Segregation induced reconstructions of LiAl(110) alloy surface

surface science ELSEVIER

Surface Science 375 (1997) L392-L396

Surface Science Letters

Segregation induced reconstructions of Li-A1 (110) alloy surface Jeong Won Kim, Yu Kwon Kim, Sehun Kim * Department of Chemistry and Centerfor Molecular Science, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea Received 21 August 1996; accepted for publication 19 November 1996

Abstract

The surface segregation of Li in A1(110)-12.7at%Li alloy and simultaneous reconstructions are investigated using Auger electron spectroscopy and low energy electron diffraction. The measured diffusion coefficient and barrier energy of Li segregation to the alloy surface was close to that of a bulk one. The saturation coverage of Li was close to 1 monolayer, that is, the ratio of the atomic number of Li to A1 is 1:1 after heat treatment. Based on the measurement of the surface Li coverages for the c(2 × 2) and (2 × 1) surface reconstructions, we suggest possible model structures. © 1997 Elsevier Science B.V. All rights reserved.

Keywords: Alkali metals; Alloys; Aluminum; Auger electron spectroscopy; Low energy electron diffraction (LEED); Surface segregation; Surface structure

Surface segregation and reconstruction in bimetallic alloys has been studied extensively due to its application to metallurgy and catalysis [1-3]. Li-A1 alloys are the promising materials for high current photocathodes, first-wall materials for the nuclear reactor, and high strength light materials for the spacecraft technology [4-7]. In their applications Li segregation after a heat treatment has been an important issue. Although several experiments on dilute A1 alloy surfaces have been carried out on segregation kinetics and their oxidations [ 8-13], the kinetic measurements of diffusion were not consistent with each other. Furthermore, the previous studies did not provide information about the surface structural phase transition with a correct coverage of segregated species. Hence, more

* Corresponding author. Fax: +82 42 869 2810; e-mail: [email protected]

detailed study on the surface structural transformation depending on the degree of segregation is necessary since the structure of Li-A1 surface under the Li segregation is important not only for basic research but also for microscopic study of the relevant oxidation mechanism. In this Letter, the surface segregation of Li on annealing the Li-AI(ll0) alloy is studied using Auger electron spectroscopy (AES). As the surface Li concentration increases, a sequential reconstruction is observed with low energy electron diffraction (LEED). We deduce the kinetic parameters of the diffusion and propose possible models for the reconstructed phases. The Li-A1 alloy was cast in a vacuum induction melting furnace using high purity A1 (99.99%) and Li (99.8%). The single crystal with diameter 13 mm was grown in a BN crucible using a modified Bridgman method [14]. The composition of the alloy has been determined to be 2.8wt% Li

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J. w. Kim et al. /Surface Science 375 (1997) L392-L396

(12.7 at%) by inductively measuring the coupled plasma emission. This Li concentration is below the solubility limit in A1 and similar to that of commercially available alloys [7]. A disk (thickness 1.5 mm) was cut from the crystal and mechanically polished using alumina down to 0.05 ILm grade. The orientation was checked using X-ray Laue diffraction within 1° from the (110) direction. The sample was connected by inserting two tantalum heating wires (45 = 0.25 mm) through holes drilled at the sample. The temperature was measured by a thermocouple attached on the back of the sample. The ultra-high vacuum chamber used in this experiment was equipped with a sputter-ion gun, a LEED optics with a digitized CCD camera, a double pass cylindrical mirror analyzer, and a concentric electron gun for AES. The base pressure of the chamber was < 1 x 10 -1° Torr. The surface cleaning was performed by repeated cycles of Ar ÷ sputtering (1-2 keV) and annealing (550 K). The most effective cleaning was the sputtering (1 keV) at an elevated sample temperature (400 K). After each experiment, the sputtering (1 keV) at 400 K and then sputtering (2 keV) at room temperature (RT) for 30 min was enough to recover the clean and nonsegregated L i - A I ( l l 0 ) surface. The surface Li concentration was determined from the AES peak-to-peak height ratio of LiKvv (52 eV) to that of A1Lvv (68 eV). The portion of AI plasmon peak at 52eV was subtracted from the LiKvv peak assuming that the ratio to the A1Lvv is constant [15]. The measurement of the surface Li concentration should be treated cautiously due to the low AES sensitivity of LiKvv transition and nonuniform depth concentration. The sensitivity factor of the LiKvv signal is 0.448 at a 3 keV electron beam while that of A1Lvv is 1.1792 [16]. We consider each AES-intensity attenuation factor and assume that the Li concentration is uniform within a few layers. So, the surface Li concentration is calculated as a lower limit. As the measured kinetic energies of Auger electrons are around 60 eV, the electron mean free path is a few ,~. Therefore, the AES intensity ratio directly measures the surface chemical composition even though there is somewhat of a depth profile.

