Surface segregation in LaNi5 induced by oxygenTh. von Waldkirch and P. Zürcher

Citation: Applied Physics Letters 33, 689 (1978); doi: 10.1063/1.90531 View online: http://dx.doi.org/10.1063/1.90531 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/33/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Particle size distribution of Ni microprecipitates in LaNi5 used for hydrogen storage Appl. Phys. Lett. 41, 999 (1982); 10.1063/1.93370 61Ni Mössbauer measurements of nickel microprecipitates produced in LaNi5 by cyclic hydrogen absorption anddesorption Appl. Phys. Lett. 40, 477 (1982); 10.1063/1.93140 Electronic structure and surface oxidation of LaNi5, Er6Mn23, and related systems J. Appl. Phys. 51, 5847 (1980); 10.1063/1.327544 Nuclear magnetic resonance in LaNi5 J. Appl. Phys. 50, 2046 (1979); 10.1063/1.327103 Magnetic resonance of Gd IN LaNi5 and LaNi5 hydride AIP Conf. Proc. 29, 686 (1976); 10.1063/1.30519

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3J. Wolter, Phys. Lett. A 42, 115 (1972). 4M. Yamanishi, T. Kawamura, S. Takada, and N. Mikoshiba, Report of Applied Electron Physics Section, the Japan Society of Applied Physics, No. 362, 1975 (in Japanese) (unpublished) .

5K. Tsubouchi, S. Minagawa, and N. Mikoshiba, J. Appl. Phys. 47, 5187 (1976).

6S. Minagawa, T. Kugaya, K. Tsubouchi, and N. Mikoshiba. 1977 Ultrasonics Symposium Proceedings, Phoenix, IEEE 77 CH 1264-1SD (IEEE, New York, 1977), p. 629.

Surface segregation in LaNis induced by oxygen Th. von Waldkirch and P. Zarcher

Laboratory of Solid State Physics, ETH Honggerberg, 8093 Zurich, Switzerland (Received 3 July 1978; accepted for publication 16 August 1978)

LEED and AES studies of LaNis single crystals cleaved in UHV and exposed to oxygen and hydrogen show that surface segregation of La is induced at room temperature by oxygen, but not by hydrogen. This segregation is driven by the chemical exchange reaction (La-Ni)+O--!(La-O)+Ni. A surface-layer model is derived supporting the important implications of the segregation for the excellent kinetics in hydrogenation and the resistance to contaminants.

PACS numbers: 68.20.+1, 64.75.+g, 82.65.Nz, 81.60.Bn

Due to their ability to store reversibly large amounts of hydrogen-a promising energy carrier of the future­some intermetallic compounds have gained high interest in the last few years. 1-3 LaNi5 is especially promising because it shows large hydrogen capacity together with good kinetics and high reSistance against contamina­tion. 1,4 While extensive studies exist on bulk features, 5,6 the nature of the surface has so far obtained little at­tention only. However, activated LaNi5 is a fine powder, and its surface therefore represents an important factor in the hydriding process. Very recently, the first surface study on polycrystalline LaNis samples by XPS and magnetic measurements was presented. 7

The air-exposed samples revealed an abundant lan­thanum content of the surface in oxide or hydrOXide form together with metallic nickel precipitations which increased with the number of hydrogenation cycleso This effect was recognized as a self-restoring mecha­nism of the active surface, since metallic nickel is­like other transition metals-known to be capable of splitting the hydrogen molecule. 8

Surface segregations in intermetallic compounds are not new. 9 The one found in LaNi5,7 however, is of special interest, since it is related to the excellent hydriding properties of this compound. We present the first Auger electron spectroscopy (AES) and LEED investigations of LaNis single crystals, cleaved in an ultrahigh vacuum (UHV) of 6X 10.10 Torr, which char­acterize the mechanism of this segregation more precisely.

The single crystals were prepared from 99.9% pure La (Reso Chern.) and 99.999% pure Ni (Koch Light) in UHV by the Czochralski method (10). The rod-shaped crystals could easily be cleaved in UHV, producing an uncontaminated yet not plane-shaped surface. The AES spectra were taken on a cylindrical mirror spectrom­eter with a retarding field analyzero The primary energy used was 2000 eV. La exhibits pronounced AES peaks at 78 eV (NsN4,502,3) and 59 eV (NsN4,s01) kinetic

energy, and a Ni peak at 61 eV (M2 3M4 5M4 5). 11 The , , , latter two lines are not resolved and must, after base-line correction, be separated by computer simulation. Since the mean free path, i. eo, the probing depth, of the 59-, 61-, and 78-eV electrons are practically the same (-5 A), 12 the peak-to-peak height (hpp) of the La (78-eV) line determines that of the La (59-eV) line. 11,12 Hence, hpp (Ni[61 eV]) can be calculated from the ex­perimental baseline-corrected value of the overlapping La/Ni line by simulated subtraction of the La (59-eV) peak. The comparison of standard spectrum h!!(Ni)1 h:;(La) ratiosll with those measured is a means to determine the Ni/La concentration ratio CN/La of the LaNis surface, 13

