INTERMETALLIC PHASES IN THE MAGNESIUM PALLADIUM SYSTEM - X-RAY ABSORPTION SPECTROSCOPIC CHARACTERIZATION OF THE AMORPHOUS ALLOY MGPDCXHY

Novel magnesium intermetallics produced via organometallic chemistry may be potential hydrogen storage systems. Nonmetallic components, i.e., interstitial carbon and hydrogen, are surmised to participate in defining both the local and the longer range order. In the magnesium-palladium system, MgPdCxHy (x = 0.4-0.6; y = 0.8-1.9), prepared in tetrahydrofuran at <25-degrees-C, remains X-ray amorphous, even after annealing. An X-ray absorption spectroscopic study of this compound, by reference to the crystalline magnesium palladium bimetallics MgPd and Mg0.9Pd1.1, as well as to PdC0.15 and metallic palladium, has allowed its local environment to be identified as closely resembling that of Mg0.9Pd1.1 (HgMn arrangement), where the body-centered site of the tetragonal unit cell (defined by palladium atoms) is partially occupied by magnesium (ca. 90%) and partially by palladium (ca. 10%). The interstitial carbon in MgPdCxHy has been found to occupy the ac face-center positions. The proximity of carbon to magnesium atoms and the retention of carbon and hydrogen on annealing may be key factors in the failure of this system to crystallize, even after prolonged annealing at 650-degrees-C. A Fourier transform difference analytical method applied to PdC0.15 and Pd foil has provided evidence for Pd-C contributions to the overall EXAFS in the former, at distances entirely compatible with the occupation by carbon of octahedral sites within the cubic cell.