Fig. 1 shows the surface Li concentl is defined here as the atomic fraction of Li, Li/(Li+A1), by the segregation as a function of annealing time at three different temperatures. Initial surface Li concentration appears to be less than bulk composition, probably because of the preferential sputtering of lighter Li atoms than A1. The amount of surface Li increases with increasing the annealing temperature and time. We measured the AES spectra with the sample held at each temperature. In the temperature range (300 550 K), the measured surface Li concentration never exceeds 0.47. The Li does not seem to form a multilayer above RT because the surface Li concentration saturates around Li:AI = 1: 1, that is 1 monolayer (ML) of Li, and the Li evaporates above T=550 K. The AES intensity ratio was retained after cooling the sample. To calculate the diffusion kinetic parameters from Fig. 1, we adopted a dilute alloy limit and an initial diffusion condition neglecting an evaporation. The diffusion coefficient was calculated by the following approximation when ( D t ) / ( ~ 2 d 2 ) < l [3, 17] Cs = eCu

1 - exp

erfc

1"21

(1)

~ ( 2/d)Cb X/N/~,

where Cs is the Li surface concentration (the number of atoms per unit volume), :~ the equilib--~ 0.6

Y 0.5

•

410KI

-t

0.4 ~ 0.3 o 0.2

~ 0.1 0.0 10 20 30 .Annealing time (min.I

40

Fig. 1. Surface Li-concentrationchange by segregation as a function of annealing time at the marked temperatures. The solid lines are the fitted curves from Eq. ( 1).

J. W. Kirn et al. / Surface Science 375 (1997) L392-L396

• the surface concentration to the bulk concentration Cb, d the lattice spacing of AI(110) ( 1.43 A), and D the diffusion coefficient. We calculated the diffusion coefficient by an Arrhenius plot of in D versus 1/T from the fitting parameters of the curves in Fig. 1. It gives the following expression:

(a)

D=Do exp (-E~ifr/RY)=2.764 × 10 -6 × exp

( - 127"7 k J / m ° l ~ (m2/s). RT /

(2)

Since our alloy sample is not an ideal dilute solution, the diffusion can be slightly deviated from the behavior expected as in Eq. (1). Our value of diffusion coefficient is one order smaller than that of bulk diffusion measured by an elastic recoil detection analysis [18], but three orders larger than the values of other AES experiments [9,12]. The different results in AES measurement from previous data are responsible for large deviations in the AES sensitivity factor of LiKvv, the initial composition of Li to A1, and the surface orientation. With the same experimental geometry as ours, Esposto et al. [9] reported that the limiting surface concentration of the segregated Li in the Al-6.5at%Li alloy was 0.17 and the diffusion coefficient was four orders smaller than that of Moreau et al. [18]. Consequently, they failed to correlate the Li coverage with the surface geometry of Li-AI(111)-~/3 x V~ assuming a Li overlayer reconstructed structure. However, the completion of the Li monolayer observed in our study is in agreement with the case of Gibbsian segregation [8] and affords the explanation of the surface geometry as will be described below. Along with the Li segregation, a series of surface reconstructions were observed. The new phases are c ( 2 × 2 ) and ( 2 × 1) reconstructions as shown in L E E D patterns of Fig. 2. Fig. 3 represents a surface phase diagram of Li-AI(110) as a function of the annealing time and the temperature. The inset represents the surface reconstructions according to the surface Li concentration. The initial diffuse (1 × 1) surface changes to the c(2 × 2) and subsequently to the (2 × 1) with the increased surface Li. The observed surface Li concentration was 0.30_+0.06 (0.44_+0.13 M L of Li) for the c(2 x 2)

(b)

Fig. 2. LEED patterns: (a) c(2 × 2) at 73 eV and (b) (2 × 1) at 70 eV. 550

.

.

.

.

Ill,,, A

soo

0.0

I

. . . . . .

i,

0.2

0.3

0.1

a .... 0.4

0.5

Surface LI concentration

:3

450 E

~-

400

350

,

0

t

10

,

I

20

,

I

,

30

I

40

Annealing time (min.)

