Structural studies of a new Laves phase alloy (Hf,Ti)(Ni,V)2 and its very stable hydride

Journal of

ALLOV5 AHD COMPOU~qD5 ELSEVIER

Journal of Alloys and Compounds 231 (1995) 90-94

Structural studies of a new Laves phase alloy (Hf,Ti)(Ni,V)2 and its very stable hydride E. R6nnebro

a, D . N o r 6 u s a, T . S a k a i b, M . T s u k a h a r a c

aDepartment of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden bOsaka National Research Institute, 1-8-31, Midorigaoka, Ikeda-shi, Osaka 563, Japan ClMRA Material R&D Co. Ltd., 5-50 Hachiken-cho, Kariya-shi, Aichi 448, Japan

Abstract

With Guinier-H~igg X-ray diffraction, a new AB 2 hafnium containing hexagonal C14 Laves type alloy has been identified, in a vanadium based, bcc, TiW3Ni0.56 solid solution alloyed with hafnium. The unit cell dimensions were determined to be a = 5.024(7) and c = 8.194(4)/~. It also forms a hydride with the expanded unit cell dimensions a = 5.234(4) and c = 8.579(7)/~, which was found to be very stable. It needed to be heated to above 600°C in vacuum to be dehydrided. From electrochemical measurements on the TiV3Ni0 56 solid solution it was concluded that both the activation and the reaction kinetics were improved by the presence of the new Laves phase. We assume that the formation of the Laves phase hydride helps to disintegrate the bulk alloy to give it a large and active surface area. From the structural refinements of the new Laves phase it was further suggested that hafnium formed a solid solution on the A-element site with one of the lighter atoms, probably titanium. Nickel was found on the two B-element sites, mixed with a light atom, probably vanadium. Keywords: X-ray diffraction; Laves phase metal hydride; Electrochemical behaviour; Rietveld refinement

1. Introduction Recently, it was shown that the favourable hydrogen storage properties of vanadium based solid solutions could lead to high specific capacities when used in metal hydride rechargeable batteries [1]. Properties related to activation and kinetics were, however, inferior to conventional A B 5 type alloys. To improve the electrochemical properties, attempts were m a d e to alloy the vanadium based solid solution with different metals. In the case of hafnium, the activation and kinetics improved coincidentally with the appearance of the new Laves phase (cf. Fig. 1). A hexagonal unit cell could index the new set of lines that a p p e a r e d in the Guinier-H~igg X-ray diffraction patterns, and by considering the p e a k intensities we could assume it to have a Laves C14 type structure. It is well known that Laves phases (AB2) are formed when large A atoms and small B atoms with the relation between their atomic diameters, dA/dB, approximately equal to 1.2-1.3 ( ~ ) combine together [2]. T h e y usually occur a m o n g the metals in the first, second and third transition series and have the type structures MgCu 2 (C15), M g Z n 2 (C14) or MgNi 2 (C36). 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

But the occurrence of Laves phases is not only determined by geometrical factors, since there are alloys within this diameter ratio that do not form Laves phases, for example HfNi 2 (dA/d B = 1.25) as stated by Elliot et al. [2]. Which of the Laves phases that are formed depends on the electron to atom ratio, s00 •

~

"

400

.~" 0~,

a00

~

200

~

loo 0~

TiV3Nio.56

~ loo

~ 200

~ 3oo

"

~ 400

~ soo Current density (A/kg)

600

700

Fig. 1. Discharge capacity as a function of discharge current for the

two alloys.

