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The thallium-iodine phase diagram
201
JOURNAL OF THE LESS-COMMON METALS Elsevier Sequoia %A., Lausanne - Printed in The Netherlands
THE THALLIUM-IODINE
PHASE DIAGRAM
D. CUBICCIOTTI Stanford
Research
(Received
January
Institute,
Menlo Park, Calif: (U.S.A.)
11 th, 197 I )
SUMMARY
The melting pointcomposition diagram for the thallium-iodine system has been determined. There are three compounds : TlI, Tl,I,, and Tl13. From Tl to TlI the solids and liquids are immiscible. From TlI to I, there is one liquid phase. Solid TlJ, melts incongruently at 260°C to a liquid (atom fraction iodine = 0.77) and solid TlI. Solid TlI, melts incongruently at 129°C to a liquid (atom fraction iodine = 0.84) and solid Tl,I,. A eufectic occurs with liquid (atom fraction iodine=0.88), solid TlI,, and solid I, at 90°C. The X-ray powder patterns of Tl,I, and TlI, are reported.
INTRODUCTION
The chemistry of thallium is similar, in many respects, to that of the heavier alkali metals and because of the interesting interactions represented in the alkali metaliodine phase diagrams’, a comparable study with thallium was undertaken. The aqueous chemistry of thallium iodide-iodine solutions has been studied by several authors2,3,4. Their results show that there are two compounds formed between TlI and 12, namely, T1314 and Tl13. In aqueous solution a complex ion, TlI, , forms in with excess I-. There is some question about the valence states of the Tl in those situations. In general, they can be formulated in each of two ways : with trivalent Tl and monovalent I or with monovalent Tl and zero-valent I. The structure of TlI, indicates that it consists of Tl+ (ref. 1) and I;. There is no information on Tl,I,. The phase diagram described below shows that Tl,I, and TlI, are also formed in the nonaqueous system. They are both incongruently melting. The freezing point depressions of the congruently melting TlI and I, are used to evaluate the nature of the species dissolved in the melt. EXPERIMENTAL
Cooling curves were obtained on samples in Pyrex glass containers sealed under vacuum. The glass cells were tubes (20 mm O.D., 60 mm long) with re-entrant thermoJ. Less-Common
Metals, 24 (1971) 201-209
D. CUBICCIOTTI
202
couple wells and 8 mm tubulations for evacuation. The tubes were filled by weighing in the desired amount of iodine and thallous iodide ; they were then evacuated and sealed off. Heating and cooling curves were obtained with the furnace and method described in ref. 5. The X-ray powder patterns were made on a Norelco diffractometer with CuKa (1.5418 A) radiation and a nickel filter. The samples were all at room temperature. The iodine used was “Baker” analyzed reagent grade. The thallous iodide was made by dissolving thallium (99.95%, from American Smelting and Relining Co.) in nitric acid and precipitating with potassium iodide. The melting points of the pure components were 113.3”C for iodine and 4416°C for TlI compared to literature values of 113.6”C6 and 441.6’C7, respectively. RESULTS AND DISCUSSION
Phase diagram
The data obtained from the thermal analysis are shown in Fig. 1. Points determined from cooling curves are shown as crosses, while those obtained from heating curves are given as circled crosses if different from the cooling data. Pure TlI melted at 441.6’C in agreement with the value we found earlier’. The thermal effect due to the transition occurred at 178°C on heating. Pronounced supercooling occurred in the cooling curves. The freezing point of TlI was lowered by the addition of IZ but the transition temperature (from heating curves) was not indicating that there was no solubility of IZ in solid TlI. The intensi~ of the thermal effect observed for the liquidus diminished as the concentration of iodine in the melt was increased so that the thermal effect at 0.68 iodine atom fraction was barely discernible and the temperature of that effect was not well delined. For compositions from 0.68 to 0.85 in iodine atom fraction, no thermal effect corresponding to the liquidus was observed on heating or cooling; therefore, the liquidus is drawn as a broken line to indicate that its exact position is not well established from 0.65 to 0.88 in iodine atom fraction. In addition to the thermal effects attributed to the liquidus and the transition in TII, thermal effects were observed at three invariant temperatures as shown in Fig. 1, namely, 260“,129”, and 90°C. From the literature one finds two compounds between TlI and IZ reported, and our X-ray results, given below, show that only those two compounds were formed. Therefore, the reactions occurring at the three invariant temperatures were taken to be: (1) At 260°C, the peritectic reaction : Tl~I~(s)=TlI(s) + liquid (atom fraction I E 0.77) (2) At 129*C, the peritectic reaction : TlI,(s) =Tl,I,(s)+ liquid (atom fraction I z 0.83) (3)At 9O”C,the eutectic reaction : TlI,(s) +12(s) =liquid (atom fraction Ir 0.88). The thermal effect for the eutectic should not have appeared in the thermal analysis of compositions poorer in I, than TlI, ; however, as seen in Fig. 1, it \?rasobserved for three compositions in that region. We presume that the thermal effect was observed there because the system was not truly in equilibrium. In particular, we presume that the peritectic reaction corresponding to the reverse of reaction (2) did not go to completion so that some liquid was still available to undergo the eutectic transformation even for compositions to the left of TII,. The reaction (reverse of eqn. (2)) was one J. Less-Common Metals,24
(1971) 201-209
THALLIUM-IODINE
203
PHASE DIAGRAM
of a liquid with a solid to form a new solid during cooling. It is reasonable to suppose that as the reaction proceeded, some of the Tl,I, became coated with the new solid (TII,) and, therefore, some of the liquid was not able to be consumed in the reaction and thus was able to undergo the eutectic reaction. The iodine was observed to melt at 113.3”C as compared with the literature value of 113.6’C6. Its freezing point was depressed by TlI down to the eutectic as indicated by the liquidus curve drawn.
