A LEED study of the (100) and (111) surfaces of the intermetallic compound FeTi

69

Surface Science 122 (1982) 69-79 North-Holland Publishing Company

A LEED STUDY OF m THE INTERMETALLIC

(100) AND (111) SURFACES COMPOUND FeTi

OF

T.E. FELTER Sattdio National Laboratories,

Livermore,

California 94550, USA

and S.A. STEWARD and F.S. URIBE Luwrence Livermore National Laboratory, Livermore, Received 7 May 1982; accepted for pub~~tion

Caiijbrnia 94.530, USA

20 July 1982

Two single crystals of FeTi, an intermetallic compound with a C&l cubic structure, have been isolated from a bulk melt. After numerous cycles of concurrent argon bombardment and heating to at least 1200 K, clean and well-ordered surfaces appeared on both crystals, as determined by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The first crystal produced a hexagonal pattern after heating to 1400 K. The measured lattice parameter is ae =4.3 20.2 A compared to 4.2 A, which is the reported X-ray value for the (111)plane of Fe%. The second crystal became ordered at a lower temperature (1200 K) showing a square pattern corresponding to FeTi( 100). The lattice parameter calculated from LEED was 3.1 r?z0.15 A, again in good agreement with X-ray data of 3.0 A. Suifur readily segregates to the surface above 1000 K and orders to produce a c(2X2) pattern on the (100) face. This is the first observation of an ordered overlayer on an intermetallic compound.

1. Introduction

Many metals, alloys, and intermetallic compounds react extensively with hydrogen, forming solid solutions or appro~mately stoichiometric compounds, called hydrides, which possess a different and distinct structure from the unreacted material. They have many technolo~cal appli~tions depending on their individual chemical and physical properties. For example, certain alloys and intermetallic compounds are being examined as reversible hydrogen storage media, because their equilibrium hydrogen vapor pressure and concentration at specific temperatures can be altered through the choice of alloy or compound composition. Also, intermetallic compounds and their hydrides are receiving interest as catalysts. Hydrogen reactivity with metals has been shown repeatedly to be affected 003%6028/82/0000-0/$02.75

Q 1982 North-Holland

70

T.E. Feher ef al. / (I 00) and (Ill)

surfaces of FeTi

by the condition of the metal surface [I], yet this important area has not been explored to any significant degree. The present work was initiated to address this neglected area for intermetallic compounds. In particular, FeTi has been chosen because it is touted as a good choice for hydrogen storage due to the natural abundance of its constituents, its high hydrogen solubility and its hydrogen vapor pressure of approximately one atmosphere near room temperature. In fact, FeTi is commercially available as a hydrogen storage medium (21. FeTi is cubic and has the undistorted CsCl structure which may be described as two inte~enetrating simple cubic lattices, one of iron and one of titanium. The iron atoms occupy the body-centered cubic positions of the titanium lattice and vice versa. The lattice constant increases from 2.97 to 2.99A over the intermetallic composition range of 49.5 to 52.5 atomic percentage tit~um [3]. Unlike alloys, in which random substitution permits large concentration ranges in a given phase, intermetallics exist over a narrow composition range, sometimes so narrow as to be called “line” compounds (41. Outside this narrow range, two-phase regions are formed. Polycrystalline specimens are easy to produce and examine. However, because of their microscopic heterogeneity it is difficult to differentiate between the surface effects of grains and grain boundaries on hydrogen reactivity. It was felt, therefore, that studies on single crystals would provide better understanding of the microchemistry involved and explain early results with polycrystalline material. The present LEED/Auger study was initiated to develop a cleaning and annealing procedure for preparing well-ordered FeTi crystals without impurities. Additionally, surface structure and lattice parameter data would be obtained. This information is prerequisite for future, planned gas exposure experiments, which are essential to understanding the activation processes for hydrogen reactivity. Unfortunately, single crystals of any intermetallic compound are rare, because they are relatively difficult to make.