E. ROnnebro et al. / Journal of Alloys and Compounds 231 (1995) 90-94

or number of outer electrons (NOE) [2,8]. This was concluded by Elliot et al. [2] when comparing different titanium and zirconium alloys. For an average NOE of about 5.4 to 7, the C14 type is formed. When NOE is below 5.4 or over 7, the C15 type is formed, They also, in studying quasi-binary systems, state that there are no Laves type ]phases in the system Ti(V, Ni)2 (dA/d B =1.14), although both the electron:atom ratio and diameter ratio are favourable. Laves phases have further interstitial sites in the close-packed metal atom layer, large enough for hydrogen atoms to enter the structure. This does not usually change the space', group, but expands the unit cell dimensions and unit cell volume by about 2.9 A 3 per hydrogen atom [3]. The stability of the hydrides depends on the hole siz~, and the size is also crucial for site occupancy. The :minimum hole size is according to Westlake [3] 0.4 jk. A minimum H - H distance of 2.1 ,~ prevents a siraultaneous occupation of all interstitial sites [4]. Our new Laves phase was found to form a very stable hydride already during the first cycle. In this paper, we want to further characterise the structure of the new Laves phase and clarify the beneficial influence it has on the hydrogen storage properties of the coexisting vanadium based solid solution.

91

ethylene-hexafluoropropylene copolymer) onto a nickel mesh at 573 K. The results of the measurements are shown in Fig. 1. The maximum discharge capacities of both alloys are about 400 A h kg -1. The high rate discharge capacity of the TiV3Ni0.56Hf0.24 alloy is considerably larger than for TiV3Ni0.56. The discharge capacity at a current density of 400 A kg -1 is 305 A h kg -1 for TiV3Ni0.56Hf0.24, while it is almost zero for TiV3Ni0.56.

2.3. SEM studies The button was cut, polished and investigated with a JEOL820 scanning electron microscope. Energy dispersive X-ray analysis (EDS) was performed to determine the surface contents. As can be seen in Fig. 2, the alloy has two phases; the dark-grey phase is the vanadium solid solution phase, while the light-grey phase is the new hafnium-containing Laves phase. The analysed composition calculated from the EDS analysis in atomic percent is as follows: • Given composition of the two phase alloy TiV3Ni0.56Hf0.24

• Analysed composition of dark-grey phase TiVs.35Ni0.30Hf0.086

• Analysed composition of light-grey phase TiVl.o9Nil.15Hf0.69

2. Experimental

2.4. X-ray diffraction analysis 2.1. Sample preparation The alloy ingots of TiV3Ni0.56 and TiV3Ni0.56Hf0.z4 were prepared by arc melting of the pure metals of titanium (purity >99%), vanadium (purity >99%), nickel (purity >99%) and hafnium (purity >98%) on a water-cooled copper hearth in argon gas. For the electrochemical measurements, the as cast ingots were pulverised by hydriding at high temperature (<673 K) under high pressure hydrogen gas (<3.3 MPa). To facilitate the structural determination, the sample composition was adjusted to maximise the content of the new Laves phase of TiV0.s9Nil.33Hf0.50. This ingot was prepared the same way as mentioned before, and was brittle enough for mechanical crushing.

2.2. Electrochemical mea:¢urements

X-ray diffraction patterns were obtained from several samples: (1) button filed and ground to a powder; (2) hydrogenated; (3) dehydrogenated. The X-ray diffraction patterns were collected with a

-B

-A

The discharge capacities of the alloy electrodes of TiV3Ni0.56 and TiV3Ni0.56Hf0.24 were measured in a

half cell at discharge rates between 25 to 600 A kg -1 down to -0.7 V vs. a I-Ig-HgO reference electrode, with a 6 M K O H electrolyte and a Ni(OH)2 counter electrode. T h e e l e c t r o d e "was p r e p a r e d b y hot pressing a mixture of 20 wt.% copper-coated alloy powder with 10 wt.% F E P p o w d e r (Daikin Co., tetrafluoro-

Fig. 2. SEM picture of a polished sample of the vanadium based TiV3Nio.56alloyed with hafnium. A, the new Laves phase (light-grey phase); B, a vanadium rich solid solution (dark-grey phase).

