FERENC E. ROSZTOCZY AND DANIELCUBICCIOTTI
124
The Bismuth Iodide-Iodine Phase Diagram'
by Ferenc E. Rosztoczy and Daniel Cubicciotti Stanford Research Institute, Menlo Park, California 94026
(Received June 20, 1964)
The Bi18-12 phase diagram was determined. It is of the simple eutectic type, the eutectic composition being very close to pure Iz. The shape of the liquidus is very close to that calculated on the basis that BiII and 1 2 are the species in the melt.
Introduction There are two ways in which bismuth(II1) iodide and iodine might react to form a compound: (a) if iodine is a sufficiently strong oxidizing agent, a higher oxidation state of bismuth might be formed; (b) iodine might combine with the iodide ions to form polyiodide ions ( i e . , Is-). Oxidation states for bismuth greater than three have been obtained through oxidation by fluorine to BiF6 and by chlorine to BiCL The formation of polyiodides is well known for the heavier alkali iodides, but no polyiodides seem to have been reported for the less ionic iodides. As may be seen below, the present thermal analysis of the Bi13-12 system indicated no compound formation in this system; hence, the problem concerning the type of compound does not arise. Experimental Thermal Analysis. Two types of sealed capsules were used for thermal analysis. One consisted of a Pyrex test tube (25-mm. o.d., 75 mm. long) with a reentrant thermocouple well in the bottom. Such cells were often found to be cracked after a cooling curve. The other type consisted of a bundle of three separate heavy-wall Pyrex tubes (each 16-mm o.d., 10 cm. long). The measuring thermocouple was placed in the channel created in the center of the bundle of three tubes. This latter type of cell was much more resistant to the stresses in the system. The capsules were filled with weighed samples of Bi and Iz and sealed under vacuum. The sealed capsules were then heated to above 410'. The heating was gradual enough so that the reaction was not vigorous. (Capsules heated too rapidly exploded.) After several cooling curves had been obtained on a sample of a given composition, the capsule was opened, Bi or IZ added to change the composition, and the capsule reThe Journal of Physical Chemistry
sealed under vacuum. When the capsule was full it was discarded and a new one filled. The amount of sample ranged from 40 to 60 g. (for one large or three small tubes). The sample capsule or bundle was placed in the center of a heavy nickel tube (72-mm. o.d., 61-mm. i.d., 22 cm. long) and insulated from it with Fiberfrax wool. The entire assembly was placed in a tube furnace and a constant temperature difference was maintained between the sample capsule and the nickel tube during a heating or cooling operation. The constant temperature difference was obtained by controlling the temperature of the furnace with a differential thermocouple between the capsule and the nickel tube which actuated a controller. The heating or cooling rate was approximately 0.5'/min. The temperature was measured with a Pt-Pt-lO% Rh thermocouple checked against a similar thermocouple calibrated by the National Bureau of Standards. The output of the thermocouple was recorded on a strip-chart potentiometer recorder with a span of 1 mv./25 cm. Conductivity Measurements. These measurements were made by a four-probe method. A fused-quartz cell was made having two bulbs connected by a tube (3-mm. i.d., 9 cm. long) which was the resistance path. Each bulb had two graphite-rod electrodes emerging from the top through closely fitting quartz tubes sealed outside the heated zone with epoxy cement. The cell was fitted with an auxiliary bulb containing iodine. The temperature of the auxiliary bulb could be varied independently of that of the main cell so that the amount of iodine in the cell (hence, the composition of the Bi-Iz sample) could be varied. The (1) This work was made possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract No. AT(04-3)-106.
THE BISMUTHIODIDE-IODIKE PHASE DIAGRAM
main cell was initially loaded with Bi13 made from the elements in another container. The resistance of the sample was measured by determining the potential drop across one pair of graphite electrodes while a measured direct current of 2 to 5 ma. was being passed through the other pair of electrodes. The conductivity of the melt was determined a t 430' with the iodine bulb a t 130, 210, 265, and 310' (measurements a t lower iodine temperatures were erratic and were discarded). The cell constant of the conductance cell was measured with mercury a t 25'. Materials. Bismuth triiodide was synthesized by direct combination of the elements. Special highpurity (99.999+0/,) bismuth (Asarco, South Plainfield, S . J.) was melted and heated under Hz in a fusedquartz tube to remove oxide. Iodine U.S.P. resublimed crystals (IZIallinckrodt) were used. The residue on resublimation was less than 0.01%.