_ h:!(La[78 eV]) h:;P(Ni[61 eV]) CN1 / La - h;~(Ni[61 eV]) h~~P(La[78 eV])'

The ratio of escape depths, the ratio of the backs cat­tering factors, and the total number of atoms per unit volume may yield systematic errors to CN1 / La deter­mined from Eq. (1).13 Application of Eq. (1) to the present data results in CNi / La"" 4.65 immediately after cleavage. The small difference to the stoichiometric value of 5 shows that these errors are below 10%. Since the linewidths remain unchanged during the ex­periment within the experimental uncertainty, the systematic errorS can be assumed as fixed. Thus, they have no influence on relative changes dealt with in this paper.

Besides La and Ni, the 51O-eV line of oxygen (KL 2L 2) and the 273-eV line of carbon (KL2L2) were monitored. The probing depth at this energy is about twice that for La and Ni.

After cleavage, a well-defined LEED pattern indi­cated a crystalline surface structure. The surface was then exposed to 3 and 10 L (1 L = 10-6 Torr sec) of 99.998% pure oxygen. During the second exposure, the LEED pattern disappeared. The AES measure-

689 Appl. Phys. Lett. 33(8), 15 October 1978 0003·6951/78/3308-0689$00.50 © 1978 American Institute of Physics 689

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5

0' OJ 0'

0 -' -'

4 -' 0 Q Q

3 Ni/La

0

-0'0_0 -0'0 ... 0

0'0-0

_o-o~o ?:a~o~-o

o 2 3 4 5 6 7 8

0' OJ

OJ I

-' I -' 0 -' 0 0 Q <.D Q

I-+-

O...Q.o_O_o 0-0 _0/1--°

9 10 II 26

:::l

-e 0

Q)

~ c: Q) 0' >.

10 >< 0

'0 £ 0'

5 11 -'" 0

If

FIG. 1. AES on LaNi5 single crystals cleaved in UHV. Upper curves: Ni (61 eV)/La (78 eV) ratio. Lower curves: oxygen (510 eV) signal. Squares: ex­periment at higher partial pressure of 02' At the times indicated, the sur­face was exposed to 1, 10, 100, and 1000 L of oxygen and 6 and 100 L of hydrogen, respectively.

Time after cleavage [hrsl--

ments were executed on the same crystal, but after separate cleavage. The experimental procedure and the AES results are seen in Fig. 1. First hpp of La, Ni, 0, and C were monitored as functions of time. The surface composition CN1ILa decreases steadily with in­creasing oxygen content. The oxygen stems from the residual gas in the URV. Then, the surface was ex­posed to increasing amounts of oxygen. Doses of 100 and 1000 L D.! show drastic effects: The oxygen content shows no saturation and CNl/La drops the more oxygen is present. After these exposures a transient effect is observed, consisting of a partial mutual "recovery" of CN1 / La and oxygen content. The carbon line first shows a slow increase due to residual gas, followed by drops with each oxygen exposure.

The results are qualitatively reproducible. Quanti­tatively CN1 / La decreases faster with more oxygen in

71 La ::l

-.. -... -...".-~-,."---.,,, ... - 6~ 5 li

t 18 > '"

=i 17 ~ .6

~ \ 0

16 -'

~ '0 15 ."

.!. .~~ •. E .S!'

> 14 '" Q) .c:

lO "'" 13

~ 0

z &'

'0 12 Ni

~ II .. .c:

10

~ 9 \.

0 2 3 4 5 6 7 8 9 10

Peal< height of oxygen (510eV)-line [arb.u.J-

FIG. 2. La (78 eV) and Ni (61 eV) (lower curve) versus ° (510 eV).

690 Appl. Phys. Lett., Vol. 33, No.8, 15 October 1978

the reSidual URV atmosphere (squares in Fig. 1). Sub­sequent exposure up to 100 L of 99.998% pure hydro­gen showed no effect on CNl/La nor oxygen content (Fig. 1).

Plotting hpp of La and Ni versus that of oxygen (Fig. 2) shows that the diminution of CN1 /La is entire­ly due to a linear decrease of the surface Ni content with increasing oxygen content, while the La stays essentially constant. The inclination of the Ni decrease is well reproducible. (Some influence of shadowing due to adsorbed oxygen may be visible at low oxygen con­tents, where the slope appears slightly steeper.)