E. R6nnebro et al. / Journal of Alloys and Compounds 231 (1995) 90-94

92

subtraction geometry Guinier-H~igg camera of diameter 80 ram, using monochromatized Cu K a I (A = 1.54060 A ) a n d silicon as internal standard, The X-ray pattern from the alloy was indexed with a hexagonal unit cell with dimensions a = 5.024(7) A and c = 8.194(4) ]k (Table 1) using the T R E O R program [5]. The observed and calculated 20 values are shown in Table 2. The alloy also has three diffuse reflexes from the dark-grey, vanadium solid solution phase (Fig. 2), indexed as a body centred cubic phase with a = 3.07 ]k. The hexagonal phase is better crystallized and is identified as the light-grey phase. This phase is isomorphous to MgZn 2, a C14-type Laves phase, space group P63/mmc (No. 194). Moreover, probably due to impurities, four reflexes were unidentified, The alloy as a button was hydrogenated by cycling the hydrogen pressure between 1 and 40 bar at 200°C in a sealed steel tube reactor, producing a powder with a very fine grain size. The sample was then cooled to

room temperature under 40 bar hydrogen pressure. The reflexes in the diffraction pattern are shifted to larger d values (A), i.e. larger cell edges with a = 5.234(4) and c -- 8.579(7) ]k (Table 1). The cell edges increased by 4.0 and 4.5% respectively, and the volume increased by 12%. The bcc phase also became hydrogenated, the a parameter increasing to 3.21 ]k. In order to prove that the increase in cell parameters is due to hydrogen atoms entering the molecular structure and not because smaller atoms have been kicked out of the lattice, we tried to dehydrogenate at elevated temperatures in vacuum. The alloy had to be heat treated at 650°C before it released the hydrogen, as could be followed by the decreasing unit cell dimensions (Table 1), thus confirming that the new Laves phase was hydrogenated. The final unit cell dimension were slightly larger than originally, indicating a small amount of hydrogen trapped in this very stable hydride.

2.5. Structure refinements Table 1 Cell parameters for (Hf,Ti)(Ni,V)2 determined from the Guinier-

Hfigg data using

the TREOR program [5]

Alloy filed and ground to a powder Hydrogen-cycling at

TiV6.z6Nil.°3Hf°.37

TiV°.s9Nil.33Hf°.s°

(dark-grey phase)

(light-grey phase)

a = 3.07 bcc

a = 5.024(7) c = 8.194(4) a = 5.234(4)

a = 3.21 bcc

200°C,40bar

c = 8.579(7)

Dehydrogenation at 650 °C

a = 3.07 bcc

a = 5.052(8)

in vacuum Table 2 Observed

c = 8.270(8)

and calculated 20 values from the TREOR program [5]

for the Laves phase,, space group. P63/mmc'.3 with a = 5.024(7) and c = 8.194(4): unit cell volume is 179.17 A (M20 = 26)

20ohs 20.405

20ca~c 20.392

4.349

002

21.719

21.673

4.089

1

101 102

23.129 29.933

23.124 29.930

3.842 2.983

2 4

103 200 1 12 20 1

38.883 41.501 42.170 42.999

38.897 41.469 42.158 42.976

2.314 2.174 2.141 2.102

100 12 98 55

004 10 4 203

44.211 49.046 53.724

44.173 49.114 53.825

2.047 1.856 1.705

5 5 4

10 5

60.221

60.297

1.535

7

300 213

64.197 66.255

64.154 66.281

1.450 1.410

5 35

115 006

68.142 68.645

68.274 68.668

1.375 1.366

3 21

10 6

72.624

72.476

1.301

6

2 14

73.814

73.827

1.283

2

220

75.649

75.644

1.256

14

2 06

83.481

83.507

1.157

17

hkl

10 0

110

205

35.730

72.101

35.709

72.092

dobs(A)