420
0 TP
0.5 TPI ATOM
0.6 TV,I, FRACTION
0.7
0.6
0.9
TP13
1.0 %I2
IODINE
Fig. 1. The phase diagram of the thallium-iodine
system.
-The T&-T11part of the phase diagram is drawn to represent two essentially immiscible materials based on the following information. The solubility of thallium in molten thallous iodide was found by Fiorani and Bombi’ to be well under OS”/, at temperatures somewhat above the melting point, from which we estimate that the freezing point of Tlf saturated with TI would be less than a degree lower than that of pure TlI. We measured the melting point of a sample of pure Tl and found that it was unchanged by the addition of 0.5 mole y0 TlI. Therefore, we are led to the conclusion that the mutual solubilities of Tl and TlI are quite small, at least near the melting points, and do not depress the melting point of one another. The TlI-I, part of the diagram is closely related to the comparable heavier alkali metal systems. In each system the incongruently melting and isostructural compound MI, is formed. The incongruent melting points of those compounds fall in the order:Cs13(2150C),Rb13(1890C),T113(1290C)and KI,(doesnotform).Thedeviation of the composition of the liquid formed on melting from stoichiometric (atom fraction iodine = 0.75) is a measure of the instability of the compound. Those values are : Cs (0.76), Rb (0.78), and Tl(O.83). Sharpe2 compared the partial pressures of iodine over J. Less-Common Meials, 24 (1971) 201L209
D. CUBICCIO~I
204
the triiodides at room temperature and found the relative order (KI,, unstable) >TII, >RbI, > CsI,. All three of these measures of stability are in good agreement. The Tl-TlI part of the phase diagram shows complete immiscibility in contrast to the complete miscibility of the heavier alkali metals with their iodides just above the melting points of the iodides. That difference in behavior can be related in part to the larger heat of vaporization of thallium compard to the alkali metals and, possibly, in part to the greater energy required to ionize Tl compared to the alkali metals. The dissolution of the metal in the salt involves overcoming the binding in the metal for which its heat of vaporization is a measure, and if the dissolved metal is ionized, the ionization energy is also important. Species diss~lued in belts
Information about the molecular nature of the dissolved species can be obtained by comparison of the freezing point depressions and values calculated for various possible solution models. For solutions in which the major constituent was TII, the solution models considered are given in Table I. The freezing points (7’) of solutions were calculated from the usual equation: 1
-=--T
1
R In X
Tf
m-
in which X is the mole fraction of the solvent; T and A& are the freezing points and heats of fusion of the pure solvents, The values used for those constants were 714.8 K and 3.52 kcal/mole for TlI (ref. 7) and 386.5 K and 3.74 kcal/mole for Iz (ref. 6). Figure 2 presents a comparison of theexper~entally determined freezing points with the calculated values. For the TII rich region the results are shown in the upper set of curves. In the region of greatest validity of the calculation, namely, the dilute solution range, the calculated curves all merge and all agree with the experimental points. At larger concentrations, model 1 results in a somewhat better lit to the data. Since molten TlI is a good electrical conductorg, one would presume that an ionic model would apply and of the ones considered, that of model 1 seems to fit the data best. In the I,-rich region the slopes of the various curves are sufficiently different that a more definite choice among the models can be made. Models 2,3, and 4 clearly do not represent the results. Models 1,5, and 6 all tit the data equally well, especially in the dilute range. Some distinction among these sets of species can be made on the basis of electrical conductance of the solutions since one of the sets of species is ionic and the others are nonionic. Therefore, a few exploratory measurements were made of the conductance of solutions of I, containing a few percent TlI compared to the conductance of solutions containing a few percent KI. In the range of 0 to about 3 mole ‘? salt in I,, the conductances of the TlI solutions were about one-tenth those of the KI solutions. Audrieth and Kleinberg” state that KI in such solutions is completely ionized. Thus, these rough conductance measurements indicate that in the I,-rich region, the TlI was only partly ionized, therefore, the predominant species must have been molecular (unionized), i.e., either TlI or Tl,I, with a small percentage of ionic species. Although these results do not allow an exact definition of the species that constitute these melts, they are in agreement with the follow~g general picture. In the iodine-rich solutions the TII is predominantly in the form of nonionic species. As the J. Less-Common Metals, 24 (1971) 201-209
THALLIUM-IODINE TABLE
PHASE DIAGRAM
205
I
SOLUTION MODELS
A. TII-RICH REGION
Model
Type of curve in Fig. 2
1
full line
Species in melt TII. I, or Tl+, I-, I,
Mole fraction of TlI* n(Tl1) i(Tll)
+n(I,)
2
dot-dashed
Tl’.