Figures 1 and 2 lead to the following conclUSions. First, the results demonstrate that oxygen, not hydro­gen, is the driving element for the segregation. At the stage when the LEED pattern disappears, the surface is entirely covered with oxygen.

Further increase of the oxygen signal can be due to the formation of further oxygen adsorbate layers or to penetration of the oxygen into the crystal. Since the La Signal does not decrease (Fig. 2), the second mecha-

c: o

~ C '" u c o

U

La-O NI-rlch layff layer

Nickel

Lanthanum

Depth

FIG. 3. Schematic model of surface-layer profile produced by the segregation, induced by oxygen. Dashed lines: surface profile at the time of cleavage.

Th. von Waldkirch and P. Zurcher 690

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nism is realized. Its start is seen from the vanishing of the LEED pattern, indicating a dissolution of the single-crystalline surface structure. The linear rela­tion between Ni and 0 content with constant La content means that the segregation is driven by the chemical exchange reaction

(La-Ni) + 0- (La + 0) + Ni. (2)

Due to the different probing depth for Ni and 0, no quan­titative reaction equation can be given from the slope of Fig. 2.

The oxidation reaction of Eq. (2) involves a steady diffusion of La to the actual surface where it forms La2~ or La(OH)3' as reported in Ref. 7. The Ni atoms stay behind and probably concentrate due to the lacking La atoms. This leads to a surface-layer structure depicted schematically in Fig. 3: The upper lanthanum­oxide layer covers a lower-lying nickel-rich layer. Due to the oxygen gradient, the nickel is presumably prevented from thorough OXidation, leading to the metal­lic nickel observed by XPS. 7 Since air does not dis­solve LaN~, the surface-layer structure becomes stable at a certain gradient. The linear segregation increase on hydrogenation7 may be caused by the widen­ing of the LaN~ lattice on hydriding, 5 favoring the La diffUSion and/or the oxygen transparency of the surface­layer structure, or by a shift of the reaction equilibriur to the right-hand side of Eq. (2) due to the heat of formation of LaNisHa. The metallic nickel can only serve as a dissociative catalyst, if the covering La-O layer is transparent for Hz molecules. This seems pOSSible, since the layer is not monocrystalline.

The observed transient effects are not due to desorp­tion of adsorbed oxygen, since the La signal shows no dip after the Oz exposures. They imply, therefore, that the equilibrium state is reached slowly only, probably due to diffusion hindrance.

In summary, the present results show that the sur­face segregation is induced by oxygen and is driven by

691 Appl. Phys. Lett., Vol. 33, No.8, 15 October 1978

the chemical exchange reaction La-Ni to La-O. It leads to a layer structure consisting essentially of lanthanum oxide and nickel, respectively. It probably enables the nickel to serve as catalyst for the dissociation of H2 molecules and prevents it from loosing this ability by oxidation. 7

The authors are indebted to L. Schlapbach and H. Scherrer for supplying the single crystals and to A. Seiler, H. C. Siegmann, F. Stucki, and W. Eib for helpful discussions.

IJ. H. N. van Vucht, F. A. Kuijpers, and H. C. A. M. Bruning, Philips Res. Rep. 25, 133 (1970); H.H. van Mal, K.H.J. Buschow and A. R. Miedema, J. Less-Common Met. 35, 65 (1974).

2J.J. Reilly and R.H. Wiswall, Inorg. Chern. 13, 218 (1974). 3J. H. Weaver, J. A. Knapp, D. E. Eastman, D. T. Peterson, and C. B. Satterthwaite, Phys. Rev. Lett. 39, 639 (1977).

40. Boser, J. Less-Common Met. 46, 91 (1976); S. Tanaka, J.D. Clewley, and T.B. Flanagan, J. Phys. Chern. 81, 1684 (1977).

5H. H. Van Mal, Philips Res. Rep., Suppl. No. 1 (1976). 6p. Fischer, A. Furrer, G. Busch, and L. Schlapbach, Helv. Phys. Acta 50, 421 (1977).

TH. C. Siegmann, L. Schlapbach, and C. R. Brundle, Phys. Rev. Lett. 40, 972 (1978).

8y. Takahashi, J. Phys. Soc. Jpn. 43, 1342 (1977) and references cited therein.

9S.H. Overbury, P.A. Bertrand, and G.A. Somorjai, Chern. Rev. 75, 547 (1975).

lOG. Busch, L. Schlapbach, and H. R. Scherrer (unpublished). IIHandbook of Auger Electron Spectroscopy (Physical Elec­

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references cited therein.

Th. von Waldkirch and P. Zurcher 691

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