Fig. 3. Phase diagram of reconstructions on the Li~l(ll0) alloy surfacevs annealingtimeand temperature. Inset is another phase diagram according to the surface Li coverage. and 0.41+0.06 (0.71_+0.17 M L of Li) for the (2 × 1). At a certain condition, the mixed phase of the c(2 × 2) and (2 × 1) was observed during the phase transformation. The final phase observed

J. W. Kim et al. / Surface Science 375 (1997) L392-L396

was always the (2 x l) in higher temperature or after a long annealing time. The (2 x 1) L E E D pattern is not as sharp as that of the c(2 x 2), but is broad and streaky in the direction of [001] as shown in Fig. 2b. Increasing the annealing time or temperature, the streak around the second order spots changed its direction from [001] to [11-0]. The final (2 x 1) surface also showed a bright background around a (00) spot. This indicates that the surface Li enrichment around grain boundaries somewhat induces a surface disordering and clustering but not to the extent of forming a multilayer or an incommensurate phase. It was previously known that alkali metals on fcc (110) metal surfaces typically induce a missingrow ( 1 x n) reconstruction [19], where the substrate atomic row is along the [11-0] direction every two or three times. However, the reconstruction of alkali metals on the A l ( l l 0 ) surface has been hardly studied. Esposto et al. reported a study of the segregation-induced surface reconstruction of the M g - A I ( l l 0 ) alloy, in which only the c(2 x 2) phase was observed [11]. However, they did not try to determine the surface atomic structure because the LEED spots were broad and also they did not suggest a reasonable Mg coverage. Therefore, the study of various reconstructions of Li~AI(I10) alloy surface induced by segregated Li will be of interest to clarify the details of the segregation induced reconstruction. The L E E D patterns of the L i - A I ( l l 0 ) alloy surface do not show any fractional spot in [001] direction as in Fig. 2, which indicates that the reconstruction is different from the missing rowtype. The measured L E E D I - V curves of the integer spots of the reconstructed phases are similar to those of the clean A I ( l l 0 ) - I x 1 surface [20,21]. These results may exclude the possibility of AI reconstruction. Therefore, as the simplest structures, we propose possible Li overlayer models for the c(2 x 2) and (2 x 1) phases shown in Fig. 4, neglecting the A1 substrate reconstruction. In these models, we did not consider any mass transport of substrate A1 atoms or bulk Li position. In the c(2 x 2) phase of Fig. 4a, we draw one of the atomic geometry of about 0.5 M L of Li adsorbed on a fourfold hollow site with a maximum coordination number. As the surface Li

(a)

(b)

114vl= [O01l Fig. 4. Possible model structures of the (a) c ( 2 x 2 ) and (b) (2 x 1) reconstructed surfaces of Li-AI(II0). Open circles, A1 atoms; shaded circles, Li atoms. Each dashed line indicates the unit cell. Arrows notify possible anti-phase boundaries.

concentration increases, the Li adsorption on the hollow site may become unstable due to the increased repulsive dipole interactions between neighboring Li atoms. In the (2 x 1) phase of Fig. 4b, the Li atoms are expected to form a zigzag chain in the [ 110] direction to release the repulsion between the Li atoms. As a result, the Li atoms adsorb on the threefold hollow sites of the fcc (111) facet plane like the case of R h ( l l 0 ) (2 × 1)-O surface [22]. The Li atom is the smallest in size and its electropositive character is the weakest of all the alkali metals. These unique properties of Li make the alkali-metal overlayer coverage up to nearly 1 ML possible. The (2 × l) LEED pattern of Fig. 2b is always broad and appears in a narrow energy window It is likely that the domain size of (2 × 1 ) is small and there are many anti-phases marked by arrows in Fig. 4. In summary, the Li segregation and reconstruction in the Li-AI(110) surface are investigated by AES and LEED. It has been shown that the surface diffusion of Li is close to the bulk case and the surface is saturated with about 1 ML of Li. We have observed the segregation-induced surface reconstruction sequentially from (l x l) to c(2 x 2) and (2 x 1). The reconstructed models are interpreted by segregated Li-overlayer structures with 0.5 and 1.0 M L coverage, respectively.

J. IF.. Kim et al. / Surface Science 375 (1997) L392-L396

~ents Financial supports have been partially provided b y the C e n t e r for M o l e c u l a r Science a n d K O S E F . W e t h a n k P r o f e s s o r J . K . P a r k for p r o v i d i n g the sample.

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