3
dashed
Tl+. I-, I;
n(TIl) -n(l,)
dashed
Tl+,Tl+“.I-
n(TI1) -“(I,)
I-, I,
n(Tl1) n(Tl1) +fn(lz)
’ I
n(TI1) 4
n(Tl1) B. 12-~1c~ REGION
Model
1
Type of curve in Fig. 2 full line
Species in melt
Mole fraction of I,*
I,, TII or
42)
I,, Tl+, I-
n(1,) +n(TlI)
2
dashed
- one dot
I,, TI+. I-
n(IJ n(I,)+2n(TlI)
3
dashed
- two dots
I,. TI+, Ij
n(I,)-n(Tl1) n(I,)+n(TlI)
4
dashed - three dots
I,, Tl+3, I-
n(IJ -rt(TlI) n(1,) + 3n(TII)
5
dotted
I,. TI,I,
n(IJ n(1,) +fn(TlI)
6
dashed
I,, TlI,
n(1,) -n(TlI) n(IJ
* In this mole fraction expression the quantities n(Tl1) and n(12) are the numbers of moles of TII and I, in the mixture. For model 1, two different sets ofspecies give the same expression for the mole fractions, namely. either molecular TII and molecular I, or ionic TIC and I- in which only the I, and I- can interchange positions and the Tl+ forms a sublattice that does not mix with I,, Model 2 assumes molecular I, and ions of Tl+ and I-. Model 3 assumes that all of the minor species (either I- or I,) are complexed as I;. Model 4 assumes that all of the I, on the TlI-rich side or Tl+ on the I,-rich side has been used to form Tl+ ‘. The mole fraction of TII for models involving ions (2,3 and 4) was assumed to equal the product (ion fraction Tl+) x (ion fraction I-).
fraction of thallous iodide is increased, and at the higher temperatures maintain the liquid, the degree of ionization of the TlI increases.
required to
X-ray investigation To establish the nature of the compounds formed, the powder patterns of several mixtures of iodine and thallous iodide were determined. The mixtures were sealed J. Less-Common Metals, 24 (1971) 201-209
206
D. CUBICCIOTTI
into evacuated tubes and heated to 280% for one hour, then cooled, ground, and Xrayed. The compositions studied are shown in the first column ofTable II. The patterns observed and the relative changes of intensities were interpreted as due to the combinations of substances given in the second column. The patterns obtained showed that two structures were formed in addition to TlI and Iz, and their maximum intensities were consistent with the formulas TI,I, and TlI,.
MOLE loo 460 f
Too
PERCENT
TPI
90
80
70
60
I
I
I
I
70
60
90
80 MOLE
PERCENT
I2
Fig. 2. Freezing point depressions for various models. Upper curves, TII region; lower curves, I, region. See Table I for significance of various types of curxs.
