Reconstruction of UIr3(100)

Surface Science 0 North-Holland

75 (1978) L385-L391 Publishing Company

RECONSTRUCTION Received

14 February

OF UIr3( 100) * 1978

Studies of ordered intermetallic compounds offer an opportunity to extend catalytically-oriented surface science work to binary metallic systems while the long-range atomic order of elemental metals is retained. Thus far, single-crystal studies have been reported only for compounds in the Au-Cu system [l-3], and these surfaces undergo order-disorder transitions. This letter reports initial results of a program to study surface properties of intermetallic compounds. The (100) face of UIra was examined using LEED and Auger and electron energy loss spectroscopies. Compounds containing iridium are of great interest because comparisons may be made with elemental Ir, a broadly-based catalyst. The equilibrium clean surface of UIrs(100) formed a stable reconstructed structure, and a model is presented with a basis similar to that used for reconstructed Ir(lOO) 141. The adsorption of oxygen and carbon monoxide on the reconstructed (100) surface of UIrs at room temperature did not produce any new diffraction features as is the case for elemental Ir [.5]. However, the interactions of this compound with oxygen at elevated temperatures paralleled the complex behavior of the U-O system [6]. The compound UIra is highly ordered; it forms in the AuCus-type structure with lattice constant a0 = 4.023 A [7]. Each Ir atom has the same number of nearest neighbors as in pure Ir (12: 4 U and 8 Ir atoms); and the interesting electronic solid state properties of Ir are retained, viz., narrow bands formed in this case by d-mf hybridization. As seen in fig. la, U atoms are at the cube corners and Ir atoms at the cube face cenrers. The primitive unit cell of the (100) face is a square with side a,,/42 = 2.84 A. If th e compound remains ordered near the surface, a c(2 X 2) LEED pattern formed by the primitive cell beams and the half-order beams associated with the superlattice of dimension a,, should be observed. The relationship between the primitive and superlattice unit cells is shown in figs. la and lb. The experiments were performed in a metal UHV system which contained Varian 4-grid LEED optics and residual gas analyzer and a Physical Electronics double-pass cylindrical mirror analyzer. The UIra sample was a single crystal sheet (1.35 X 0.40 X 0.05 cm) oriented to within 1” of the (100) surface. It was mounted on a strip of 0.013 cm thick tantalum foil which was spot welded to Ta rods. The Ta was joule heated, and temperature was obtained from a Pt-Pt/RhlO% thermocouple attached to the foil near the sample. * Work supported

by the US Department

of Energy.

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*,a T. W. Orent et al. /Reconstruction

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(i

--9

- -\

0

0

0

(a)

Fig. 1. (a) Structure of UIr3 (solid circles arc U atoms and open circles are II atoms). The compound consists of (100) planes which are alternately 50% Ir-50% U and 100% 11. A primitive unit cell is indicated by the heavy solid lines and a superlattice unit cell by the dashed lines. (b) A c(2 X 2) LEED pattern which is predicted for an ordered 50% Ir-50% U surface of UIrs(100). (c) Proposed model for the top layer of a (100) surface in its reconstructed form. The periodicity of the layer is c(2 X 8) with respect to the superlattice. The underlying layer of pure Ir is represented by the extended square net. The hexagonal close-packing of the Ir surface atoms is indicated by dashed lines and the three-fold coordination of the U surface atoms b) heavy solid lines.

Initially the sample was heated briefly to 5OO”C, and a c(2 X 2) LEED pattern with large diffuse spots was observed. The lattice constant of the primitive unit cell was the same as the bulk value for a (100) plane to within 10%. The presence of the (l/2, l/2) superlattice beams indicated that the ordered state of the bulk alloy was retained in the near-surface region. Auger analysis, however, indicated that the surface was contaminated with sulphur, carbon and oxygen at a total coverage estimated to be in the monolayer range. These impurities were removed by argon ion bombardment (250 eV, 25°C) to within the sensitivity limits of Auger analysis which are estimated to be 2% of a monolayer for oxygen and 5% for carbon and sulphur due to overlap with Ir and U Auger signals. The argon ion bombardment disordered the surface as expected and removed the half-order superlattice beams. The sample had to be annealed at 600°C in order to produce a sharp c(2 X 2) LEED pattern, but the Auger spectrum again indicated the presence of sulphur. Sulphur was detected whenever the sample was heated above 550°C for over fifteen minutes.

