New ternary rare earth based intermetallic compounds of the composition RENi4Ga (RE = Dy, Ho, Er) were prepared and studied using X-ray powder diffraction. All compounds were found to be hexagonal and to crystallize in the space group P6/mmm with the CaCu5 type of structure, i.e., the same symmetry as their prototype binaries RENi5. The interaction with hydrogen was also studied. It was found that all intermetallics react readily and reversibly with hydrogen to form hydrides with hydrogen contents of up to 3.48 hydrogen atoms per alloy formula unit at room temperature. The entropy, the enthalpy and the Gibbs free energy of formation have been extracted from the equilibrium plateau in the pressure composition desorption isotherms. The thermodynamic parameters and the hydrogen capacity of the RENi4Ga–hydrogen systems are compared with the corresponding values for their aluminium analogues and briefly discussed

Antun Drašner

Božica Sorgić

Želimir Blažina

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ISSN-0011-1643CCA-2598 Original Scientific PaperThe RENi4Ga (RE = Dy, Ho, Er) IntermetallicCompounds – Crystal Structure and HydrideProperties*Bo`ica [orgi}, @elimir Bla`ina,** and Antun Dra{nerLaboratory for Solid State Chemistry, Institute Ru|er Bo{kovi}P. O. Box 1016, 10001 Zagreb, CroatiaReceived September 28, 1998; revised April 12, 1999; accepted April 13, 1999New ternary rare earth based intermetallic compounds of the com-position RENi4Ga (RE = Dy, Ho, Er) were prepared and studied us-ing X-ray powder diffraction. All compounds were found to be hexa-gonal and to crystallize in the space group P6/mmm with theCaCu5 type of structure, i.e., the same symmetry as their prototypebinaries RENi5. The interaction with hydrogen was also studied. Itwas found that all intermetallics react readily and reversibly withhydrogen to form hydrides with hydrogen contents of up to 3.48 hy-drogen atoms per alloy formula unit at room temperature. The en-tropy, the enthalpy and the Gibbs free energy of formation havebeen extracted from the equilibrium plateau in the pressure com-position desorption isotherms. The thermodynamic parametersand the hydrogen capacity of the RENi4Ga–hydrogen systems arecompared with the corresponding values for their aluminium ana-logues and briefly discussed.Key words: rare earth intermetallics, hydrogen sorption, crystalstructure, thermodynamic properties.INTRODUCTIONThe rare earth based intermetallic compounds of the composition RENi5(RE = rare earth including misch metal) have attracted a great deal of at-CROATICA CHEMICA ACTA CCACAA 72 (2–3) 567¿574 (1999)* Dedicated to Professor Boris Kamenar on the occasion of his 70th birthday.** Author to whom correspondence should be addressed. (E-mail: blazina@rudjer.irb.hr)tention as possible hydrogen storage materials. This is because many of themreact reversibly and absorb large quantities of hydrogen at modest pressureand moderate temperature. For example, the superior representative of theclass, LaNi5, forms LaNi5H6.7 at room temperature and under 200 kPa ofhydrogen.1 Substitution of nickel in RENi5 by other metals or metalloids fre-quently influences the crystal structure of the prototype RENi5 and thethermodynamic properties of the corresponding RENi5–hydrogen system.Some examples are the systems LaNi5-xAlx–hydrogen,2 LaNi5-xSnx–hydro-gen,3,4 CeNi5-xAlx–hydrogen,5 TbNi5-xAlx–hydrogen,6 HoNi5-xAlx–hydrogen7and NdNi5-xAlx–hydrogen.8However, aluminium seems to be the best material for tailoring a me-tal–hydrogen system for a particular application.9,10 Accordingly, it was pos-sible to prepare some LaNi5 based compounds that are today commerciallyused as hydrogen storage materials and as negative electrode in environ-ment friendly rechargeable nickel–metal hydride (Ni/MH) batteries.Since