Results and Discussion The cooling curves had the following general characteristics. For pure Bi13 the curves showed an arrest of about 70-min. duration with an initial supercool region of a few minutes followed by a flat region (taken as the freezing point) that dropped only 0.1' for the first 10-min. duration. There were no heat effects below the freezing point down to 70'. For pure IZ the curve was similar except the over-all duration was about 40 min. For conipositions from 75 atom yo I ( i e . , Bi13) to 85 atom % the curves showed, a t the upper thermal effect, an initial supercool followed by a curve that was initially almost horizontal and then curved downward. This almost horizontal portion became less and less horizontal with increasing iodine content. The supercool made it difficult to determine the temperature of the beginning of the thermal effect more precisely than 1O . From 90 to 98 atom % I the curves showed no supercool ; however, the change in slope, due to the heat effect, was small, and the beginning of the heat effect could be established with the same accuracy. A eutectic halt was observed at about 113' for all compositions other than 1 2 and Bi13. The length of the halt increased with iodine content indicating the eutectic composition was close to pure 1 2 The heating curves were in general agreement with the cooling curves but were not used to determine the temperatures of the t hernial effects. The temperatures of the heat effects observed are given in Table I. Our interpretation of the phase diagram based on these observations is the Bi13-12 portion of the entire Bi-Iz phase diagram in Figure 1. (The Bi-BiI3 portion is from the work of Yosini, et a1.2) The Bi13--12system is a simple eutectic one with the eutectic very close to
*
125
Table I : Thermal Effects Observed --
Composition-Atom % I Mole % 1%
75 75 75 76 77 77 79
0
0 00 3 50
44 87 31 05 97 06
6 9 15 21 27 38 50 61 68 75 80 86 91 93 95 100
81 00 83 50 86 00
88 00 90 92 94 96 96 98 100
Temp., OC.---First break Eutectic h a l t
7 -
00 00 00 00 78 00 00
408 405 402 399 394 388 381 366 349 332 319 304 291 275 251 239 208 113
70 94 2
3 9 7 7 1 4 0 9 4
3 1 8 00
I
5 3 4
112 112 112 112 113 113 112 113 113 113 112 113 113 113 112 113
4
0 4 0 4 0 7 4 8 9 4 5 1 6 3
1 4 7 9 0 1 9 1 1 1 8 1 2 0 8 0
m Bi I , + I2
I
Bi
20
40 B i I
I 1
60 BiI3f 80
11212
ATOM PERCENT Figure 1. The Bi-12 phase diagram. Data to left of Birr from Yosim, e l al. Present data = points to right of Bi18.
pure 1 2 . There is no evidence for any compound formation (congruently or incongruently melting) since the eutectic halt occurs a t the same temperature over the whole composition range. van Klooster3 reported some nieasureiiients on the Bi13-12 system. His results are much below those we reported above. His nieasuremexits were made in open tubes, and he reported heavy loss of Iz; therefore, we did not consider those data in constructing the phase diagram. (2) S. J. Tosim, L. D. Ransom, It. A. Sallach. and L. E. Topol, J . P h y s . Chem., 66, 30 (1962). (3) H. S. van Klooster, 2. anorg. Chem., 80, 104 (1913.)
Volume 69, Sumber I
January 1965
FERENC E. ROSZTOCZY AND DANIELCUBICCIOTTI
126
To substantiate this interpretation of the thermal data, three samples in sealed tubes were equilibrated for about 70 hr., quenched, and X-rayed. The compositions and temperatures of these equilibrations are marked by circled crosses in Figure 1. The patterns observed for all three samples were those of BiI3 and I2with no unidentified lines. Comparison of the experimental freezing point lowering of BiIs by Iz with that expected for various solution models is shown in Figure 2. The freezing points expected for the several solution models were calculated from
The heat of fusion of %I3 (AHf = 9.41 kcal./mole) was taken from Yosim, et d 2 The effective mole fraction of BiI3 (ie., X(Bi13)) was calculated for the models assumed as in Table 11. Table I1
The phase diagrams of three similar systems have been reported: AsI3-1~ and Sb13-Iz by Jaeger and Doornbo~ch,~ and A113-12 by X i ~ h n i k . ~These systems are also of the simple eutectic type. In Figure 3 the experimental freezing points for the liquidus in equilibrium with hII3 are compared with curves calculated for the same types of solution models. For this purpose the heats of fusion of the iodides were determined by drop calorimetry. The values obtained were 5.21, 5.44, and 3.86 kcal./mole of Ad3, Sb13, and AII3, respectively.6 It was also possible in these three systems to calculate the liquidus in equilibrium with solid 1 2 since that branch of the curve was long enough to be measured. The heat of fusion’ of 1 2 was taken as 3.74 kcal./mole. The mole fractions of I2 substituted in eq. 1 were calculated for the various solution models as in Table 111. Table I11 Species assumed
Species assumed
X(Bi1a)‘
X(Id
MIs, Iz
Bi18, IZ
M+*, I-, 11
Bi+a,I-, 11MzIs, BizIs, IZ
L
MIz+, I-, Ia
‘ [BiIa] and [Iz] represent moles of BiIo and 1 2 originally introduced into sample. The comparisons of Figure 3 indicate that the only solution model that fits both liquidus branches reasonably well for all three systems is the one in which the species are assumed to be RI13 and Iz. This was also the case with Bi13. (It may well be that a t compositions near the pure components some other solution model would give a better representation of the properties. For instance, with small concentrations of Iz in SbI3 the latter may act as a dimer. However, the liquidus for the whole phase diagram is in each case in I50 Bi13
I
I
I
1
1
20
40
60
80
‘2
MOLE PERCENT
Figure 2. Comparison of experimental freezing points with various models for solution: , BiIa, 12; ’ . ., Bi3+, I-, IS-; - - - -, Bi& 12.
