaces are not always im.mobile is fairly recent. Several surface tensions of solid metals have been obtained3 from (1) Based on a dissertation submitted to the faculty of the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy, December, 1960. (2) American Cyanamid Company, Stamford, Connecticut. (3) See. e Q., H. Udin, A. J. Shaler and J. Wulff, Trans. Am. Inst. Mining Metal. EnQrs.,186,185 (1949):E.R.. Hayward and A. P. Greenough, J. Inst. Metals, 88, 217 (1960): A. J. Shaler, “The Mechanical Properties of Crystalline Metal Surfaces,” in “Structure and Properties of Solid Surfaces.” edited by R. Gomer and C. S. Smith, Unirersity of Chicago Press, 1953.
the weight on a vertical wire which is necessary to just keep the wire from contracting under the influence of surface tension. It also has been shown4 that solid-solid and solid-liquid interfaces may have sufficient mobility to establish equilibrium among their free energies and that the energies are of basic importance in determining the actual distribution of phases and the properties of polycrystalline bodies. In a polycrystalline solid, the crystals (or grains) (4) C. S. Smith, Trans. Am. Inst. Minina Metal Enprs., 115, 15 (1948).