Professor Sharpe has studied the compounds Ti,I, and TlI, formed by precipitation from aqueous solutions. We compared the photographic Elms he obtained with the powder patterns from the present work and found good agreement, so that the materials prepared by our methods are structurally similar to those prepared by Sharpe from aqueous solutions. The powder patterns for TII and I, were in good agreement with the ASTM cards for those substances”. The powder patterns for TII, and Tl,I, do not appear in the literature. The pattern for T1314was established from those of composition 0.1 and 0.14 mole fraction I,. The former was used to determine the d values compared to TII as a standard, while the lines attributable to Tl,I, alone and their relative intensities were derived from the latter sample. The.powder pattern for TI,I, established in that manner is given in Table III. J. Less-Common
Metals, 24 (1971) 201-209
THALLIUM-IODINE
207
PHASE DIAGRAM
For Tl13, the pattern was determined in a similar fashion using the results from compositions 0.5 and 0.8 mole fraction I, with I, as a reference. That pattern is given TABLE
II BYX-RAY
PHASESFOUND Composition
Patterns observed
(mole fraction
12) Tl,I,
0.1
strong,
TII weak 0.14
Tl ,I, only
0.2
Tl,I, strong TlI, weak
0.4
Tl,I,+TlI,
0.5
TIIj
0.6
TlI,
only
0.67
TlI,
strong
0.80
TlI,+I,
only
I, weak
TABLE
III
POWDER PATTERN FOR Tl,I,
4
1.738
18
1.700
11 7
3.44
4
1.692
6
3.36 3.10
15
1.680
4
100
1.622
2.85
67
1.561
7 4
4.67 4.20
2.72
3
1.559
3
2.49
7
1.498
3
2.41
11
2.28
9
1.492 1.478
4 7
2.10
21
1.423
4
9
1.362 1.340
4 2
2.03 1.889 1.792
9 6
in Table IV. Hazell12 found TIIJ to be orthorhombic of space group P,,,,, and unit cell dimensions : 9.45,10.56 and 6.48 A. Several values of the d spacing were used in a coniputer program deviced by Dr. G. Martin to evaluate the unit cell dimensions from the powder pattern and the knowledge that the structure was orthorhombic. The dimensions derived were 9.42,10.58 and 6.40 & in reasonable agreement with the results of Hazell. The hkl values were assigned to the d values in Table IV from a calculation of all possible values for an orthorhombic structure with the unit cell dimensions given above and consistent with the P,,,,,, space group. J. Less-Common
Metals, 24 (1971) 201-209
D . CUBICCIOTTI TABLE IV POWDERPATI'ERNFOR TIE,
5.26 4.71 4.14 3.74* 3.57* 3.52* 3.3@ 3.20* 3.08 2.94 2.82 2.72 2.70 2.64 2.58 2.55 2.48 2.45 2.37
4 5 9 13 26 7 14 56 100 5 9 6 5 21 16 5 2 6 5
002 020 021 102 112 121 022 013 200 122 113 023 131 220 004 123 014 132 104 222 114
2.35 2.30 2.10 2.06 2.04
4 15 2 6 12
2.02 1.991 1.869 1.827 1.759 1.735 1.706 1.691 1.670 1.601 1.576 1.569 1.540
5 2 7 12 3 6 5 6 4 4 3 2 3
040 024 310 232 204 142 034 302 143 322 044 215 323 135 243 400 333 341 036
* Values used to determine unit cell dimensions.
ACKNOWLEDGEMENTS
The author is gratefulto Drs. T. Hopkins and G. Martin of Stanford University for interpretation of X-ray patterns, to Mr. T.Imai for obtaining the patterns, and to Prof. A. G. Sharpe of Cambridge University for the loan of X-ray photographs. This work was supported by the Research Division of the U.S. Atomic Energy Commission under Contract No. AT(O4-3)-1~.
REFERENCES 1 2 3 4 5 6
F. A. L. F. F. F.
E. ROSZTOCZY AND D. CUBICCIOTTI,J. Phys. Chem., 69 (1965) 1687. G. SHARPE,J. Chem. Sm., (1952) 2165. JOHANSSON,Acta Chem. Sand., 20 (1966) 2156. YA. KUL’BA, AND V. E. MIRONOV, Zh. Neorg. Khim., 8 (1957) 1741. E. ROSZTOCZY AND D. CUBICCIOTTI,J. Phys. Chem., 69 (1965) 124. D. ROSSINI,D. D. WAGMAN, W. H. EVANS, S. LEVXNEAND 1. JAFFB,Natl. Bur. Srds.
Govt. Printing Office, Washington, D.C., 1952. 7 D. CIJBICCZIOT~ AND H. EDING, J. Chem. Eng. Data, IO (1965) 343. 8 M. FIORAN~AND G. G. BOMBI, Ber. B~se~ge~. Phys. Chem., 69 (1965) 605. 9 L. F. GRANTHAMAND S. J. YOSIM,J. Chem. Phys., 45, (1966) 1192. J. Less-Common Metals, 24 (1971) 201-209
Circ.
500,
U.S.
~ALI~~U~-IODINE
PHASE DIAGRAM
209
10 L. F. ALJDRIETII AM) J. KLEINBERG, Non-Aqueous Sobents, Wiley, New York, 1943, p. 252. See also A. G. SHARPE, Chap. 7 in T. C. WADDINGTO&(ed.), Non-Aqueous Solvent Sy.vtems, Academic Press, London, 1965. 1I Powder Dgfraction File, ASTM, Philadelphia, Cards 6-279 (TII) and 5-559 (I, ). 12 A. C. HAZELL, Acta Cryst., I6 (1963) 71. J. Less-Common
Metals, 24 (1971) 201-209
JOURNAL OF THE LESS-COMMON METALS Elsevier Sequoia %A., Lausanne - Printed in The Netherlands
THE THALLIUM-IODINE
PHASE DIAGRAM
D. CUBICCIOTTI Stanford
Research
(Received
January
Institute,
Menlo Park, Calif: (U.S.A.)