2.511

1.309

lo,s 3

52

20

T O improve the structure refinements, X-ray powder diffraction data were collected on a Stoe Stadi/P diffractometer, which gives better defined line shapes than Guinier-H~igg data. The structures were refined using the Rietveld program DBW3.2S [6]. Since

titanium and vanadium have very similar X-ray s c a t tering factors we could not differentiate them in the refinement procedure. However, when comparing atomic radii and electronegativities, it seems reasonable that titanium (r = 1.47 ]k, Xp = 1.5) substitutes hafnium ( r = 1.67 /*k, XP = 1.3) at the A-site, and vanadium (r --- 1.34 2k, Xp = 1.6) substitutes nickel (r = 1.24/*k, Xp = 1.8) at the B-sites. The C14 Laves phase is described as space group P63/mmc (No. 194) with three atomic positions 2a, 4f and 6h, where the last two have one refineable atomic parameter each (cf. Table 3). In the first refinement, hafnium was put in the 4f position and nickel in the 2a and 6h positions. The derived Bragg R factor was 0.148, R v = 0 . 1 5 6 and Rw = 0.064. Then titanium and vanadium were substituted in the hafnium and nickel positions, leading to an R B value of 0.082, R F = 0.089 and R w = 0.072 (cf. Table 3). The number of refined parameters was 15,

including cell parameters, half-width parameters (U,V,W),coordinates, occupancy factors and temperature factors. However, the haft-width parameters could not be refined at the same time. The observed and calculated diffraction pattern and the difference plot from the Rietveld refinement procedure is shown in Fig. 3. From the occupancy factors (cf. Table 3), the formula was calculated to be (I-Ifo.57Ti0.43)(Ni0.85V0.15)2. Comparing this composition with the one determined

E. ROnnebro et al. I Journal of Alloys and Compounds 231 (1995) 90-94

93

Table 3 Atomic parameters for (Hf,Ti)(Ni,V):, space group P63/mmc (Z = 4), refined from X-ray powder diffraction data using the DBW3.2S program [2]: R B = 0.082, R F = 0.089 and R w = 0.072

Atom

Site

x

y

z

B~so (~2)

N

Hf (Ti,V)I Nil (Ti,V)2 Ni2 (Ti,V)2

4f 4f 2a 2a 6h 6h

1/3 1/3 0 0 0.827(2) 0.827(2)

2/3 2/3 0 0 -0.827(2) -0.827(2)

0.062(1) 0.062(i) 0 0 1/4 1/4

0(2) 0(2) 0(2) 0(2) 0(2) 0(2)

2.30 1.70 0.78 1.22 5.92 0.08

r,

0

r

i=

o

'

z6

. . . . . . . . .

36

. . . . . . . . .

46

. . . . . . . . .

~6

. . . . . . . . .

66

. . . . . . . . .

76

. . . . . . . . .

86

'

'

Fig. 3. The final plot from the Rietveld refinement for the Laves phase. The calculated (top) and observed (center) diffraction patterns, together with the difference plot, are shown in the figure. The four impurity peaks are marked in the observed pattern.

from EDS, i.e. comparing Ti0.43V0.30Ni1A7Hf0.57 with TiV1.09NiH5Hf0.69, the result from the Rietveld refinement should be more reliable, since using E D S implies larger standard deviations as the three ZAF factors (atomic number effect (Z), absorption (A) and fluorescence (F)) cannot be calculated so precisely. Proceeding from this model, except using the cell parameters of the hydride;, the metal atom structure of the Laves phase hydride, was refined to R B = 0.224, R F = 0.211 and R w = 0.1,39. The poor agreement we attribute to line width problems of the Bragg peaks due to poor crystallinity in the hydride. The tempera-

ture factors, Biso, had to be locked and U,V,W could not be refined at the same time. The z-coordinate in the 4f position increased from z = 0.062(1) to z = 0.067(2). The x-coordinate in the 6h position decreased from x = 0.827(2) to x = 0.822(6).

