Scientific paper
Crystal Structure snd Hydrogen Sorption Properties
of the YNi5_XGaX Alloys
Antun Drasner and Zelimir Blažina*
Laboratory for Solid State Chemistry, Department of Materials Chemistry, Ruler Bošković Institute,
PO Box 1016, 10001 Zagreb, Croatia
* Corresponding author: E-mail: blazina@irb.hr Phone: +385 1 4561 111; Fax: +385 1 4680 098
Received: 31-03-2008
Dedicated to the memory of Professor Ljubo Golic
Abstract
Alloys of the general composition YNij ^Ga^ were prepared and studied by X-ray powder diffraction. It was found that in the YNi5-xGax system the single phase region exists up to the composition YNi2Ga3. The hexagonal structure of the prototype compound YNij (CaCuj type, space group P6/mmm) is preserved in all alloys for x < 2. Within the composition region YNi3Ga2 - YNi2Ga3 the existing single phase alloys crystallize also in the hexagonal symmetry with the same space group P6/mmm, but with a larger unit cell of the YCo3Ga2 type. All single phase alloys have been exposed to hydrogen gas, whereby binary YNij, and ternary alloys with the YCo3Ga2 type of structure do not show any significant hydrogen absorption up to 5 MPa of hydrogen. All ternary alloys having the CaCuj structure react readily with hydrogen. The pressure composition desorption isotherms for the corresponding alloy-hydrogen systems were determined and it was found that the equilibrium pressure and the hydrogen capacity decrease with the increased gallium content. The change of entropy, the change of enthalpy and the change of the Gibbs free energy of formation for the alloy-hydrogen systems have also been extracted from the pressure composition desorption isotherms. The relevant thermodynam-ic parameters confirmed that the stability of the hydride increases with the increased amount of gallium.
Keywords: Yttrium-nickel-gallium alloys; crystal structure; hydrogen sorption
1. Introduction
Since hydrogen was assigned as a promising fuel source for the future, considerable interest has been raised for both fundamental and practical research of intermetal-lic compounds. This is because many intermetallic compounds absorb large amounts of hydrogen and are therefore considered as potential materials for hydrogen storage purposes.
Among different intermetallic compounds, those of the general composition AB5 play an important role and therefore have been extensively investigated. Primarily, this is due to the ability that LaNi5 absorbs large amounts of hydrogen (up to 6.7 hydrogen atoms per formula unit), and exhibits at moderate pressures and at room temperature a wide equilibrium plateau1. At present some of the LaNi5-based compounds are commercially traded as hydride-forming alloys with a wide vari-
ety of applications including fuel cells and negative electrode materials for rechargeable nickel-metal hydride (Ni/MH) batteries.
The effect of replacement of either components in selected RET5 compounds (RE = rare earth metal, T = transition 3d metal) has frequently shown that the sub-stituent strongly affects the crystal structure of the prototype compound and the thermodynamic characteristics of the corresponding RET5- hydrogen system. Some typical examples can be found for the alloys of the composition RENi5-xAlx2-4, where the stability of a particular crystal structure type is composition dependent, while the equilibrium pressure, as well as the hydrogen capacity of the RENi5-xAlx - hydrogen system, is generally drastically reduced with the increased aluminium content. Some other examples of influence of substitution on the structural, hydrogen sorption and magnetic properties of AB5 compounds can be found in the systems
RECo5-xGax5, CeNÌ5-xGax6'7, CeNÌ5-xCux8, LaNÌ5-xSnx9, MmNi5 xSnx (Mm = mìshmetal)10 DyNì5 ^Al^11 and
5X X	^ 5x x
DyNÌ5-xGax12.
The results on the crystal structure and hydrogen sorption properties of the YNÌ5-XGaX alloys reported here represent the contÌnuatÌon of our systematÌc studÌes on structural and hydrogen sorptÌon propertÌes of selected RENÌj compounds where nÌckel has partÌally been replaced by other metals or metalloÌds. To the best of our knowledge a systematÌc study of the YNÌ5-xGax -hydrogen system has not been performed so far. The only pub-lÌshed data for thÌs system report on the CaCuj type of structure for YNÌj13 and for YNÌ4Ga14, and on hydrogen sorptÌon propertÌes for YNÌ4Ga14. There are also some data on absorptÌon of about 4 H atoms per YNÌj formula unÌt at pressures above 30 MPa11516 as well as on absorptÌon of 4.4 H atoms at 1.2MPa17.