Rather than spending considerable effort attempting to produce a single crystal directly, we were able, after modest effort, to select a few small crystals by screening a great number of similarly-sized pieces originating from a large melt. The crystals were l-5 mm on a side which after preparation provided enough area for surface experiments. We assume that such large crystallites formed due to the slow-cooing provided by the large rnulti~lo~~ melt employed. The melt was produced via inductive heating. Surprisingly, the only suitable crystals came from a melt that included 0.30 wt% oxygen. No crystals

T. E. Fefter et al. / (I 00) and (Ii 1) surfaces of FeTi

11

could be found in similar material containing less oxygen. We have no ready explanation for this result. The small individual crystals were polished in a standard manner, with a final vibratory polish using 0.1 pm alumina slurry chosen to minimize surface damage. The largest polished crystal was characterized by Laue X-ray backscattering, which indicated the well-ordered specimen had cleaved along the (100) plane. This and a second specimen, oriented along a (111) face, were selected for the LEED/Auger investigation. The stainless steel UHV surface analysis system used for these studies had a base pressure near 1 X lo- ” Torr. The system contained a Varian 4-grid LEED optics, a UT1 residual gas analyzer, and a Physical Electronics single-pass cylindrical mirror analyzer. The samples were mounted with a molybdenum mask on a Varian heater. The temperature was measured via an iron-constantan thermocouple spot-welded to the single crystal. Rather extensive exploration of heating and cleaning cycles was needed to determine the required method for surface ordering. Initially, titanium readily segregated to the surface of mono- and polyc~stalline FeTi upon heating to about 700 K. Without subsequent or concurrent ion sputtering titanium and oxygen covered the surface with only a hint of iron present in the Auger spectrum, as shown in fig. 1. Near 1000 K severe sulfur segregation occurred,

c i_ (I

100

200

300

m ELECTnON

500 ENERGY.

Fig. 1. Auger electron spectrum of FeTi after heating sputtering. Eb = 2 keV; modulation=2 V peak-to-peak.

600

700

Boa

rV.

to 700 K for several

minutes

without

72

T.E. Felter et al. / (100) and (II I) surfaces of FeTi

masking all other surface atoms. Long periods of concurrent sputtering and heating above these temperatures, were required to minimize the segregation of titanium, oxygen, and sulfur, when the sputtering was stopped. The low energy iron (49 eV) and titanium (28 eV) peaks were used as a measure of the surface cleanliness and the relative concentration of each species. The mean free path of such electrons is about 2-3 A or about one atomic layer [5].

3. Results After heating the first specimen to over 1200 K with considerable argon sputtering, a “square” LEED pattern characteristic of a (1 X 1) structure was observed. The image became sharper after more heating and sputtering cycles. This process also produced an Auger spectrum (fig. 2) that minimized titanium, carbon, sulfur and oxygen surface segregation. Some segregation, particularly of sulfur, occurred during cooling to obtain better LEED patterns. However,

Fe

i

Fig. 2. Auger electron spectrum of clean/annealed

FeTi. E, =2 keV; modulation=2

V peak-to-

T.E. Feller et al. / (100) and (III) surfaces of FeTi

73

the sample cooled to below 800K within a minute, allowing little time for segregation. The ion beam energy was reduced to 500 eV for a time before the heating was stopped to minimize surface disordering due to sputtering. Eventually, after many cycles, the pattern could be observed even during heating to 1200 K, despite the loss of intensity in the diffraction spots due to the Debye-Wailer factor, and interfering light radiating from the sample. Once the “square” pattern had been formed, titanium segregation decreased. Fig. 3 shows the “square” LEED pattern obtained with an incident beam energy of 62 eV and after cooling from 1300 K. The spots are sharp and the background intensity is low, indicating a well ordered sample. The four-fold symmetry of the pattern immediately identified the surface as the (100) face of a cubic material. Furthermore, we were able to quite accurately determine the

Fig. 3. FeTi(lOO)(l X 1) LEED pattern at 62 eV.