The curve calculated on the basis of Bi13 and IZas the species in the melt agrees quite well with the experimental data. The Journal of Physical Chemiatry
(4) F. M. Jaeger and H. J. Doornbosch. 2. anorg. Chem., 7 5 , 261 (1912). Phase diagrams for these systems have also been reported by E. Quercigh, Atti accad. Lincei, (5) 21, 786 (1912), whose data are in general agreement with those of Jaeger and Doornbosch and also by E. Montignie, Bull. soc. chim. France, 8, 542 (1941), who did not present his data points. (5) A. T. Niahnik, Zh. Obshch. Khim., 7 , 1935 (1937). (6) H. Eding and D. Cubicciotti, “Heat Contents for ASIS,SbIs, and AlIa above Room Temperature,” to be published. (7) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I . Jaffe, National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952.
THEBISMUTHIODIDE-IODINE PHASE DIAGRAM
200
127
1 .o
[
0.0
z 0
3 2 0.6 n
z
s Y 0.4
LL w 0
a
v)
0.2 12 TEMP
0
60
70
-
1 1
'1
t
130' 21P 205'
72
74 BiI3 76
310' 70
00
ATOM PERCENT I IN MELT
Figure 4. Electrical conductance of melts near BiIs composition a t 430". Dashed curve from data of Grantham and Yosim.
-1 I60
-
140
-
120 100
-
Sb
RiT,
13
80
60 40 60 80 MOLE PERCENT Figure 3. Comparison of freezing points of Jaeger and Doornbosch and Nizhnik with various , MI3, 12; - - - -, M+3, I-, 12; solution models: ---, M A , 12; - . -, MIZ+, I-, 1 2 . MI3
20
law for the melt. The results, which are to be considered as qualitative, are shown in Figure 4. The data show that the conductance of the melt decreases with increasing 1%content; that is, the solution of Iz in BiIs does not involve the production of appreciable concentrations of conducting species. Also, the data join reasonably well with those of Grantham and Yosims and indicate that no abrupt change in conduction occurs at the composition of the compound
'2
best accord with the assumption that MIS and IZ are the species in solution.) A brief investigation of the effect of added Iz on the electrical conductivity of molten Bi13 was made. The sample of BiI3 was maintained at 430' and the Iz bulb temperature increased from about 130 to 310'. The approximate composition of the melt was calculated from the Iz pressure in the bulb assuming Raoult's
Tn f n d tho urarliinl CIoprpscw in m-"dptanro with
increase in Iz content is very similar to that observed in the melts of the Sb-Sg and Bi-S'O systems near the M2S3 composition with increasing S content. As seen above, the species which can be considered to determine the thermodynamic properties of these solutions are BiI3 and 1 2 . Since these are uncharged, there must also be some minor species in the melt to carry the current. The exact nature of these species is as yet unspecified ; their concentration apparently decreased as 11was added to the melt.
Acknowledgment. The authors are grateful to Dr.
J. W. Johnson for his contributions, especially in the conductance work. (8) L. F. Grantham and 5. J. Yosim, J . Chem. Phys., 38, 1671 (1963). (9) F. S. Pettit, J . Phys. Chem., 6 8 , 9 (1964). (10) J. W. Johnson, "Electrical Conductance of Bi-S Melts," to be
published.
Volume 69, Number 1 January 1966
