Structure and thermal stability of the new intermetallics MgPd2, MgPd3, and Mg3Pd5 and the kinetics of the iodine-catalyzed formation of MgPd2

Three new binary intermetallics in the palladium-rich part of the magnesium-palladium system have been identified and characterized by combining preparative methodologies with powder X-ray diffraction techniques and thermochemical analyses. MgPd2 decomposing in the solid state at 628(2)degreesC adopts a Co,Si-type structure: space group Pnma, a = 5.4421(2), b = 4.1673(2), c = 8.0129(3) Angstrom. MgPd3 crystallizes in a Al3Zr-type structure: space group a = 3.92263(7), c = 15.6527(4) Angstrom. It decomposes in a peritectoid reaction at 704(2)degreesC. The kinetics of the iodine-catalyzed synthesis of MgPd2 have been studied. The reactions seem to follow Ostwald's step rule, Four intermediate phases were identified, among them was Mg,Pd,, a previously unknown phase with a Co2Si-type structure: space group Pnma, a = 5.4421(2), b = 10.588(5), c = 4.1304(12) Angstrom. (C) 2001 Academic Press