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T. E. Felter et al. / (100) and (1 II) surfaces of FeTi

surface lattice parameter by comparing to an in situ standard. The standard we employed was a segment of the exposed portion of the molybdenum heater that had also become crystalline during the bombardment/annealing sequence. By accurately translating the sample vertically and horizontally, and comparing to the shape and size of the sample, we were able to ascertain that the entire surface was well ordered FeTi(100). In a similar way, the sample, molybdenum heater, and mask were “mapped out” using AES. Both maps accurately reproduced the semicircular shape (and size) of the exposed sample. By comparing the separation of the spots for the Mo( 100) and FeTi( 100) and taking into account the inverse square root dependence of electron wavelength on incident beam energy, we found a 3.10 * 0.15 A lattice parameter for FeTi, in good agreement with the X-ray data of 2.98A. By comparison, the lattice

Fig. 4. ~(2x2) structure of sulfur on FeTi(~~)

at 92 eV.

T.E. Feller et al. / (IOO)and (111) surfacesof FeTi

15

parameters for bee &Ti and o-iron are 3.31 and 2.86 A respectively, which lay outside our measured value. Much of the cleaning cycle was designed to remove sulfur which initially segregated to the surface above 1000 K. It was impossible to remove it entirely, because it diffused to the surface rapidly at the temperature needed for ordering. If the sulfur segregation was allowed to continue on the (100) face at 1300 K without sputtering, (l/2, l/2) spots developed in the center of each reciprocal mesh. At 92 eV four such spots are obtained, as shown in fig. 4. This pattern is characteristic of a c(2 X 2) structure and to our knowledge is the first observation of an ordered overlayer structure on an intermetallic. Fig. 4 was obtained for the sulfur Auger peak at 151 eV to have approximately the same peak to peak amplitude as the titanium peak at 418 eV. Even brighter (l/2, l/2) spots were obtained for sulfur coverages approximately 30 to 40% larger by briefly heating the specimen to 1350 K. The c(2 X 2)-S structure could be sputtered away and recreated repeatedly. It is found on the (100) face of many pure metals, including the iron (100) surface [6]. Carbon and oxygen were the other major contaminants encountered. In preliminary adsorption experiments, carbon monoxide and hydrogen did not readily form superperiodicity on the (100) face, but increased gas exposures of several hundred Langmuirs at room temperature degraded the clean surface pattern. Because increased oxygen was observed with AES, and residual gas analysis revealed water during dosing, this contaminant is likely responsible for the degradation. The water formation mechanism is uncertain. Others have seen water formation when the hot tungsten filaments of an RGA contact hydrogen after it is introduced into the analysis system. Also, reactions with H+ on stainless steel surfaces can produce water [7j. The second crystal required heating to 14OOK before any ordering was apparent. A hexagonal pattern over most of the sample became clearer after several heating and sputtering cycles at this temperature. The pattern is shown in fig. 5 at 85 eV. As before, it was possible to determine the lattice parameter of the surface by comparing the separation of the spots in the hexagonal pattern to an internal standard, taking into account the E-Ii2 dependence of the electron wavelength. In addition, because real-lattice vectors are not parallel to reciprocal lattice vectors in hexagonal symmetry, it was necessary to take into account the 30” angle between them. We found the nearest neighbor distance to be 4.3 * 0.2 A. This can be compared to the nearest neighbor distance in the (111) plane of FeTi which is simply r!? times the reported X-ray value for the bulk lattice parameter, i.e., 4.2 A. By comparison, hexagonal a-Ti and TiFe, have lattice parameters in the a direction of 2.95 and 4.79 A respectively, clearly outside our measured value. This crystal was heated to 1470 K in an attempt to clarify the hexagonal image, since it was not as sharp as the square pattern. Unfortunately, the crystal melted at this temperature, 100 K below the expected melting temperature of 1600 K. Examination of the

76

T E. Felter et al. / (100) and (I 11) surfaces of FeTi

Fig. 5. FeTi( 1 1 1x1 X 1) LEED pattern at 85 eV.

phase diagram - 200 K [4].