11 th, 197 I )
SUMMARY
The melting pointcomposition diagram for the thallium-iodine system has been determined. There are three compounds : TlI, Tl,I,, and Tl13. From Tl to TlI the solids and liquids are immiscible. From TlI to I, there is one liquid phase. Solid TlJ, melts incongruently at 260°C to a liquid (atom fraction iodine = 0.77) and solid TlI. Solid TlI, melts incongruently at 129°C to a liquid (atom fraction iodine = 0.84) and solid Tl,I,. A eufectic occurs with liquid (atom fraction iodine=0.88), solid TlI,, and solid I, at 90°C. The X-ray powder patterns of Tl,I, and TlI, are reported.
INTRODUCTION
The chemistry of thallium is similar, in many respects, to that of the heavier alkali metals and because of the interesting interactions represented in the alkali metaliodine phase diagrams’, a comparable study with thallium was undertaken. The aqueous chemistry of thallium iodide-iodine solutions has been studied by several authors2,3,4. Their results show that there are two compounds formed between TlI and 12, namely, T1314 and Tl13. In aqueous solution a complex ion, TlI, , forms in with excess I-. There is some question about the valence states of the Tl in those situations. In general, they can be formulated in each of two ways : with trivalent Tl and monovalent I or with monovalent Tl and zero-valent I. The structure of TlI, indicates that it consists of Tl+ (ref. 1) and I;. There is no information on Tl,I,. The phase diagram described below shows that Tl,I, and TlI, are also formed in the nonaqueous system. They are both incongruently melting. The freezing point depressions of the congruently melting TlI and I, are used to evaluate the nature of the species dissolved in the melt. EXPERIMENTAL
Cooling curves were obtained on samples in Pyrex glass containers sealed under vacuum. The glass cells were tubes (20 mm O.D., 60 mm long) with re-entrant thermoJ. Less-Common
Metals, 24 (1971) 201-209
D. CUBICCIOTTI
202
couple wells and 8 mm tubulations for evacuation. The tubes were filled by weighing in the desired amount of iodine and thallous iodide ; they were then evacuated and sealed off. Heating and cooling curves were obtained with the furnace and method described in ref. 5. The X-ray powder patterns were made on a Norelco diffractometer with CuKa (1.5418 A) radiation and a nickel filter. The samples were all at room temperature. The iodine used was “Baker” analyzed reagent grade. The thallous iodide was made by dissolving thallium (99.95%, from American Smelting and Relining Co.) in nitric acid and precipitating with potassium iodide. The melting points of the pure components were 113.3”C for iodine and 4416°C for TlI compared to literature values of 113.6”C6 and 441.6’C7, respectively. RESULTS AND DISCUSSION
Phase diagram
The data obtained from the thermal analysis are shown in Fig. 1. Points determined from cooling curves are shown as crosses, while those obtained from heating curves are given as circled crosses if different from the cooling data. Pure TlI melted at 441.6’C in agreement with the value we found earlier’. The thermal effect due to the transition occurred at 178°C on heating. Pronounced supercooling occurred in the cooling curves. The freezing point of TlI was lowered by the addition of IZ but the transition temperature (from heating curves) was not indicating that there was no solubility of IZ in solid TlI. The intensi~ of the thermal effect observed for the liquidus diminished as the concentration of iodine in the melt was increased so that the thermal effect at 0.68 iodine atom fraction was barely discernible and the temperature of that effect was not well delined. For compositions from 0.68 to 0.85 in iodine atom fraction, no thermal effect corresponding to the liquidus was observed on heating or cooling; therefore, the liquidus is drawn as a broken line to indicate that its exact position is not well established from 0.65 to 0.88 in iodine atom fraction. In addition to the thermal effects attributed to the liquidus and the transition in TII, thermal effects were observed at three invariant temperatures as shown in Fig. 1, namely, 260“,129”, and 90°C. From the literature one finds two compounds between TlI and IZ reported, and our X-ray results, given below, show that only those two compounds were formed. Therefore, the reactions occurring at the three invariant temperatures were taken to be: (1) At 260°C, the peritectic reaction : Tl~I~(s)=TlI(s) + liquid (atom fraction I E 0.77) (2) At 129*C, the peritectic reaction : TlI,(s) =Tl,I,(s)+ liquid (atom fraction I z 0.83) (3)At 9O”C,the eutectic reaction : TlI,(s) +12(s) =liquid (atom fraction Ir 0.88). The thermal effect for the eutectic should not have appeared in the thermal analysis of compositions poorer in I, than TlI, ; however, as seen in Fig. 1, it \?rasobserved for three compositions in that region. We presume that the thermal effect was observed there because the system was not truly in equilibrium. In particular, we presume that the peritectic reaction corresponding to the reverse of reaction (2) did not go to completion so that some liquid was still available to undergo the eutectic transformation even for compositions to the left of TII,. The reaction (reverse of eqn. (2)) was one J. Less-Common Metals,24
(1971) 201-209
THALLIUM-IODINE
203
PHASE DIAGRAM
of a liquid with a solid to form a new solid during cooling. It is reasonable to suppose that as the reaction proceeded, some of the Tl,I, became coated with the new solid (TII,) and, therefore, some of the liquid was not able to be consumed in the reaction and thus was able to undergo the eutectic reaction. The iodine was observed to melt at 113.3”C as compared with the literature value of 113.6’C6. Its freezing point was depressed by TlI down to the eutectic as indicated by the liquidus curve drawn.
