Hyperfme Interactions9 (1981) 531-546 INVITED PAPER North-HollandPublishingCompany HYDROGENABSORPTIONMECHANISM AND LOCATION IN INTERMETALLIC COMPOUNDS* G. K. SHENOY, B. D. DUNLAP, P. J. VICCARO, and D. NIARCHOS Argonne National Laboratory Argonne, I l l i n o i s 60439 USA It has recently been shown that many intermetallic com- pounds of d- and f- shell elements reversibly absorb large amounts of hydrogen to form ternary hydrides at easily ac- cessible temperatures (0-100 ~ C) and hydrogen pressures (.01-10,000 torr). Such ternary hydrides have been consid- ered in many energy conversion or storage applications. Two fundamental questions may be addressed regarding this class of hydrides. The first one is related to the ease with which hydrogen is absorbed from the gas phase by these in- termetallics, and the second to the location of hydrogen atoms once it is absorbed in the intermetallic lattice. Hyperfine interaction measurementson a large number of systems have been carried out in relation to these ques- tions. In many cases, measurementsof this type are shown to contribute rather direct and at times unique information concerning these two aspects of hydrogen absorption. In particular, we demonstrate that conversion electron M~ssbauer spectroscopy has the potential of providing evidence for the chemical and magnetic state of the surface layers in the intermetallic. This information, in turn, has direct bearing on the hydrogen absorption mechanism. Furthermore, since hyperfine interaction measurementsare microscopic probes, they offer the possibility of distin- guishing different local hydrogen environments surrounding a given metal atom. Information of this type can be used in conjunction with results from bulk measurements to de- termine hydrogen location in the metal lattice. Examples illustrating both aspects of ternary hydride formation will be discussed. INTRODUCTION With the increasing demandand cost of petroleum, much consideration is currently being given to alternate forms of energy. One of the most frequently discussed candidates is hydrogen [ i ] which, when burned in air, produces a very clean form of energy. However,one of the first considerations that arises in regard to large scale use of hydrogen concerns its storage. In small quantities, hydrogen is stored in bulky, high-pressure gas tanks, but this is of little use for massive applications. The energy required to liquify hydrogen makes this option also unattractive. In recent years, numerous intermetallic compounds have *Work supported by the U. S. Department of Energy 531