suggests a 2% excess of titanium

can lower the melting

point by

4. Discussion We have found only four LEED studies of single crystal intermetallic compounds. Semiconductor compounds such as GaAs have been excluded since they are covalently bonded and not true intermetallics. These four have concentrated on the Cu-Au system [8-lo] and on UIr, [ll] a homologous compound to Cu, Au. The Cu-Au system is interesting in that it has three compounds, two of which are classic examples exhibiting order-disorder

T.E. Feller et al. / (100) and (11 I) surfaces of FeTi

71

transformation. The Cu,Au structure is face-centered cubic except that gold atoms are at the comers of the cube while copper atoms are at the center of the faces. Above the order-disorder temperature the structure remains facecentered cubic, but the atoms are randomly located with some short range order likely [12]. UIr, has the Cu,Au structure, but is not an order-disorder compound. The CuAu(1) ordered structure is similar to Cu,Au, except copper atoms at the center of the top and bottom faces are replaced by gold, resulting in alternating copper and gold layers [ 131. This may be described as a tetragonally distorted CsCl structure. There is also a CuAu(I1) structure, in which the gold and copper atom switch locations every five unit cells along the horizontal or b axis [14]. Knowledge of the surface composition of FeTi single crystals is useful in comparisons to polycrystalline samples, in analyses of surface contamination and in determinations of surface structure. Quantitative AES and other surface techniques, have been studied and used with limited success. The most comprehensive studies are those of Hall and Morabito [ 15,161, who consider the matrix dependent variables, peak shape, sputtering, atomic density, escape depth and backscattering, in Auger analysis of binary systems. They have determined correction factors for the last three from published work and discussed estimates for peak shape and sputtering. The accuracies of the compositions are inherently improved, because only ratios of factors between the elements are involved, i.e., gross changes or errors tend to cancel out. Summarizing the results of Hall and Morabito and using their nomenclature, CR.3

pTi,s

pFe

-=CF”,”

pFe.3

-F(Fe, pTi

Ti)“,

(1)

where C is surface concentration (at%) for the element indicated, .r means the value after sputtering and F is the matrix correction factor from their tables. The peak heights, PFe and PTi, refer to relative peak heights of pure materials (418 eV for titanium and 703 eV for iron). For compounds or alloys of iron and titanium, P and F are known. The peak height ratio of the pure elements is 0.48 [17] and the matrix correction factor is 0.78 or 0.84 depending on the data used for its calculation. We used an average value of 0.8 1. The reduced equation for the iron- titanium system is crLs/cre,s

=

0.39pTi,s/pFe.s.

(2)

The product of F and PFe/PTi is known as the relative elemental sensitivity factor, P,,,(FeTi) for this system. Eq. (2) is valid for unsputtered systems as well, e.g., after heating causes surface segregation. This result indicates that the peak ratio (PTi,s/PFe,s) for an equiatomic surface, i.e., FeTi, is 2.6. That is indeed the case for the spectrum of the ordered surface in fig. 2. The often used

78

T. E. Feiier et ai. / (100) and (Iii)

surfaces of FeTi

peak height ratio of the pure elements is 2.1 (l/0.48), causing an error of about 20% in the concentration. The relative peak heights of the pure elements are also called the relative elemental sensitivity factors in ref. [17], a point of possible confusion in this discussion. There is a distinct difference between the peak height ratios of clean, sputtered surfaces of the two crystals at room temperature. The square (100) surface shows an average peak ratio of 1.9 at these conditions, indicating an iron rich surface. The hexagonal (1 I 1) surface exhibits a peak ratio between 2.6 and 3.0, indicating an ~quiato~c or slightly titanium-~ch surface. Since the top layer of an ideal (100) or (111) surface contains only one species, it is tempting to believe that these observations of peak ratios indicate the topmost layer of (100) to be Fe while the top layer of (111) to be titanium. We believe such a conclusion to be premature, however, because the data had considerable scatter and represent only two crystals. It is also possible that iron and titanium substitute for each other in the surface region. We have no way of determining such an effect with any precision. Heating without sputtering increases the titanium on both surfaces. This increase varies according to temperature and time heated. The importance of this difference may become more apparent for gas adsorption experiments.