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Eventually a reconstructed surface structure characteristic of clean UIra(100) was prepared by argon bombardment at room temperature followed by briefly annealing the sample above 700°C. The LEED pattern from this surface is shown in fig. 2. The periodicity of the reconstructed surface is c(2 X 8) with respect to the superlattice unit cell and occurs in two orthogonal domains (the LEED pattern from a single domain was observed on several occasions). The surface was free of impurities within the limits of detectability and was the only clean structure that could be obtained reproducibly. Visual observation of the LEED pattern as the clean, reconstructed surface cooled from 900°C indicated that the structure was thermally stable. There was no surface order-disorder transformation as has been found for AuCua( 100) [l-3]. The Ir5&J7a Auger peak height ratio for the reconstructed surface was the same as that found for the c(2 X 2) surface; hence, there was no evidence for changes in the surface concentrations of either component accompanying reconstruction. A plausible structural model that accounts for the observed c(2 X 8) reconstruction can be developed using the same ideas employed to explain reconstruction on the (100) faces of Au [8], Pt [9] and Ir [4]. Those results have been interpreted qualitatively in terms of the formation of hexagonal, close-packed overlayers on otherwise undisturbed bulk lattices. The ordered array shown in fig. lc would produce a c(2 X 8) LEED pattern with respect to the superlattice unit cell. The Ir atoms form an hcp array with lattice constant 4.59 A and the U atoms occupy

$1

>III, /

b Fig. 2. (a) LEED pattern of the reconstructed UIr3(100) surface at 58 eV. (b) Schematic LEED pattern showing a primitive unit cell (-- - - ), a superlattice unit cell ( -) and the LEED pattern from a single domain of the overlayer (X’s) which has c(2 X 8) periodicity with respect to the superlattice unit cell.

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00)

every other three-fold site. Each U atom is surrounded by three Ir atoms at a distance of 2.68 a which is only 6% shorter than the 2.84 A tlearest-neighbor distance in bulk UIrJ and which is comparable to the nearest-neighbor distances of 2.71 and 2.75 ,& in pure Ir [IO] and U [I 11, respectively. The U-Ir distance would be larger if the overlayer undulated so as to follow the morphology of the underlying square mesh more closely. The interactions of CO and O2 with the clean, reconstructed UIra(100) surface were investigated also. Carbon monoxide adsorbed without forming ordered LEED overlayer structures. A 1O3 L dose of CO at room temperature caused a reduction in the intensity of the original c(2 X 8) fractional-order beams but did not eliminate them. This suggested submonolayer CO coverage as opposed to multilayer adsorption and a sticking coefficient 5 10S3. Exposure of the CO adlayer to the lo-PA primary electron current used in Auger analysis resulted in apparent partial decompositiorl of the CO; as a result, CO adsorption was monitored nondestructively via electron energy loss spectroscopy using a 20 nA primary current. Two CO-induced losses appeared at 8 and 14 eV (see fig. 3). The intensities of these losses decreased by about 50% upon evacuation of CO from the belljar and decreased totally upon flashing the sample to 500°C. Similar energy losses have been observed previously on a variety of metal substrates covered with molecular CO [12,13]. Hence, the loss spectra suggested the presence of molecular CO on UIra(lOO), but the simultaneous presence of some dissociated species could not be ruled out. The 14 eV loss conventionally is assigned to an intramolecular electronic excitation from the degenerate In and 50 CO orbitals to the vacant 27r* orbital. The 8 eV loss is considered to be a charge-transfer excitation from the substrate valence band (d-f) to the CO 2n* orbital [12,13].

Fig. 3. UIr3flOO) loss spectra eV, Zp = 20 nA, pulse counting

for increasing exposures to CO at room temperature mode). The spectra arc displaced vertically for clarity.

(Ep = 200

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Three different types of surface oxygen were detected. First, reversible solution of an oxygen species into the bulk was observed via Auger on several occasions during the initial cleaning process. The species was present at a concentration of approximately one tenth of a monolayer, and all of it diffused into the bulk when the sample temperature was raised above 550°C and then reappeared as the sample cooled. The second oxygen species was formed by exposing the clean, reconstructed UIra(100) surface to O2 at room temperature. Only a small amount of O2 adsorbed. Low O2 sticking coefficients have been observed also for Ir(100)-(5 X 1) [5] and Pt(100)-(5 X 20) [14]. Finally, exposure of the reconstructed surface to O2 at 600°C resulted in oxidation. This was accompanied by the elimination of all but the specular LEED beam and the appearance of dramatic changes in the Auger spectrum (see fig. 4). A saturation oxygen Auger signal developed, and the Ir54/U,2 peak height ratio decreased by an order of magnitude which indicated the enrich-

I

I

I I

I

.

