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)°C adopts a Co2Si-type structure: space group Pnma, a=5.4421(2), b=4.1673 (2), c=8.0129(3) ?. MgPd3 crystallizes in a Al3Zr-type structure: space group I4/mmm, a=3.92263(7), c=15.6527(4) ?. It decomposes in a peritectoid reaction at 704(2)°C. The kinetics of the iodine-catalyzed synthesis of MgPd2 have been studied. The reactions seem to follow Ostwaldʹs step rule. Four intermediate phases were identified, among them was Mg3Pd5, a previously unknown phase with a Ge3Rh5-type structure: space group Pbam, a=5.427(2), b=10.588(5), c=4.1304(12) ?.