THE ALUMINIUM - ALUMINIUM BROMIDE AND ALUMINIUM - ALUMINIUM IODIDE SYSTEMS

JOMAR THONSTAD Department of Melallurgical Engineering, Universily of Toronto, Toronlo, Ontario

Received July 20, 1964

ABSTRACT

The A1-A12Br-6 and Al-Al21~ binary systems have been investigated by cryoscopy and solubility measurements. Pure AI,Br6 melts a t 98.01 f 0.02 "C; the eutectic is found a t 97.91 f 0.02 "C and 0.017 wt. % Al; a t 254 "C the solubility of aluminium increases to 0.0195 wt. %. Pure A1216 melts a t 188.32 f 0.02 OC; the eutectic is found a t 188.17 f 0.02 OC and 0.018 wt. % Al; a t 320 OC the solubility increases to 0.021 wt. % Al. Frorn the results i t is concluded that dissolved aluminium is present in the melt either as A120 dimer species or as associated molecules of AIdBr6 and AlrIs.

INTRODUCTION

Aluminium bromide and aluminium iodide are white crystalline compounds that melt into colorless liquids. The melting points are given as 97.1 "C (I) to 97.5 "C (2) for the bromide and 191 "C (2) for the iodide. The boiling points are 255 and 381 "C (3) respective1 y.

The structure of aluminium bromide has been found (4) to consist of bromine in closest hexagonal packing, with the aluminium in adjacent tetrahedral holes. This corresponds to A12Brs molecules where aluminium is tetrehedrally surrounded by four bromine atoms, the two tetrehedra sharing an edge. A corresponding but somewhat distorted bimolecular structure is found (5) in the vapor phase of both bromide and iodide, although a certain dissociation takes place a t high temperatures. Raman spectra (6) show that the bimolecular configuration is also maintained in the liquid phase for the bromide and the iodide. The covalent molecular state of the melts has been confirmed by measurements of specific conductivities (2) which were found to be extremely low.

The solubility of aluminium in aluminium iodide has been measured (7) and was found to be 0.G mole yo (A1 in Al2I6) a t 423 "C and 0.0 mole yo a t 380 "C. Apart from these measurements the solubilities of metals in metal halides of covalent n~olecular structure have not been studied. In view of this fact it was found to be of interest to investigate the solubility of aluminium in AlzBr6 and Alz16 in some detail.

EXPERIMENTAL

Aluminium bromide and iodide were made by direct reaction of the component elements, The aluminium was of 99.998% purity (Refined ALCAN aluminium). The bromine and iodine were "Baker Analysed" reagents with the following analyses: bromine 99.8% Br, 0.01% non-volatile matter, 0.15% Cl, 0.05% I , 0.001% SO, .OO1yo heavy metals; iodine 0.002% non-volatile matter, O.OO1yo Cl + Br.

Both compounds were extremely hyroscopic and had to be synthesized in evacuated pyrex tubes. The reactants were transferred to the reaction tubes in a dry-box under a n inert nitrogen atmosphere.

The rates of reaction between the aluminium and the halide vapors were kept under control by maintaining the tubes a t temperatures well below the boiling points of bromine and iodine respectively. The resulting compounds were distilled twice and could then be used directly for the solubility measurements. For the melting point determinations further purification was needed, as will be described subsequently.

Solubility Measz~rements The distilled product was allowed to equilibrate with aluminium metal for 24 h in a furnace kept a t

constant temperature. At the end of this period the tube was tilted to separate the metal from the melt and was quenched in air. The quenched sample had a grayish tinge, probably because of its metal content.

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2740 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42. 1964

The sample was then transferred to an analysis apparatus where it was dissolved in alcohol and water. T o prevent the initially occurring violent reaction of the anhydrous salt with water, alcohol was added first. The excess metal content reacted with alcohol or water with the evolution of hydrogen

which was determined by a combustion technique. T o this purpose the evolved hydrogen, mixed with nitrogen as a carrier gas, was led through a liquid air trap to a heated tube containing CuO a t 300 OC.The water vapor produced was absorbed in an absorption tube and weighed. The method was tested with blank runsand by usingsamples of dirferent weights. The blanks gave no weight increase of the absorption tube, while the runs with samples of different weights showed a precision as given for the results.

Melti?zg Point Determinations The co~llpounds were kept in capillary tubes immersed in a thermostatically heated oil bath. The tem-

perature was measured with a Beckmann thermometer calibrated against an ERTCO precision thermometer, calibrated according to ASTM standards.

By manual control the temperature of the oil bath could easily be kept within f 0.005 "C. The melting points were determined by increasing the temperature in steps of 0.01 "C a t intervals of 1-2 h, until the crystals in the capillary had melted.

The distilled samples did not melt a t a definite temperature but over a range of 0.2-0.3 "C, either because of the influence of impurities or because of an excess of halogen or metal. For further purification the samples were kept within the observed solidus-liquidus range a t gradually increasing temperatures, the molten phase being discarded each time. An alternative procedure was to let the compound crystallize out from the molten samples by keeping them within the corresponding liquidus-solidus range. The presence of seed crystals was necessary in this case because the melt had a strong tendency to undercooling. Undercooling was generally about 8 "C for AI2Br6 and about 12 "C for A1216.

Pure samples which melted completely a t one definite temperature were finally obtained. Samples saturated with metal were made by equilibrating the pure samples with metal for 24 h before being transferred to the capillary. Intermediate compositions were obtained by adding the pure compound to the saturated mixtures. The samples were finally analyzed for their metal content.