3. Structural aspects of the C14 Laves phase and its hydride The new Laves type phase we wish to describe with the composition (Hf,Ti)(Ni,V)2 belongs to the hexagon-

94

E. ROnnebro et al. / Journal of Alloys and Compounds 231 (1995) 90-94

al C14-type (MgZnz). This phase fulfills the criteria for the occurrence of a Laves phase as the average of d A / d B is 1.22, which is very close to the ideal value and N O E was calculated to be 7.5 (4 + 2(10- 0.85 + 5. 0.15)=22.5; 22.5/3=7.5). Obviously, the combination of these elements forms a suitable geometrical arrangement for a Laves phase to be formed, This Laves phase, (Hf,Ti)(Ni,V)2, of space group P 6 3 / m m c (No. 194), has its z-coordinate in the 4f position and x-coordinate in the 6h position (Table 3) close to the ideal values of 1/16 and - 1 / 6 respectively for a close-packed structure (c/a = 1.631; the ideal value is 1.633). The smaller B atoms (Ni, V) occupy the corners of tetrahedra, joined alternately point-to-point and base-to-base in vertical rows throughout the structure (Berry et al. [7]). This leads to a network of tetrahedra which include holes in which the larger A atoms (Hf,Ti) are situated. The A atoms are arranged in a hexagonal pattern in the order A B A B A B . In order to estimate how many hydrogen atoms the molecule can absorb, one can use the simple criterion that the volume expansion is about 2.9 A 3 per hydrogen atom [3], which in this case gives 2 hydrogen atoms per molecule (AV= 24.4 ] k 3 ~ 8.4 H atoms per unit cell ~ ca 2 H atoms per molecule as Z = 4). Another way to calculate and predict the hydrogen affinities in 3d transition metals of cubic and hexagonal phases was outlined by Bernauer et al. [8]. As the end of the equilibrium plateau is reached when the 3d band is half-full, the equation H / M = 5 - D E C can be applied to estimate the number of stored hydrogen atoms (H) per number of metal atoms (M). D E C stands for the d-electron concentration. H / M was for this Laves phase calculated to 1.9 in the following way: F o r m u l a : (Hf0.57Ti0.43)(Ni0.ssV0.15)2

Sum of 3d(5d) electrons: (0.57 +0.43)2 +((0.3 + 0.04)/0.4)8 +(0.06/0.4)3 = 9.25 D E C = 9.25/3 = 3.1 ~ H / M = 1.9 So, both calculations estimate the composition to be (Hf,Ti)(Ni,V)2H ~ for the Laves phase hydride. T o determine the positions for the hydrogen atoms, we are now proceeding with neutron diffraction experiments.

4. Conclusion For a metal hydride to be suitable for energy storage application, it should be fairly unstable so the sorption-desorption process can proceed close to ambient temperatures and pressures. The Laves phase hydride (Hf,Ti)(Ni,V)2H 2 is very stable so it does not participate in the hydrogen-dehydrogenation cycling, but is easily hydrogenated. It becomes a hydride already during the first activation cycle and thus helps to disintegrate the bulk alloy, giving it a large active surface area. The role of this new Laves phase (Hf,Ti)(Ni,V)2, as a second phase, is to improve the activation and kinetics for the vanadium based solid solution alloy Ti~Ni0.56.

Acknowledgment The Swedish National Board for Industrial and Technical Development is acknowledged for financial support.

References [1] M. Tsukahara, K. Takahashi, T. Mishima, I. Uehara, T. Sakai, K. Oguro, N. Kuriyama and H. Miyamura, Nippon kinzoku gakkai koen gaiyou (Abstracts of the Japan Institute of Metals), 133 (1993) 480 (in Japanese). [2] R. P. Elliot and W. Rostocker, Trans. Am. Soc. Met., 50 (1958) 617. [31 D.G. Westlake, J. Less-Common Met., 90 (1983)251. [41 A.C. Switendick, Z. Phys. Chem. N.F., 117 (1979) 891 [5] P.-E.Werner, L. Eriksson and M. Westdahl, J. Appl. Crystallogr., 18 (1985) 367. 16] D.P. Wiles, A. Sakthivel and R.A. Young, Program DBW3.2S, School of Physics, Georgia Institute of Technology, Atlanta,

1988. [7] R.L. Berry and G.V. Raynor, Acta Crystallogr., 6 (1953) 178. [81 O. Bernauer, J. T6pler, D. Nor6us, R. Hempelmann and D. Richter, Int. J. Hydrogen Energy, 14 (1989)187.