Low-temperature, mercury-mediated synthesis of aluminum intermetallics

The preparative-scale synthesis of intermetallic phases in the Al-Au and Al -Pd systems can be achieved at the relatively low temperature of boiling Hg (similar to 350 degrees C at 1 atm). In particular, Al2Au, AlAu, AlAu2, Al 3Pd2, and AlPd can be made in high yield either as single products or as mu ltiple products by dissolution of the elements in liquid Hg, followed by di stillation of the solvent. The reaction products were determined by X-ray d iffraction and electron microprobe analysis. A systematic study reveals tha t the products of these reactions depend strongly on initial mole ratios. A thermodynamic phase stability analysis was conducted using estimated free energies for the intermetallic phases. From these calculations and from the observed products and their sequence of formation, some of the reactions a re shown to be kinetically controlled. Direct synthesis reactions tend to b e favored for concentration ratios that correspond to intermetallic phases that exhibit a coexistence tie line with the Hg solution.