2. Experimental
The startÌng materÌals used Ìn thÌs ÌnvestÌgatÌon were supplÌed by Johnson Matthey, UK (yttrÌum and nÌckel of 3N purÌty; gallÌum 5N purÌty) and Messer CroatÌa Gas, CroatÌa (hydrogen of 99.999% purÌty). Sample alloys of the weÌght of about 2.0 g were prepared by argon arc meltÌng. To ensure homogeneÌty the alloys were turned upsÌde down and remelted several tÌmes. The weÌght loss of the materÌal was checked and was found to be neglÌgÌble. Therefore Ìt Ìs assumed that the startÌng composÌtÌon of the alloys Ìs preserved. Good sÌngle phase alloys were obtaÌned after annealÌng Ìn vacuum at 1123 K for 7 days.
The X-ray powder dÌffractÌon patterns were ob-taÌned on a PhÌlÌps PW 1880 dÌffractometer usÌng mono-chromated Cu Ka radÌatÌon. The lÌne posÌtÌons were corrected usÌng sÌlÌcon powder (5N purÌty, Johnson Matthey, UK) as an Ìnternal standard. The ÌntensÌtÌes were calculated wÌth the "Lazy PulverÌx" program18.
The pressure-composÌtÌon Ìsotherm (PCI) desorp-tÌon measurements were carrÌed out Ìn a staÌnless-steel apparatus of a SÌeverts type Ìn a temperature range from 253 K to 400 K and wÌth hydrogen gas at pressures up to 3.5 MPa. The apparatus has been descrÌbed Ìn detaÌls elsewhere19. PrÌor to PCI measurements the alloys were actÌ-vated by heatÌng under hydrogen (700 K, 5 MPa). It was assumed that the actÌvatÌon process has been completed when the repeated amount of released hydrogen remaÌned constant. DesorptÌon measurements were made on alloys completely saturated wÌth hydrogen by releasÌng small amounts of the gas. The equÌlÌbrÌum pressure was measured after 15 mÌn, assumÌng that then the equÌlÌbrÌum was reached. The hydrÌde composÌtÌon was calculated from the pressure-volume-temperature data, and the thermody-namÌc data of the alloy-hydrogen system were extracted from the PCI data applyÌng the Van't Hoff equatÌon.
3. Results and Discussion
3. 1. Structure
Alloys of the composÌtÌon YNÌ5-xGax (x = 0.5, 0.75, 1.25, 1.5, 2, 2.5 and 3) were prepared and theÌr phase equÌlÌbrÌum was studÌed. The correspondÌng data for the alloys wÌth x = 0 and x = 1 whÌch have been publÌshed elsewhere14 are here also Ìncluded for comparÌson. The X-ray powder dÌffractÌon data revealed that all these sÌngle phase materÌals are of the hexagonal symmetry but adopt dÌfferent structure types. All alloys wÌth x <2 are of the CaCuj type (space group P6/mmm), but the alloys wÌth x > 2 crystallÌze wÌth a larger unÌt cell of the YCo3Ga2 type (space group P6/mmm) 20. ThÌs Ìs true for both as-cast and annealed alloys. These two structure types are closely related (Table 1). BrÌefly, Ìn the CaCu5 type there exÌst two layers of atoms. The basal layer at z = 0 contaÌns nÌckel atoms (2c sÌtes) and rare earth atoms (1a sÌtes), whÌle the equatorÌal layer at z = 1/2 contaÌns nÌckel atoms only (3g sÌtes). The YCo3Ga2 type can be regarded as beÌng derÌved from the CaCu5 type by shÌftÌng one thÌrd of the rare earth
Table 1. AtomÌc coordÌnates for varÌous YNÌ5-xGax alloys
Atom Position
Coordinates x	y	z
Occupation
YNÌ5
(Space group P6/mmm,CaCu5 type)
Y	1(a)	0	0	0	1
NÌ	2(c)	1/3	2/3	0	1
NÌ	3(g)	1/2	0	1/2	1
YNÌ3Ga2					
(Space group P6/mmm, YCo3Ga2			type)		
Y	1(b)	0	0	1/2	1
Y	2(c)	1/3	2/3	0	1
NÌ	6(m)	0.175	0.35	1/2	0.33
NÌ	3(g)	1/2	0	1/2	0.33
Ga	6(m)	0.175	0.35	1/2	0.67
Ga	3(g)	1/2	0	1/2	0.67
NÌ	6(i)	0.31	0	0	1
YNÌ2,5Ga2,5					
