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MÖSSBAUER STUDIES OF
HYDROGEN-ABSORBING RARE-EARTH INTERMETALLICS
Richard Cohen
To cite this version:
Richard Cohen. MÖSSBAUER STUDIES OF HYDROGEN-ABSORBING RARE-EARTH INTERMETALLICS. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-333-C1-336.
�10.1051/jphyscol:19801124�. �jpa-00219607�
JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n O 1, Tome 41, janvier 1980, page Cl-333
Richard L. Cohen
Be22 Laboratories, &ray HiZZ, N. J. 07974, USA.
ABSTRACT
Mossbauer spectroscopy has been used to study both the physics and phenomenology of hydrogen absorption in rare-earth
intermetallics. This paper surveys experi- ments 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 revers- ibly large amounts of hydrogen at conven- ient temperatures (0-100°C) and pressures (0.1-100 atm) with rapid kinetics. These properties have become the basis of hydro- gen 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 d e - sorption 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 sta- bility 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 thermo- dynamic 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 concentra- tions. 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 spectros- COPY.
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 temper-
ature, 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 simi- larly.
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 ~ d 1 5 5 reso- nance as a probe in LaNi5, the measure- ments 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 show- ing 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 Hydride
x= Conclusions Group, References
Greatly reduced Fe-Fe exch.
slightly incr, Heff for YFe2H4. Very broad llnes.
Isomer shift changes.
Local inhomogeneities giving very broad lines no a-Fe.
E U ~ + + E U ~ + (EuNi5, EuRhz]
I.S., Heff, Tc for hydrides close to EuH values. Only saturated an3 dehydride phases. "Molecular complex1' model of hydride formation.
DyTm2Hx I.S. like that of DyH2, DyH3. Reduced cond.
el. density. Greatly reduced TM-Dy exchange.
Support of RE-H ucomplex"
model.
Reversibly absorbed Hz decreases after a few cycles - formation of fixed hydride.
Reversibly absorbed HZ decreases after hundreds of cycles. Formation of a fixed hydride
Spectra show only distinct hydride phases. Reduced s-density at Gd nucleus Greatly reduced Tc and Fe-Er exchange. kgligible Fe moment and Heff in ErFe2H4.12.
Reduced Dy-Fe exchange.
Philips 5, 6
Bell-Philips 7, 8 EuPd, EuNi5,
EuRh2, EuMg2
Bell 11 Lao. $do. 1Ni5
gGdO. lC05 ErFe2
Hebrew Univ.
12 Ar gonne
13
field is reatly reduced in the higher hydrides, 5 ~ 3 resulting in a decrease in the hf field at the RE nucleus (Fig. 3).
Hydriding does not change the Hundt's rule 4f ground state of the
RE^+
ions, so the local moment and hf field should in prin- ciple be little affected. However, the RE-RE exchange is greatly reduced by the reduced conduction electron density in the hydrides (see below). Thus, magnetic ordering temperatures are greatly reduced in the hydrides. Ionic ground states are determined more by crystal field and less by exchange, so that hf fields and ordered- state moments are greatly reduced.Hydriding greatly reduces the electron density at the nuclei of the RE ions. For all intermetallics studied, the electron density was reduced from a value typical for that in metallic systems, to a value close to that observed in the binary RE.
hydrides 7~8,9,10,11 (see Fig. 4). This has been proposed as evidence that the hydrogen bonds primarily to the RE ions, and that the electronic structure of the hydrided intermetallics is like that of the binary hydrides. The reduced magnetic ordering temperatures and electrical con- ,ductivity15 of the hydrided intermetallics
4 - 3 - 2 - 1 0 1 2 3 4 VELOClTI ( m m l t e t )
Fig. 1. Quadrupole-split ~d~~~ spectra in LaNi5 and hydrides. Spectrum of the partially hydrided material is com- posed of components from the unhydrided material and the saturated hydride
(Ref. 12).
-
-1.0 VELOCITY 0 (rnmls) 1 .oFig. 2. ~e~~ spectrum of ErFe2H4.12. Iron sub- lattice appears nonmagnetic, even at 1.7 K (Ref. 13).
- 6 0 -40 -20 0 20 40 60 VELOCITY Irnmfs)
Fig. 3. ~r~~~ spectra of hydrided ErFe2. In the H4.12 material, exchange from the Fe sublattice is negligible, and the splitting at 4.2'~ comes from para- magnetic hfs (Ref. 13).
are evidence that d electrons, as well as s electrons, are removed from the conduc- tion band by the hydriding.