gallium substituted RENi5 alloys have not been studies so far andgallium and aluminium have similar atomic sizes and configuration of va-lence electrons, we were interested in performing structural and thermody-namic studies of gallium substituted RENi5 alloys and their hydrides, someresults of which have been published elsewhere.11,12 The results reportedhere represent the continuation of our systematic study on the structuraland hydrogen sorption properties of selected RENi5 compounds wherenickel is partially replaced by other metals. The aim is to select potentialmaterial for hydrogen storage purposes.EXPERIMENTALThe starting materials used in this investigation were supplied by Johnson Mat-they, UK (rare earth with purity of 99.9%; gallium with purity of 99.99%) and CarloErba, Italy (nickel, 99.5%; main impurity iron). The alloys of the general composi-tion RENi4Ga (RE = Dy, Ho, Er) were prepared by arc melting under an argon at-mosphere. To ensure homogeneity, the alloys were inverted and remelted severaltimes. The weight loss of the material was checked and was found to be negligible.Good single phase material was obtained after annealing in vacuum at 1173 K for atleast 75 hours.The X-ray powder diffraction patterns were obtained with a Philips PW 1050 dif-fractometer equipped with a graphite monochromator and nickel-filtered Cu-K ra-diation. The intensities were calculated using the »Lazy-Pulverix« program.13All alloys were exposed to hydrogen supplied by Jesenice, Slovenia (purity,99.999%). The pressure composition desorption isotherm (PCDI) measurementswere carried out in a stainless-steel apparatus that enables work in a temperaturerange from 77 to 800 K, in vacuum and/or with hydrogen at pressures up to 15 MPa.Prior to PCDI measurements, the alloys were activated by heating under hydrogen568 B. [ORGI] ET AL.(700 K, 10 MPa). After cooling, the absorbed hydrogen was removed by heating andevacuating. This procedure was repeated several times and the equilibrium condi-tions were assumed to be reached when the amount of released hydrogen remainedconstant. The PCDI measurements were made on activated samples, completelysaturated with hydrogen, by releasing small quantities of hydrogen from the reactor.The equilibrium pressure was measured after 15 min and the procedure was re-peated until the pressure dropped to 10 kPa, when the samples were heated and ad-ditional amounts of released hydrogen were measured. The composition was calcu-lated from the pressure–temperature–volume data.RESULTS AND DISCUSSIONThe X-ray powder diffraction data of the RENi4Ga (RE = Dy, Ho, Er) in-termetallic compounds indicated that these single phase materials are ofhexagonal symmetry and of the CaCu5 type of structure (space groupP6/mmm). The unit cell parameters for RENi4Ga are listed in Table I, andthe relevant values for the binary RENi5 and the ternary RENi4Al interme-tallic compounds containing the same kind of rare earth metal are also in-cluded for comparison.7,14,15 Figure 1 illustrates the unit cell volume for allthree series of compounds. The unit cell parameters of the RENi4Ga com-pounds are comparable with those of the aluminium analogues but both se-ries of parameters are expanded in comparison with the corresponding val-ues of the binary compounds. This should be attributed to the size of theHYDROGEN IN RARE EARTH INTERMETALLICS 569TABLE ICrystallographic data and hydrogen capacity (n) at room temperature for theRENi5, RENi4Ga and RENi4Al (RE = Dy, Ho, Er) compoundsCompositionaAcAVA3n(H atoms/alloyformula unit)Ref.DyNi5 4.869 3.956 81.22 – 14DyNi4Ga 4.933 4.038 85.10 3.48DyNi4Al 4.933 4.037 85.07 3.68 14HoNi5 4.873 3.963 81.50 – 7HoNi4Ga 4.931 4.037 85.03 3.19HoNi4Al 4.935 4.044 85.29 3.75 7ErNi5 4.866 3.977 81.55 – 15ErNi4Ga 4.909 4.032 84.17 3.01ErNi4Al 