420
0 TP
0.5 TPI ATOM
0.6 TV,I, FRACTION
0.7
0.6
0.9
TP13
1.0 %I2
IODINE
Fig. 1. The phase diagram of the thallium-iodine
system.
-The T&-T11part of the phase diagram is drawn to represent two essentially immiscible materials based on the following information. The solubility of thallium in molten thallous iodide was found by Fiorani and Bombi’ to be well under OS”/, at temperatures somewhat above the melting point, from which we estimate that the freezing point of Tlf saturated with TI would be less than a degree lower than that of pure TlI. We measured the melting point of a sample of pure Tl and found that it was unchanged by the addition of 0.5 mole y0 TlI. Therefore, we are led to the conclusion that the mutual solubilities of Tl and TlI are quite small, at least near the melting points, and do not depress the melting point of one another. The TlI-I, part of the diagram is closely related to the comparable heavier alkali metal systems. In each system the incongruently melting and isostructural compound MI, is formed. The incongruent melting points of those compounds fall in the order:Cs13(2150C),Rb13(1890C),T113(1290C)and KI,(doesnotform).Thedeviation of the composition of the liquid formed on melting from stoichiometric (atom fraction iodine = 0.75) is a measure of the instability of the compound. Those values are : Cs (0.76), Rb (0.78), and Tl(O.83). Sharpe2 compared the partial pressures of iodine over J. Less-Common Meials, 24 (1971) 201L209
D. CUBICCIO~I
204
the triiodides at room temperature and found the relative order (KI,, unstable) >TII, >RbI, > CsI,. All three of these measures of stability are in good agreement. The Tl-TlI part of the phase diagram shows complete immiscibility in contrast to the complete miscibility of the heavier alkali metals with their iodides just above the melting points of the iodides. That difference in behavior can be related in part to the larger heat of vaporization of thallium compard to the alkali metals and, possibly, in part to the greater energy required to ionize Tl compared to the alkali metals. The dissolution of the metal in the salt involves overcoming the binding in the metal for which its heat of vaporization is a measure, and if the dissolved metal is ionized, the ionization energy is also important. Species diss~lued in belts
Information about the molecular nature of the dissolved species can be obtained by comparison of the freezing point depressions and values calculated for various possible solution models. For solutions in which the major constituent was TII, the solution models considered are given in Table I. The freezing points (7’) of solutions were calculated from the usual equation: 1
-=--T
1
R In X
Tf
m-
in which X is the mole fraction of the solvent; T and A& are the freezing points and heats of fusion of the pure solvents, The values used for those constants were 714.8 K and 3.52 kcal/mole for TlI (ref. 7) and 386.5 K and 3.74 kcal/mole for Iz (ref. 6). Figure 2 presents a comparison of theexper~entally determined freezing points with the calculated values. For the TII rich region the results are shown in the upper set of curves. In the region of greatest validity of the calculation, namely, the dilute solution range, the calculated curves all merge and all agree with the experimental points. At larger concentrations, model 1 results in a somewhat better lit to the data. Since molten TlI is a good electrical conductorg, one would presume that an ionic model would apply and of the ones considered, that of model 1 seems to fit the data best. In the I,-rich region the slopes of the various curves are sufficiently different that a more definite choice among the models can be made. Models 2,3, and 4 clearly do not represent the results. Models 1,5, and 6 all tit the data equally well, especially in the dilute range. Some distinction among these sets of species can be made on the basis of electrical conductance of the solutions since one of the sets of species is ionic and the others are nonionic. Therefore, a few exploratory measurements were made of the conductance of solutions of I, containing a few percent TlI compared to the conductance of solutions containing a few percent KI. In the range of 0 to about 3 mole ‘? salt in I,, the conductances of the TlI solutions were about one-tenth those of the KI solutions. Audrieth and Kleinberg” state that KI in such solutions is completely ionized. Thus, these rough conductance measurements indicate that in the I,-rich region, the TlI was only partly ionized, therefore, the predominant species must have been molecular (unionized), i.e., either TlI or Tl,I, with a small percentage of ionic species. Although these results do not allow an exact definition of the species that constitute these melts, they are in agreement with the follow~g general picture. In the iodine-rich solutions the TII is predominantly in the form of nonionic species. As the J. Less-Common Metals, 24 (1971) 201-209
THALLIUM-IODINE TABLE
PHASE DIAGRAM
205
I
SOLUTION MODELS
A. TII-RICH REGION
Model
Type of curve in Fig. 2
1
full line
Species in melt TII. I, or Tl+, I-, I,
Mole fraction of TlI* n(Tl1) i(Tll)
+n(I,)
2
dot-dashed
Tl’.