Most of the technological applications of these hydrogen-absorbing materials require them to go through many absorption-desorp- tion cycles without a change of properties.
After early experiments10 in which it was shown that EuRh2 lost its reversibility in just a few cycles, additional experimentsll
*
EuH2P EuNi5 V EuMg2 0 EuRh2 0 EuPd
Fig. 4. EU''~ IS and Hint for Eu intermetallics.
The open points are for the fresh materials, the filled points for the saturated hydrides. Por all compounds studied, both IS and Hint for the hydrides are close to those of EuH2
(Ref. 8).
were carried out on LaNis doped with small amounts of Eu and Mn. This material has essentially the same absorption properties as LaNi5. Control experiments (Fig. 5 A, B, C) show that the Eu is tri alent in the original material, becomes EuYf in the hydride, and returns to E U ~ + when the hydrogen is removed. It thus serves as an excellent indicator of the amount of hydro gen in the sample.
Measurements carried out with automated cycling apparatus showed that after 1500 absorption-desorption cycles, the material retained only 29% of its original capacity to absorb hydrogen reversibly, measured by the evolved gas volume. The MBssbauer measurements (Fig. 6 D, E) showed that this effect resulted from the formation of a permanent hydride phase, which could not be decomposed at room temperature. The Mdssbauer and gas volume measurements were in quantitative agreement on the amount of the perman nt hydride phase. Since the peak of Euq' in E u ~ 0 3 is easily resolved from that of E U ~ + in LaNi5, the spectrum (Fig. 5 D) showed that very little oxida- tion had occurred during the cycling.
This confirmed that the degradation was the result of an intrinsic mechanism, rather than simp1 oxidation. Thermo- dynamic argumentsrl show that the formation of RE hydride and nickel metal is ener- getically preferred to formation of the ternary hydride. Thus, in principle, this effect should occur. In fact, it has been observed in ~ a ~11 However, our obser- ~ i ~ . vations are the flrst of this kind in an intermetallic normally considered to be a reversible absorber.
'21-336 JOURNAL DE PHYSIQUE
Fig. 5. Eul'l spectra of (Lao. gEuO. 1)Nil. @boo 4.
A, original material; B, hydrided;
C, dehydrided, after 10 absorption- desorption cycles. After 1500 cycles, D shows hydrided sample, E is spectrum of same sample with all reversible hydrogen removed. Remaining E U ~ + in E is from permanent hydride (Ref. 11).
The material above shows that there is a great deal that can be learned via MBssbauer spectroscopy about the physics and phenomenology of hydrogen absorption.
Once the materials and hydrogen volume apparatus are at hand, the measurements are relatively easy and provide useful information. Areas w4ich would be particu- larly appropriate for further work would be (1) ~e''~ studies of Fe doped LaNis, (2) calculations o f wave functions for the hydrides, for comparison with the results of MSssbauer experiments, and (3) RE
Mossbauer measurements to detect oxidation in intermetallics used as hydrogen absorb- ing electrodes in contact with aqueous electrolytes.
REFERENCES
Many papers are published in "Proceedings of the International Symposium on Hydrides for Energy storage," A. F. Andresen and A. J.
Maeland, eds. (Pergamon, N. Y., 19781.
H. H. van Ma1 and A. R. Miedema, in Ref. 1.
H. C. Siegmann, L. Schlapbach, and C. R.
Brundle, Phys. Rev. Letters (1978) 972.
P. Fischer, A. Furrer, G. Busch, and L.
Schlapbach, Helv. Physica Acta 2 (1977) 421.
K. H. J. Buschow and A. M. van Diepen, Solid State Commun. 2, (1976) 79-81.
A. M. van Diepen and K. H. J. Buschow, Solid State C o m n . 22 (1977) 113-115.
K. H. J. Buschow, R. L. Cohen, and K. W. West, J. Appl. Phys. 48, (1977) 5289.
F. W. Oliver, K. W. West, R. L. Cohen, and K. H. J. Buschow, J. Phys. F 8 , (1978) 701.
F. W. Oliver, K. W. West, R. L. Cohen, and K. H. J. Buschow, to be published.
R. L. Cohen, K. W. West, and K. H. J. Buschow, Solid State Comm. 25 (1978) 293
R. L. Cohen, K. W. West, and J. H. Wernick, to be published, J. Less. Common Metals.
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P. J. Viccaro, J. M. Friedt, D. Niarchos, B. D. Dunlap, G. K. Shenoy, A. T. Aldred, and D. G. Westlake, J. Appl. Phys. 2 (1979) 2051.
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