5. Summary The (100) and (111) surfaces of FeTi, an intermetallic compound, have been studied. The two crystals used were isolated from a bulk melt. FeTi has a CsCI-type cubic structure. The analysis chamber for the AES and LEED studies had a base pressure of about 1 X lo- r” Torr. Clean and well-ordered surfaces appeared on both crystals, as determined by AES and LEED, after numerous argon bombardment and annealing sequences to at least 1200 K. The first crystals became ordered at 1200 K, showing a square pattern corresponding to FeTi( 100). The lattice parameter calculated from LEED was 3.1 -C 0.15 A, in good agreement with X-ray data of 3.0 A. Sulfur readily segregates to the surface above 1000 K and orders to produce a c(2 X 2) pattern on the (100) face. The second crystal produced a hexagonal pattern after heating to 1400 K. The measured lattice parameter is a, = 4.3 5 0.2 A, compared to 4.2 A, which is the X-ray value for the (Ill) plane of FeTi. Carbon and oxygen were the other major cont~nants encountered. Carbon monoxide and hydrogen did not readily form supe~e~odicity, but increased exposures of several hundred langmuirs degraded the clean surface pattern. Because increased oxygen was observed with AES and residual gas analysis revealed water during dosing, this contaminant is likely responsible for the degradation.

T.E. Fe&e?et ai. / (100) and (1 I I) surfacesof FeTi

79

The authors thank Carlos Colmenares for the use of his LEED system and for a critical reading of the manuscript, and Gary Sandrock of INCO Ltd. for supplying the iron titanium samples. S.A.S. and F.S.U. thank F. Dee Stevenson and John Burnett of the Office of Basic Energy Science, US Department of Energy for their support. This work was performed by Lawrence Livermore National Laboratory and Sandia National Laboratories under DOE contract numbers W-7405Eng-48 and DE-AC04-76-DP00789 respectively.

References [l] J.J. Reilly and R.W. Wiswall, Inorg. Chem. 13 (1974) 218. [2] International Nickel Corporation, V~uums~hmel~e Hanau, and Gesellschaft fiir Elektrometailurgie, Niirnberg. [3] R.P. Eliot, Constitution of Binary Alloys, Vol. II, 1st Suppl. (McGraw-Hill, New York, 1965) p. 438. [4] A. Brown and J.H. Westbrook, in: Intermetallic Compounds, Ed. J.H. Westbrook (Wiley, New York, 1967) p. 304. [5] D.R. Penn, J. Electron Spectrosc. 9 (1976) 29, [6] K.O. Legg, F. Jona, D.W. Jepsen and P.M. Marcus, Surface Sci. 66 (1977) 25. [7] C. Colmenares, Lawrence Livermore National Laboratory, private communication. [8] VS. Sundaram, B. Farrell, R.S. Alben and W.D. Robertson, Phys. Rev. Letters 31 (1973) 1136. [91 VS. Sundaram, R.S. Alben and W.D. Robertson, Surface Sci. 46 (1974) 653. [‘Ol H.C. Potter and J.M. Blakely, J. Vacuum Sci. Technol. 12 (1975) 635. [Ill T.W. Orent, SD. Bader and M.B. Brodsky, Surface Sci. 75 (1978) L385. Electrons, Atoms, Metals and Alloys, 3rd ed. (Dover, New York, 1963) p, iI21 W. Hume-Rothery, 337. W.B. New [I41 A.E. lW P.M. 1161 P.M. (171 L.E. Eden

Pearson, The Crystal Chemistry and Physics of Metals and Alloys (Wiley-Interscience, York, 1972) p. 90. Dwight, J.W. Downey and R.A. Conner, Jr., Acta Cryst. 14 (1961) 7.5. Hall and J.M. Morabito, CRC Critical Rev. Solid State Sci. 8 (1978) 53. Hall and J.M. Morabito, Surface Sci. 83 (1979) 391. Davis et al., Handbook of Auger Electron Spectroscopy (Physical Electronic Industries, Prairie, MN, 1976) p. 11.

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