3
dashed
Tl+. I-, I;
n(TIl) -n(l,)
dashed
Tl+,Tl+“.I-
n(TI1) -“(I,)
I-, I,
n(Tl1) n(Tl1) +fn(lz)
’ I
n(TI1) 4
n(Tl1) B. 12-~1c~ REGION
Model
1
Type of curve in Fig. 2 full line
Species in melt
Mole fraction of I,*
I,, TII or
42)
I,, Tl+, I-
n(1,) +n(TlI)
2
dashed
- one dot
I,, TI+. I-
n(IJ n(I,)+2n(TlI)
3
dashed
- two dots
I,. TI+, Ij
n(I,)-n(Tl1) n(I,)+n(TlI)
4
dashed - three dots
I,, Tl+3, I-
n(IJ -rt(TlI) n(1,) + 3n(TII)
5
dotted
I,. TI,I,
n(IJ n(1,) +fn(TlI)
6
dashed
I,, TlI,
n(1,) -n(TlI) n(IJ
* In this mole fraction expression the quantities n(Tl1) and n(12) are the numbers of moles of TII and I, in the mixture. For model 1, two different sets ofspecies give the same expression for the mole fractions, namely. either molecular TII and molecular I, or ionic TIC and I- in which only the I, and I- can interchange positions and the Tl+ forms a sublattice that does not mix with I,, Model 2 assumes molecular I, and ions of Tl+ and I-. Model 3 assumes that all of the minor species (either I- or I,) are complexed as I;. Model 4 assumes that all of the I, on the TlI-rich side or Tl+ on the I,-rich side has been used to form Tl+ ‘. The mole fraction of TII for models involving ions (2,3 and 4) was assumed to equal the product (ion fraction Tl+) x (ion fraction I-).
fraction of thallous iodide is increased, and at the higher temperatures maintain the liquid, the degree of ionization of the TlI increases.
required to
X-ray investigation To establish the nature of the compounds formed, the powder patterns of several mixtures of iodine and thallous iodide were determined. The mixtures were sealed J. Less-Common Metals, 24 (1971) 201-209
206
D. CUBICCIOTTI
into evacuated tubes and heated to 280% for one hour, then cooled, ground, and Xrayed. The compositions studied are shown in the first column ofTable II. The patterns observed and the relative changes of intensities were interpreted as due to the combinations of substances given in the second column. The patterns obtained showed that two structures were formed in addition to TlI and Iz, and their maximum intensities were consistent with the formulas TI,I, and TlI,.
MOLE loo 460 f
Too
PERCENT
TPI
90
80
70
60
I
I
I
I
70
60
90
80 MOLE
PERCENT
I2
Fig. 2. Freezing point depressions for various models. Upper curves, TII region; lower curves, I, region. See Table I for significance of various types of curxs.