(Space group P6/mmm, YCo3Ga2			type)		
Y	1(b)	0	0	1/2	1
Y	2(c)	1/3	2/3	0	1
NÌ	6(m)	0.183	0.366	1/2	0.25
NÌ	3(g)	1/2	0	1/2	1
Ga	6(m)	0.183	0.366	1/2	0.75
NÌ	6(j)	0.33	0	0	0.5
Ga	6(i)	0.33	0	0	0.5
YNÌ2Ga3					
(space group P6/mmm, YCo3Ga2 type)					
Y	1(b)	0	0	1/2	1
Y	2(c)	1/3	2/3	0	1
Ga	6(m)	0.184	0.368	1/2	1
Ga	3(g)	1/2	0	1/2	1
NÌ	6(i)	0.31	0	0	
Table 2. Unit cell parameters and cell volumes for the YNÌ5_xAlx alloys
Composition	a (±0.002 A)	c (±0.001 A)	da	Y (±0.004 A3)	Literature
YNi5	4.892	3.975	0.813	82.38	[14]
YNi5 YNi4,5Ga 0.5	4.891	3.961	0.810	82.06	[13]
	4.907	4.012	0.818	83.66	
YNi4.25Ga 0.75 YNi4Ga	4.914	4.036	0.821	84.40	
	4.924	4.039	0.820	84.90	[14]
YNi3 75Ga125	4.935	4.044	0.819	85.29	
YNi3.5Ga,5	4.984	4.046	0.812	87.04	
YNi3Ga2	8.680	4.130	0.475	269.47	
YNi2.5Ga2.5	8.845	4.076	0.461	276.15	
Ni2Ga3	8.778	4.143	0.472	276.45	
atoms from the basal plane along the z-axis into the equatorial plane (1b sites), whereby the parameter a is increased by a factor of V3, while the parameter c is increased up to 2.5%. The mutual structure relationship of these crystal structures has been discussed in details else-where3, 20-22. The unit cell parameters and cell volumes for all alloys are given in Table 2. It can be seen that they increase with the increasing content of gallium. This should be ascribed to the larger atomic radius of Ga (1.41 À) compared to that of Ni (1.24 À).
A detailed intensity analysis was carried out. For the alloys of the CaCu5 type of structure it was determined that gallium atoms replace nickel atoms within both available nickel sites (3g and 2c sites; Table 1).
For the alloys with the larger unit cell of the YCo3Ga2 type substitution of gallium for nickel atoms (Table 1) takes place either preferentially within the equatorial layer (YNi3Ga2, YNi2Ga3) or within 6m and 6j nickel sites at both layers (YNi25Ga25). The atomic arrangement of gallium (nickel) atoms over these two layers can probably explain the rather odd values of cell parameters at the composition YNi2.5Ga2.5. Namely, partial moving of larger gallium (compared to nickel) atoms from the equatorial into the basal plane at this particular composition is related with an increase of parameter a and a decrease of parameter c. At higher gallium content (composition YNi2Ga3) the removing of gallium atoms into the equatorial layer only, reflects again in decrease (increase) of parameters a (c), respectively.
for YNi4Ga which have been published elsewhere14 are also here included for comparison reasons. The maximum hydrogen capacity for the alloys which absorb hydrogen was extracted from the PCI data and it was found that they absorb up to 3.9 hydrogen atoms per alloy formula unit (YNi4.5Ga0.5 at 253 K).
The change of entropy, ^S, and the change of enthalpy, AH, have been calculated for the metal-hydrogen systems at the ratio of 2 hydrogen atoms per alloy formula unit. A least-square fit of the Van't Hoff equation lnpe^ = AH/RT - AS/R, where is the plateau pressure, R the universal gas constant equal to 8.314 J/K mol, and T the temperature, was applied. The change of the Gibbs free energy of formation at room temperature (AG) was calculated according to AG = AH - TAS. The corresponding values for the change of entropy, the change of enthalpy and the change of the Gibbs free energy of formation are summarized in Table 3.