)XIDIZED

CLEAN

Fig. 4. Auger

spectra

of clean and oxidized

UIr3(100)

(Ep = 2 kV, Ip = 10 MA, 3 Vpeak_to_peak

modulation). The spectra are displaced vertically for clarity. The Ir&U72 peak height ratio decreased by an order of magnitude and the major U valence band Auger peaks were chemically shifted downard -4 eV upon oxidation.

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ment of uranium in the surface region. Also, the major U valence band Auger peaks (04VV and OsW) were chemically shifted by about 4 eV to lower energies. Nearly identical U Auger peak shape changes and chemical shifts were observed by Ellis [15] during his study of oxygen on polycrystalline U and were attributed to the formation of UOZ on the surface. On several occasions annealing the oxidized sample at 900°C led to the formation of a surface structure which was characterized by a ringlike LEED pattern with a radius of 3.5 A. On other occasions there was also some LEED evidence for the existence of a fourth-order structure with a less than saturation oxygen coverage. Several structurally different oxide phases could coexist on the surface. The complexity of the U-O system, which is due in part to the multiplicity of uranium oxidation states of comparable stability, makes even qualitative interpretation of the results very difficult. The present study was undertaken as part of an effort to compare the surface properties of intermeta~ic compounds with those of elements known to be catalysts. A comparison of UIra(100) with Ir(lOO) [4] shows that both surfaces reconstruct in a manner that can be attributed to the formation of an hcp overlayer on an otherwise undisturbed bulk layer. The reconstruction is irreversible for clean UIr,(lOO); while, a clean, unreconstructed Ir(lOO) surface can be prepared by a pretreatment procedure that involves oxygen exposure [S] . This difference between Uira and Ir can be attributed to the stronger and more complex interaction of oxygen with the uranium-containing substrate. The major difference between the proposed model for the reconstruction of UIra( 100) and those for Ir( loo), Pt( 100) and Au(100) is that the former involves a 13% reduction in density of the surface layer and a lowering of the coordination from four-fold to three-fold within the surface layer; while, the latter involves increased densities of the surface layers with respect to the bulk layers. This difference may be a consequence of a tendency in UIrs toward more extreme directionality of bonding which could be due to the admixture of the highly localized and antisymmetric uranium f-orbitals into the transition-metal d band. We thank Professor JM. Blakely for pro~d~g the sample and B.F. Addis at Cornell University for growing, cutting and polishing the sample. We also thank R.J. Friddle and R.L. Panosh for technical assistance. T.W.

ORENT,

S.D.

BADER

and M.B.

BRODSKY

Materials Science Division, Argonne NatiotaatLaboratory, Argonne, ~~~i~ois 60439, USA

References [ 11 VS. Sundaram, B. Farrell, R.S. Alben and W.D. Robertson, Phys. Rev. Letters 3’1 (1973) 1136.

T. W. Orent et al. /Reconstruction [2] [3] (41 [5] [6] (71 [8] [9] [lo] [l l] [ 121 [13] [14] [15]

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V.S. Sundaram, R.S. Albert and W.D. Robertson, Surface Sci. 46 (1974) 635. H.C. Potter and J.M. Blakely, J. Vacuum Sci. Technol. 12 (1975) 635. A. Ignatiev, A.V. Jones and T.N. Rhodin, Surface Sci. 30 (1972) 573. T.N. Rhodin and G. Broden, Surface Sci. 60 (1976) 466. F.A. Shunk, Constitution of Binary Alloys, 2nd Suppl. (McGraw-Hill, New York, 1969) p. 580. A.E. Wright, J.W. Downey and R.A. Conner, Jr., Acta Cryst. 14 (1961) 75. D.G. Fedak and N.A. Gjostein, Surface Sci. 8 (1967) 77. H.B. Lyon and G.A. Somorjai, J. Chem. Phys. 46 (1967) 2539. H.P. Singh, Acta Cryst. A24 (1968) 469. C.S. Barrett. M.H. Mueller and R.L. Hitterman, Phys. Rev. 129 (1963) 625. F.P. Netzer, R.A. Wille and J.A.D. Matthew, Solid State Commun. 21 (1977) 97. S.D. Bader, J.M. Blakely, M.B. Brodsky, R.J. Friddle and R.L. Panosh, Surface Sci. 74 (1978) 405, and references therein. C.R. Helms, H.P. Bonzel and S. Kelemen, J. Chem. Phys. 65 (1976) 1773. W.P. Ellis, Surface Sci. 61 (1976) 37.