Professor Sharpe has studied the compounds Ti,I, and TlI, formed by precipitation from aqueous solutions. We compared the photographic Elms he obtained with the powder patterns from the present work and found good agreement, so that the materials prepared by our methods are structurally similar to those prepared by Sharpe from aqueous solutions. The powder patterns for TII and I, were in good agreement with the ASTM cards for those substances”. The powder patterns for TII, and Tl,I, do not appear in the literature. The pattern for T1314was established from those of composition 0.1 and 0.14 mole fraction I,. The former was used to determine the d values compared to TII as a standard, while the lines attributable to Tl,I, alone and their relative intensities were derived from the latter sample. The.powder pattern for TI,I, established in that manner is given in Table III. J. Less-Common
Metals, 24 (1971) 201-209
THALLIUM-IODINE
207
PHASE DIAGRAM
For Tl13, the pattern was determined in a similar fashion using the results from compositions 0.5 and 0.8 mole fraction I, with I, as a reference. That pattern is given TABLE
II BYX-RAY
PHASESFOUND Composition
Patterns observed
(mole fraction
12) Tl,I,
0.1
strong,
TII weak 0.14
Tl ,I, only
0.2
Tl,I, strong TlI, weak
0.4
Tl,I,+TlI,
0.5
TIIj
0.6
TlI,
only
0.67
TlI,
strong
0.80
TlI,+I,
only
I, weak
TABLE
III
POWDER PATTERN FOR Tl,I,
4
1.738
18
1.700
11 7
3.44
4
1.692
6
3.36 3.10
15
1.680
4
100
1.622
2.85
67
1.561
7 4
4.67 4.20
2.72
3
1.559
3
2.49
7
1.498
3
2.41
11
2.28
9
1.492 1.478
4 7
2.10
21
1.423
4
9
1.362 1.340
4 2
2.03 1.889 1.792
9 6
in Table IV. Hazell12 found TIIJ to be orthorhombic of space group P,,,,, and unit cell dimensions : 9.45,10.56 and 6.48 A. Several values of the d spacing were used in a coniputer program deviced by Dr. G. Martin to evaluate the unit cell dimensions from the powder pattern and the knowledge that the structure was orthorhombic. The dimensions derived were 9.42,10.58 and 6.40 & in reasonable agreement with the results of Hazell. The hkl values were assigned to the d values in Table IV from a calculation of all possible values for an orthorhombic structure with the unit cell dimensions given above and consistent with the P,,,,,, space group. J. Less-Common
Metals, 24 (1971) 201-209
D . CUBICCIOTTI TABLE IV POWDERPATI'ERNFOR TIE,
5.26 4.71 4.14 3.74* 3.57* 3.52* 3.3@ 3.20* 3.08 2.94 2.82 2.72 2.70 2.64 2.58 2.55 2.48 2.45 2.37
4 5 9 13 26 7 14 56 100 5 9 6 5 21 16 5 2 6 5
002 020 021 102 112 121 022 013 200 122 113 023 131 220 004 123 014 132 104 222 114
2.35 2.30 2.10 2.06 2.04
4 15 2 6 12
2.02 1.991 1.869 1.827 1.759 1.735 1.706 1.691 1.670 1.601 1.576 1.569 1.540
5 2 7 12 3 6 5 6 4 4 3 2 3
040 024 310 232 204 142 034 302 143 322 044 215 323 135 243 400 333 341 036
* Values used to determine unit cell dimensions.
ACKNOWLEDGEMENTS
The author is gratefulto Drs. T. Hopkins and G. Martin of Stanford University for interpretation of X-ray patterns, to Mr. T.Imai for obtaining the patterns, and to Prof. A. G. Sharpe of Cambridge University for the loan of X-ray photographs. This work was supported by the Research Division of the U.S. Atomic Energy Commission under Contract No. AT(O4-3)-1~.
REFERENCES 1 2 3 4 5 6
F. A. L. F. F. F.
E. ROSZTOCZY AND D. CUBICCIOTTI,J. Phys. Chem., 69 (1965) 1687. G. SHARPE,J. Chem. Sm., (1952) 2165. JOHANSSON,Acta Chem. Sand., 20 (1966) 2156. YA. KUL’BA, AND V. E. MIRONOV, Zh. Neorg. Khim., 8 (1957) 1741. E. ROSZTOCZY AND D. CUBICCIOTTI,J. Phys. Chem., 69 (1965) 124. D. ROSSINI,D. D. WAGMAN, W. H. EVANS, S. LEVXNEAND 1. JAFFB,Natl. Bur. Srds.
Govt. Printing Office, Washington, D.C., 1952. 7 D. CIJBICCZIOT~ AND H. EDING, J. Chem. Eng. Data, IO (1965) 343. 8 M. FIORAN~AND G. G. BOMBI, Ber. B~se~ge~. Phys. Chem., 69 (1965) 605. 9 L. F. GRANTHAMAND S. J. YOSIM,J. Chem. Phys., 45, (1966) 1192. J. Less-Common Metals, 24 (1971) 201-209
Circ.
500,
U.S.
~ALI~~U~-IODINE
PHASE DIAGRAM
209
10 L. F. ALJDRIETII AM) J. KLEINBERG, Non-Aqueous Sobents, Wiley, New York, 1943, p. 252. See also A. G. SHARPE, Chap. 7 in T. C. WADDINGTO&(ed.), Non-Aqueous Solvent Sy.vtems, Academic Press, London, 1965. 1I Powder Dgfraction File, ASTM, Philadelphia, Cards 6-279 (TII) and 5-559 (I, ). 12 A. C. HAZELL, Acta Cryst., I6 (1963) 71. J. Less-Common
Metals, 24 (1971) 201-209