Table 3.The thermodynamic parameters AH, AS and AG for the
YNi5-xGax - hydrogen system at 2 H atoms / alloy formula unit
Composition	AH	AS	AG
	( kJ/mol H2)	(J/mol H2)	(kJ/mol H2)
			(at 293 K)
YNi3.5Ga„.5	23.98	-109.73	+8.171
YNi3.25Gao.75	-29.46	-122.20	+6.339
YNi4Ga	-30.76	-113.90	+2.613
YNi3.75Ga,25	-30.19	-100.59	-0.723
YNi3.5Ga,5	-26.35	-86.46	-1.021
3. 2. Hydride Properties
In order to determine the thermodynamic properties of the YNi5-xGax - hydrogen systems single phase alloys were crushed and pulverized into powder which was then exposed to hydrogen gas at different pressures and temperatures.
Ternary alloys with the CaCu5 structure and with 0.5 < x < 2 were easily activated. However, up to 700 K and up to 5 MPa of hydrogen binary YNi5 and ternary alloys with 2 < x < 3 are inert to hydrogen. Fig. 1 illustrates the results of the desorption PCI measurements. The results
A brief analysis of the relevant data permits us to extract the following features of the YNi5-xGax - hydrogen system. The alloys of the CaCuj structure within the YNi4.5Ga0.5 and YNi3.5Ga1.5 composition region react easily with hydrogen. With the increasing gallium content the hydrogen equilibrium pressure decreases as does the hydrogen capacity. The change of the Gibbs free energy of formation (Table 3) decreases as the gallium content in the alloys increases. This indicates that the relative stability of the alloy-hydrogen systems increases with the increasing content of gallium since it is known that the reaction is
spontaneous in the direction of decreasing free energy. The above is in good agreement with the observed trend of the equilibrium pressure (Fig. 1), which also can be regarded as the measure of the stability of the corresponding alloy-hydrogen system.
Binary YNi5 and ternary alloys with 2< x < 3, i.e., those with the larger unit cell of the YCo3Ga2 type, are inert to hydrogen up to 700 K and 5 MPa. Since binary YNi5 does not absorb hydrogen under the above mentioned condition, but ternary alloys with small amounts of gallium do, it can be assumed that YNi5 absorbs hydrogen at very high pressures, i.e., that the data from refs.1, 15, 16 are the accurate ones. However, although gallium substitution for
Figure 1. Pressure composition desorption isotherms for the a) YNi4 5Ga05 - hydrogen, b) YNi4 25Ga0 75 - hydrogen, c)YNi4Ga - hydrogen, d) YNi3 75Gaj 25 - hydrogen and e) YNi3 5Gaj5 - hydrogen system.
nickel drastically reduces the very high equilibrium pressure of the binary YNi5 without decreasing too much the hydrogen capacity, the lack of a reasonable wide plateau at ambient temperatures, does not assign this system as suitable material for hydrogen storage purposes. One further point. At the moment it is not quite clear how the structural change influences the hydrogen sorption properties of the YNi5-xGax alloys. It seems that the explanation should be in tetrahedra which are built up of different kind of atoms in the larger unit cell than those in the smaller cell and are more distorted in the former case. However, for an even general conclusion much more relevant studies must be performed.
4. Conclusion
The structural study of the YNi5-xGax alloys revealed their single phase nature up to the composition YNi2Ga3. All single phase alloys are of the hexagonal symmetry, but depending on the gallium content they crystallize with the CaCuj or the YCo3Ga2 type of structure. Both structures are closely related, whereby the
smaller cell of the CaCu5 type is stabilized at lower gallium contents, while the larger cell of the YCo3Ga2 is stable at higher gallium contents. Only alloys of the CaCu5 type of structure and within the composition range YNi4 5Ga0 5 and YNi3 5 Ga! 5 react readily and reversibly with hydrogen. The hydrogen capacity and the equilibrium pressure decreases with the increased content of gallium. This behavior is consistent with other related RENi5-xGax systems.
5. Acknowledgement
The support of this research by the Ministry of Science Education and Sport of the Republic of Croatia under Project 098-0982904-2941 is gratefully acknowledged.
