Mössbauer Studies of Hydrogen-absorbing Rare-earth Intermetallics

Mossbauer spectroscopy has been used to study both the physics and phenomenology of hydrogen absorption in rare-earth intermetallics. This paper surveys experiments that have been carried out so far, and discusses the results obtained. * * * * * * * * LaNi5 and similar intermetallic compounds have been of great interest recentlyl because of their ability to absorb reversibly large amounts of hydrogen at convenient temperatures (0-100°C) and pressures (0.1-100 atm) with rapid kinetics. These properties have become the basis of hydrogen pumps ,l batteries ,l gas absorption air conditioners,l and bulk hydrogen storage facilities .l One of the major advantages of LaNi5 is that the temperature and pressure dependence of its hydrogen desorption curve can be lldesigned" for a particular application by alloying other elements with the La or Ni. An effective semi-phenomenological theory has been developed to explain the stability of the ternary hydrides, and, to a great extent, to predict possible new families of useful material^.^ LaNi5 and its pseudobinary alloys have been studied well enough to allow prediction of thermodynamic properties which are of importance for most applications listed above. Studies by photoelectric spectroscopy~ and neutron diffraction4 have also been carried out to determine the microscopic mechanism by which hydrogen is incorporated into these materials. An important aspect of these materials is that the hydrides form as ternaries with a distinct and well-defined hydrogen swichiometry. This is in contrast to, e:g., Ta, which dissolves hydrogen over a wide range of solid-solution concentrations. Some materials form only one hydride (e.g., LaNi5 + LaNigH6.7), while others form hydrides with two or more hydride phases. These can be observed by x-ray crystallographic studies, the gas evolution isotherm, or Mossbauer spectrosCOPY. MBssbauer spectroscopy has the advantage of being a truly microscopic probe of materials. It is capable of studying these intermetallics over a wide range of temperature, and while they are actua1.l~ under hydrogen pressure. Since neither La nor Ni are desirable MSssbauer isotopes, the studies carried out to date have all been on LaNi5 with a small quantity of another rare-earth,(RE) substituted for the La, or on other RE-transition metal intermetallics which could be expected to behave similarly. Five basic phenomena have been studied in experiments to date (see Table I) : 1. In polyphase samples, the individual phases can be identified and quantified. 2. The magnetism of the sublattices can be studied via the hf interaction. 3. The isomer shift provides a unique indicator of changes in the electronic structure under hydriding. 4. Oxidation can be readily observed. 5. The Mossbauer spectrum can be used to assay the amount of absorbed hydrogen, and thus to diagnose degradatibn mechanisms. In the discussion below, these points are illustrated by selections from the work listed in Table I. A clear early example of the first point is ref. 12, some results of which are shown in Fig. 1. Using the ~d155 resonance as a probe in LaNi5, the measurements show that the spectrum of Gd:LaNi5H6.7 (the saturated hydride) differs in I.S. and quadrupole splitting fram that of Gd:LaNi5, and that the spectrum of a partially hydrided sample can be explained as being a sum of spectra of the intermetallic and the saturated hydride. This result is important in showing that even on a microscopic scale the hydride is not continuous solid solution. Similar results were found for Lacog, but in that case, more than one hydride is formed. Several subsequent experiments with Eu compounds7~8,9,10 showed similar results. For RE intermetallics with Fe, the hf field and moment at the Fe site usually change slightly for the lower hydrides (H/RE ~ 4 ) , but decrease strongly for the higher hydrides. The iron sublattice in ErFe2H4.12 is virtually nonmagnetic, even at 1.7 K (see Fig. 2). In Fe-RE systems in which the iron sublattice provides a large exchange field for the RE ions, that Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801124 C1-334 JOURNAL DE PHYSIQUE TABLE I: MOSSBAUER STUDIES OF HYDROGEN-ABSORBING RARE-EARTH INTERMETALLICS Intermetallic Isotope Hydridex=ConclusionsGroup, References Greatly reduced Fe-Fe exch.slightly incr, Heff forYFe2H4. Very broad llnes.Isomer shift changes. Local inhomogeneitiesgiving very broad linesno a-Fe. EU~++EU~+(EuNi5, EuRhz]I.S., Heff, Tc for hydridesclose to EuH values. Onlysaturated an3 dehydridephases. "Molecular complex1'model of hydride formation. DyTm2Hx I.S. like that ofDyH2, DyH3. Reduced cond.el. density. Greatlyreduced TM-Dy exchange.Support of RE-H ucomplex"model. Reversibly absorbed Hzdecreases after a fewcyclesformation of fixedhydride. Reversibly absorbed HZdecreases after hundreds ofcycles. Formation of afixed hydride Spectra show only distincthydride phases. Reduceds-density at Gd nucleus Greatly reduced Tc and Fe-Erexchange. kgligible Fe momentand Heff in ErFe2H4.12. Reduced Dy-Fe exchange.Philips5, 6