4.910 4.034 84.22 2.63 15atomic radius of the metals. The atomic radii of gallium (1.41 Å) and alu-minium (1.43 Å) are comparable, but larger than the atomic radius of nickel(1.24 Å). One further point. Generally, in none of the three series of com-pounds the unit cell parameters follow the lanthanide contraction, i.e., a de-crease in unit cell parameters with the increasing atomic number of therare earth metal. In contrast, an increase of cell parameters for binary al-loys and a nonlinear decrease of cell parameters for ternary alloys havebeen observed, whereby HoNi4Ga exhibits a maximum in the gallium series.A detailed X-ray intensity analysis was carried out and the atomic coor-dinates for the RENi4Ga compounds were determined. Two crystallographi-cally inequivalent nickel sites: exist in the CaCu5 type of structure 2(c) inthe basal layer at z = 0 (atomic coordinates 1/3, 2/3, 0) of mixed atoms (Ni +RE) and 3(g) in the equatorial layer at z = 1/2 (atomic coordinates 1/2, 0,1/2) containing nickel atoms only. The rare earth atoms occupy the 1(a) sitesin the basal layer (atomic coordinates 0, 0, 0). It was found that the replace-ment of gallium atoms for nickel atoms takes place statistically and prefer-entially within the equatorial layer only. The unit cell expansion, largeralong the c-axis than along the a-axis, also indicates the substitution ofsmaller nickel atoms by larger gallium atoms preferentially within theequatorial layer.In order to determine the thermodynamic characteristics of theRENi4Ga–hydrogen systems, the alloys were exposed to hydrogen gas at dif-ferent pressures and temperatures. All alloys were easily activated, andwere found to absorb large quantities of hydrogen. Hydrogen capacities atroom temperature are given in Table I. It should be noted that the corre-570 B. [ORGI] ET AL.Figure 1. Unit cell volumes for the RENi5 (), RENi4Ga () and RENi4Al () (RE =Dy, Ho, Er) alloys.sponding binary compounds RENi5 do not show any significant hydrogenabsorption under the experimental conditions applied during the course ofour studies (below 700 K; less than 15 MPa).7,14,15Figures 2–4 illustrate the results of the PCDI measurements. The ther-modynamic parameters, the entropy and the enthalpy were determinedfrom the equilibrium pressures, at a ratio of 2 hydrogen atoms per alloy for-HYDROGEN IN RARE EARTH INTERMETALLICS 571Figure 2. PCDI for the DyNi4Ga–hydrogen system. () 255 K, () 273 K, () 293 K,() 313 K.Figure 3. PCDI for the HoNi4Ga–hydrogen system. () 255 K, () 273 K, () 293 K,() 313 K.mula unit (Table II). The corresponding values were calculated using theleast-square fit of the van’t Hoff equation ln peq = H/RT – S/R, where peq= plateau pressure, H = reaction enthalpy, R = universal gas constant, T =temperature, and S = reaction entropy. The Gibbs free energy of formation(G) was calculated according to G = H – TS and the corresponding val-ues for room temperature are also included in Table II. The set of relevantvalues for the RENi4Al–hydrogen systems is also included in Table I andTable II for comparison.572 B. [ORGI] ET AL.TABLE IIThermodynamic parameters and equilibrium pressures for the RENi4Ga–hydrogenand the RENi4Al–hydrogen (RE = Dy, Ho, Er) systems at the ratio of 2 hydrogenatoms per alloy formula unitSystemHkJ (mol H2)–1SkJ (mol H2)–1 K–1GkJ (mol H2)–1at 293 KPeqkPaat 293 KRef.DyNi4Ga–H2 –25.64 –105.59 +5.30 847HoNi4Ga–H2 –27.90 –111.41 +4.74 674ErNi4Ga–H2 –23.05 –104.76 +7.64 2390DyNi4Al–H2 –35.38 –116.40 –1.27 58 14HoNi4Al–H2 –34.30 –114.73 –0.68 92 7ErNi4Al–H2 –32.38 –122.22 +3.43 467 15Figure 4. PCDI for the ErNi4Ga–hydrogen system. () 255 K, () 273 K, () 293 K.The investigated RENi4Ga–hydrogen systems exhibit the following fea-tures. Ternary RENi4Ga compounds react easily with hydrogen to form un-stable hydrides at room