2. Experimental Procedure
The general nature of the diagram was first established with a commercial DTA apparatus2, using a microholder with Al2O3, as a reference material and a 10°/min heating rate. The final diagram (fig. 1) was refined by determining the liquidus and solidus curves by visual observation. The chlorides of aluminum and sodium were obtained as Reagent Grade. The NaCl was analyzed to be 99.7 ± 0.1 percent pure. The AlCl3 was resublimed at 135-150 °C under vacuum from molten NaCl · AlCl3 mixtures containing Al-metal strips [8]. The final purity of AlCl3 was analyzed to be 99.8 ± 0.1 percent. Atomic absorption analysis for Na confirmed a content of < 0.1 percent Na in the AlCl3. To prevent hydration of AlCl3, various mixtures of NaCl and AlCl3 were prepared in glove bags flushed with anhydrous N2 and sealed under vacuum in Pyrex tubes. Compositions not forming two liquids, i.e., those below 80 mol percent AlCl3, were subjected to three cycles of grinding and sealing in the glove box, followed by heating to promote homogeneity. Samples which had been reground several times appeared pure white in color; but when they were heated to the appropriate invariant temperature of either 153 or 107 °C, they turned sharply to a gray hue.
The two liquid region of the diagram was studied visually by introducing the sealed sample into a nichrome-wound Pyrex tube furnace (40 mm O.D.) insulated by an outer Pyrex tube. Sealed samples with 80 mol. percent AlCl3 and less were studied visually in a silicone oil bath, stirred with a magnetic stirrer. Heat control was accurate enough to permit heating and cooling rates of 6 °C per hour or less. The liquidus temperatures were determined by observing under a magnifying glass the disappearance of tiny crystals approximately 0.5 mm in their largest dimension. Several determinations were averaged, and all temperatures are estimated accurate to ±0.5 °C. The Chromel-Alumel thermocouple was calibrated using pure Sn (mp 231.89 °C) and the normal freezing and boiling points of water. At times a calibrated Hg thermometer was used also in the oil bath.
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The solid phases were identified by x-ray diffraction powder techniques, using a dry mount with Mg(ClO4)2 as desiccant. All compositions could be interpreted in terms of the components NaCl and AlCl3, and one intermediate compound NaAlCl4. The x-ray powder pattern of NaAlCl4 was indexed on the basis of an orthorhombic cell [7], with a = 10.33 Å, b = 9.91 Å, c = 6.18 Å, within a few hundreths Ångstroms of the dimensions reported by Baenziger [7] and Semenenko et al. [9]. Small irregular variations in interplanar spacings with different compositions are attributed to hydration effects rather than to an indication of slight solid solubility of AlCl3 in NaAlCl4.
A two liquid region, as reported by Kendall et al. [1], and Shvartsman [3], was found to exist between a composition of 80.25 ± 0.25 and 99.6 mol percent AlCl3 at 191.3 °C. Two liquids were observed visually to form in several samples containing 80.5 mol percent AlCl3 or greater (fig. 1). In repeated experiments, two liquids were never observed in samples of 80.0 mol percent AlCl3 or less, i.e., 79.5, 77.9, and 75 percent. Atomic absorption analysis for Na in the upper liquid segregated at ~ 192 °C revealed the composition to be 0.4 mol percent NaCl. The composition of the AlCl3-rich liquid, therefore, is 99.6 mol percent AlCl3.
Several attempts were made to study the closure of the two-liquid dome. Samples of composition 80.6 and 90.2 mol percent AlCl3 were heated in sealed glass tubes to form two liquids. After heating at a rate of 5 °C/day and reaching temperatures of above 210 °C, the tubes burst. The triple point of AlCl3 is reported [10] at 193.3 °C and 2.33 atm.
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