Melting points could be determined with a precision better than &0.01 "C. Thus freezing point depressions could be determined with a precision better than h0.02 OC. Because of possible errors in the calibration of the thermometer, the accuracy of the melting points presented herein is given a s f 0 . 0 2 OC.

RESULTS

From the combined results of the melting point and the solubility measurements, the liquidus curve for solutions of aluminium in aluminium bromide and aluminium iodide melts \\;as determined.

The results for the Al-AlzBr6 systeim are given in Table I.

TABLE I

Melting points and solubilities in the A1-A12Br6 system

Temperature, Composition, "C wt. % Al

From solubility measurements 145 198 241 254

The observed melting point for pure aluminium bromide is 98.01 f 0.02 "C, which is 0.5-0.9 "C higher than the values given in the literature (1, 2). The results indicate a eutectic a t 97.91 f 0.02 "C and 0.017 wt. yo Al. On a molar scale (atom % A1 in A12Br6) this composition corresponds to 0.34 mole yo Al. Within the precision of the measure- ments, the freezing point depression of the eutectic is 0.10 f 0.02 "C.

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THONSTAD: ALUMINIUM -ALUMINIUM HALIDE SYSTEMS 2741

Five solubility determinations a t 198 "C gave results between 0.0177 wt. % and 0.0188 wt. yo Al. At higher temperatures the precision apparently was not so good.

The results for the A1-Alz16 system are given in Table 11.

TABLE I1 Melting points and solubilities in the Al-Al216 system

Temperature, Composition, "C wt. ??, A1 . "

From melting ooint determinations 188 .32f 0 .02 0

From solubility measuren~ents 202 265 320

The observed melting point for pure aluminium iodide is 188.32 f 0.02 "C, which is about 2.7 "C lourer than the value given in the literature (2). The results indicate a eutectic a t 188.17 f 0.02 "C and 0.018 wt. % Al, corresponding to 0.54 mole % Al. The freezing point depression of the eutectic is 0.15 f 0.02 "C. The precision of the solubility measurenlents was not as good as in the bromide system.

DISCUSSION

The illeasured melting points and the solubility data give the liquidus curves of the phase diagrams of the two systems, as shown in Figs. 1 and 2. The shape of the curves is similar to those available for some metal-salt systems (8), but the solubilities are in this case unusually low. This is probably due to the covalent molecular structure of the melts.

Extrapolation of the liquidus curve in Fig. 2 to 423 "C indicates that the solubility a t that temperature is -0.023 wt. % or -0.7 mole yo A1 in Alz16. This is in fair agreement with the rsleasurement by Corbett and Winbush (7) giving a solubllity of 0.6 mole % a t 423 "C. The other measurement by these authors, indicating no solubility a t 380 "C, is apparently in error.

From cryoscopic calculations some information can be obtained about the species that are present in the melt. Assuming ideal behavior and the absence of solid solubility, the simplified equation is :

H f log N = - --7 AT, R Ti

where N , Hi, and Ti are the mole fraction, the heat of fusion, and the melting point respectively of the solvent (aluminium bromide or iodide), and AT is the corresponding freezing point depression.

The heats of fusion of the compounds are (9) 5.4 kcal/mole A12Br6 and 7.6 kcal/mole Alz16 respectively. The uncertainty of the data is estimated (9) to be j~0.4-0.6 kcal/mole.

Assuming different schemes of solution, corresponding values for N and AT can be calculated and the latter can be compared to the measured values. The following schemes are considered (X stands for Br or I , n is the number of moles).

la. Free aluminium atoms:

A1 -+ A10

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2742 CANADIAN JOURNAL O F CHEMISTRY. VOL. 42, 1964

w t % Al-

FIG. l. Liquidus curve of the A12Br6-A1 system. 0, From melting point determinations; 0, from solubility measurements.

FIG. 2. Liquidus curve of the AIzIF,-AI system. 0, From melting point determinations; 0, from solubility measurements.

FIG. 3. Measured and calculated freezing point depressions in the AlzBre-A1 system. FIG. 4. Measured and calculated freezing point depressions in the A1216-Al system,

l b . Associated A1 atoms: A1 + A12Xe -+ A13X6

2a. Dimer aluminium atoms: 2AI -+ A120

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THONSTAD: ALUMINIUM -ALUMINIUM HALIDE SYSTEMS

2b. Associated dimers:

The resulting liquidus curves calculated for the eutectic points are given in Figs. 3 and 4. Schemes 2a and 2b give almost identical results, and it may be seen that either scheme is in good agreement with the experilnentally measured depressions of the freezing points. For the bromide system the calculated depression for the eutectic composition is 0.085 f 0.006 "C as against the measured 0.10 f 0.02 "C. For the iodide system the calculated depression is 0.15 f 0.01 "C and the measured is 0.15 f 0.02 "C. I t should be noted tha t the same results are obtained if the bromides and iodides are taken as monomolecular (AIBr3 and All3). Testing of schemes other than those listed here confirms tha t 2a and 2b are the only conceivable models that agree with the experimental data.

From the preceding treatment it appears tha t the AlzBr6-A1 and AlzI6-Al systems contain either neutral diatomic groups of aluminium or associated dimers like AllBrc and A1416.

ACKNOWLEDGMENT

The author wishes to thank Professor S. N. Flengas, University of Toronto, for helpful suggestions and for critically reading the manuscript.

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