temperature (peq > 101.3 kPa). This should be as-cribed to the replacement of one nickel atom in RENi5 by gallium, sincebinary RENi5 compounds are inert to hydrogen up to 700 K and 15 MPa.The hydrogen capacity of RENi4Ga decreases (Table I) with the increas-ing atomic number of the rare earth metal. A maximum hydrogen capacityof 3.48 hydrogen atoms per alloy formula unit at room temperature was ob-served for DyNi4Ga. The entropy, the enthalpy and the Gibbs free energy offormation increase (with a discrepancy observed for the holmium containingalloy), as does the hydrogen desorption equilibrium pressure, with the in-creasing atomic number of the rare earth metal.The results obtained for the RENi4Ga–hydrogen systems were comparedwith those for the RENi4Al–hydrogen systems (Tables I and II). One can seethat the systems with aluminium form more stable hydrides (lower valuesof H, S, G and desorption equilibrium pressures) but exhibit higher hy-drogen capacities than the corresponding systems with gallium. Further-more, the thermodynamic parameters of gallium and aluminium containingsystems generally follow different patterns.However, to extract a more general conclusion about the thermodynamicbehaviour during hydrogenation of these classes of intermetallic com-pounds, much more relevant data on similar systems should be gathered.REFERENCES1. J. H. N. van Voucht, F. A. Kuijpers, and H. C. A. M. Bruning, Philips Res. Rep. 25(1970) 133–139.2. M. H. Mendelsohn, D. M. Gruen, and A. E. Dwight, Nature 269 (1977) 45–47.3. J. S. Cantrell, T. A. Beiter, and R. C. Bowman, J. Alloys Compd. 207/208 (1994)372–376.4. S. Luo, W. Luo, J. D. Clewley, T. B. Flangan, and L. A. Wade, J. Alloys Compd. 231(1995) 467–472.5. F. Pourarian and W. E. Wallace, J. Less-Common Met. 87 (1982) 275–280.6. @. Bla`ina, B. [orgi}, and A. Dra{ner, J. Phys.: Condens. Matter 9 (1997) 3099–3105.7. @. Bla`ina, B. [orgi}, and A. Dra{ner, J. Mater. Sci. Lett. 16 (1997) 1683–1685.8. J. L. Bobet, S. Pechev, B. Chevalier, and B. Darriet, J. Alloys Compd. 267 (1998)136–141.9. E. L. Houston and G. A. D. Sandarac, J. Less-Common Met. 74 (1980) 435–443.10. M. Wad, Science and Technology in Japan 51 (1994) 54–57.11. @. Bla`ina and A. Dra{ner, J. Phys.: Condens. Matter 10 (1998) 4777–4783.12. @. Bla`ina, B. [orgi}, and A. Dra{ner, J. Mater. Sci. Lett. 17 (1998) 1585–1587.HYDROGEN IN RARE EARTH INTERMETALLICS 57313. K. Yvon, W. Jeitschko, and E. Parthè, J. Appl. Crystallogr. 10 (1977) 73–74.14. B. [orgi}, @. Bla`ina, and A. Dra{ner J. Phys.: Condens. Matter 7 (1995) 7209–7215.15. B. [orgi}, A. Dra{ner, and @. Bla`ina, J. Alloys Compd. 232 (1996) 79–83.SA@ETAKIntermetalni spojevi RENi4Ga (RE = Dy, Ho, Er) – kristalna strukturai svojstva hidridaBo`ica [orgi}, @elimir Bla`ina i Antun Dra{nerPrire|eni su novi trokomponentni intermetalni spojevi sastava RENi4Ga (RE =Dy, Ho, Er) i istra`eni metodom rentgenske difrakcije na prahu. Na|eno je da su svispojevi heksagonski i da kristaliziraju u prostornoj skupini P6/mmm s tipom struk-ture CaCu5, tj. istom simetrijom kao i njihovi prototipni dvokomponentni spojeviRENi5. Istra`ivana je i interakcija s vodikom. Na|eno je da svi intermetalici reagi-raju lagano i reverzibilno s vodikom i stvaraju hidride koji sadr`e do 3,48 vodikovihatoma po formulskoj jedinki slitine na sobnoj temperaturi. Entropija, entalpija iGibbsova energija stvaranja dobivene su iz vrijednosti ravnote`nih platoa u desorp-cijskim izotermama tlak–sastav. Termodinami~ki parametri i kapacitet vodika u su-stavima RENi4Ga–vodik uspore|eni su s odgovaraju}im vrijednostima dobivenim zaanalogne sustave s aluminijem i ukratko prodiskutirani.574 B. [ORGI] ET AL.

The RENi4Ga (RE = Dy, Ho, Er) Intermetallic Compounds – Crystal Structure and Hydride Properties

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