6. References
1.	J. H. N. van Voucht, F. A. Kuijpers and H. C. A. M. Bruning, Philips Res. Rep. 1970, 25, 133-139.
2.	M. H. Mendelsohn, D. M. Gruen and A. E. Dwight, Nature 1977, 269, 45-47.
3.	T. Takeshita, S. K. Malik and W. E. Wallace, J. Solid State Chem. 1978, 23, 271-274.
4.	B. Šorgić, A. Drasner and Ž. Blažina, J. Alloys Comp. 1996, 232, 79-83.
5.	Ch. Routsi, J. Alloys Comp. 1998, 27, 275-277.
6.	J. Tang, L. Li, C. J. O'Connor and Y. S. Lee, J. Alloys Comp. 1994, 207/208, 241-244.
7.	H. Flandorfer, P. Rogl, K. Hiebl, E. Bauer, A. Lindbaum, E. Gratz, C. Godart, D. Gignoux and D. Schmitt, Phys. Rev. B 1994, 50, 15527-15541.
8.	A. T. Pedziwiatr, F. Pourarian and W. E. Wallace, J. Appl. Phys. 1984, 55, 1987-1989.
9.	J. M. Hughes, J. S. Cantrell and R. C. Bowman, Jr, Int. J. Hydrogen Energy 1997, 22, 347-349.
10.	V. losub, M. Latroche, J. M. Joubert, A. Percheron-Guegan, Int. J. Hydrogen Energy 2006, 31, 101-108.
11.	G. I.Miletic, Ž. Blažina, J. Magn. Magn. Mater. 2004, 268, 205-211.
12.	G. I. Miletić, A. Drašner and Z. Blažina, J. Alloys Comp. 2000, 296, 170-174.
13.	A. E. Dwight, Trans Am. Soc. Metals 1961, 53, 479-500.
14.	Ž. Blažina, B. Šorgić and A. Drasner, J. Mater. Sci. Lett. 1998,17, 1585-1587.
15.	T. Takeshita, K. A. Gschneidner, Jr., D. K.Thome and O. D. McMasters, Phys. Rev.B, 21 (1980) 5636-5641
16.	T. Takeshita, K. A. Gschneidner, Jr., J. F. Lakner, J. Less-Common Met. 1981, 78, 43-47.
17.	R. H. van Essen and K. H. J. Buschow, J. Less-Common Met., 70 (1980) 189-198
18.	K. Yvon, W. Jeitschko and E. Parthé, J. Appl. Crystallogr. 1977, 10, 73-74.
19.	A. Drasner and Ž. Blažina, J. Less-Common Met. 1990,163, 151-157.
20.	M. A. Fremy, D. Gignoux, J. M. Moreau, D. Paccard and I. Paccard, J. Less-Common Met. 1985, 106, 251-255.
21.	Ch. D. Routsi, Y. K. Yakinthos and E. Ressouche, J. Alloys Comp. 1997, 256, 61-64.
22.	P. Schobinger-Papamantellos, D. P. Middleton and K. H. J. Buschow, J. Alloys Comp. 1994, 206, 189-193.
Povzetek
Pripravili smo zlitine s splošno sestavo YNi5_xGax in jih preiskali s praskovno rentgensko difrakcijo. Ugotovili smo, da v sistemu YNi5_xGax obstoja le ena faza do sestave YNi2Ga3. Heksagonalna struktura prototipne spojine YNi5 (CaCu5 tip, prostorska skupina P6/mmm) se ohrani v vseh zlitinah za x < 2. Znotraj področja sestave YNi3Ga2 - YNijGaj kristalizirajo obstoječe enofazne zlitine s heksagonalno simetrijo in z isto prostorsko skupino P6/mmm vendar z večjo osnovno celico tipa YCo3Ga2. Binarne YNi5 in ternarne zlitine strukturnega tipa YCo3Ga2 ne absorbirajo znatnih množin vodika do 5 MPa. Vse ternarne zlitine strukturnega tipa CaCu5 pa z vodikom reagirajo. Določili smo desorpcijske izoter-me vodika v odvisnosti od sestave zlitine in tlaka vodika in ugotovili, da se ravnotežni tlak in absorpcijska kapaciteta znižujeta pri povečani vsebnosti galija. Iz desorpcijskih izoterm smo določili tudi spremembe entropije in entalpije ter spremembe Gibbsove proste energije nastanka sistema zlitina-vodik. Termodinamski parametri so potrdili, da se stabilnost hidrida povečuje z visjo vsebnostjo galija.