Chem. Mater. 1993,5, 1762-1771
1762
Oxide Thermal Desorption from the LaB6( 100) Surface following Reaction with 02 J o h n S. Ozcomertt and Michael Trenary* Department of Chemistry, MIC 111, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607- 7061 Received July 19, 1993. Revised Manuscript Received September 22, 199P
We have used temperature-programmed desorption (TPD) to study the oxidation of the LaBS(lO0) surface by 0 2 . We find that the surface oxide is removed through the desorption of BO and L a 0 between 1300 and 1500 K and the desorption of B202 between 1100 and 1500 K. The desorption of each of the three oxides shows a distinctly different dependence on 02exposure. For all but the lowest coverages, L a 0 desorption follows zero-order desorption kinetics with an activation energy of 775 kJ/mol. At the highest oxide coverages, B202 follows first-order kinetics with a constant desorption peak temperature of 1170 K. The first-order rate constant corresponding to this B202 peak is (1.3 X 109 s-l) exp[-(233 kJlmo1)lRTl. Similarly, a firstorder desorption peak for BO at 1485-1500 K has a first-order rate constant of (6.3 X 109 8-1) exp[-(303 kJ/mol)/RTl. The results are discussed in terms of various surface reactions involving B203 and La203.
Introduction Lanthnum hexaboride has been the subject of numerous surface science studies with most work concerned with properties of the clean surface1-16and its interaction with 02.17-27 These studies have been largely motivated by the widespread use of L a 6 as a high-performancethermionic t Present address: Department of Physics, University of Maryland, College Park, MD 20742. Author to whom correspondence should be addressed. * Abstract published in Advance ACS Abstracts, November 1,1993. (1)Yamauchi, H.;Takagi, K.; Yuito, I.; Kawabe, U. Appl. Phys. Lett. 1976,29(lo),638. (2) Aono, M.; Tanaka, T.; Bannai, E.; Oshima, C.; Kawai, S. Phys. Rev. B 1977,16(8), 3489. (3)Aono, M.; Oshima, C.; Tanaka, T.; Bannai, E.; Kawai, S. J.Appl. Phvs. 1978.49(5). 2761. i4) Aono, M:; Nishitani, R.; Oshima, C.; Tanaka,T.; Bannai, E.; Kawai, S. J. AppZ. Phys. 1979,50,4802. (5) Swanson, L. W . ;McNeely, D. R. Surf. Sci.1979,83,11. (6) Futamoto, M.; Kawabe, U. Surf. Sci. 1980,93,L117. (7) Nishitani.R.: Aono, M.: Tanaka, T.: Kawai, S.: Iwasaki, H.: Oshima, C.; Nakamura, S. Surf. Sci.1980,95,341. (8) Nishitani, R.; Aono, M.;Tanaka, T.; Oshima, C.; Kawai, S.; Iwasaki, H.;Nakamura, S.Surf. Sci. 1980,93,535. (9)Nakazawa, M.; Futamoto, K.; Usami, K.; Kawabe, U. J. Appl. Phys. 1981,52 (ll),6917. (10) Swanson, L.W . ;Gesley, M. A,; Davis, P. R. Surf. Sci.1981,107, 263. (11)Watson, R. E.;Perlman, M. L. Surf. Sci.1982,122,371. (12)Futamoto, M.; Nakazawa, M.; Kawabe, U. Vacuum 1983,33,727. (13) Gesley, M.; Swanson, L. W . Surf. Sci.1984,146 (2-3), 583. (14) Davis, P. R.; Gesley, M. A.; Schwind, G. A.; Swanson, L. W . ; Hutta, J. J. Appl. Surf. Sci.1989,37,381. (15)Ozcomert, J. S.;Trenary, M. Surf. Sci.1992,265,L227. (16)Ozcomert, J. S.;Trenary, M. J. Vac.Sci.Technol. A 1992,lO(4), 2581. (17)Oshima, C.; Kawai, S. Appl. Phys. Lett. 1973,23,215. (18)Swanson, L. W . ;Dickinson, T. Appl. Phys. Lett. 1976,B (lo), 578. (19)Berrada, A.; Mercurio,J. P.; Etourneau, J.;Alexandre, F.; Theeten, J. B.; Duc, T. M. Surf. Sci. 1978,72,177. (20) Goldatein, B.; Szostak, D. J. Surf. Sci. 1978,74 (2),461. (21) Nishitani, R.; Kawai, S.; Iwasaki, H.; Nakamura, S.; Aono, M.; Tanaka, T. Surf. Sci. 1980,92,191. (22) Davis, P. R.; Chambers, S. A. Appl. Surf. Sci. 1981,8,197. (23)Bae, E.B.;Hafner, P.; Klauser, S. R o c . 7th Znt. Congr. 3rd Znt. Conf. Solid Surf. Vienna 1977,881-884. (24) Chambers, S.A.; Davis, P. R.; Swanson, L. W . Surf. Sci.1982, 118, 75. (25) Chambers, S.A.;Davis,P. R.; Swanson,L. W.Surf.Sci.1982,118 (1-2),93.
emitter for use in electron optical instruments. Three properties of La& make it useful as a cathode: (1)a low work function, (2) a low rate of evaporation relative to electron emission at high temperature, and (3) congruent vaporization so that its composition and hence its properties remain constant as material evaporates. For optimal performancethe emitting surface should have a uniformly low work function, which has led to the use of low-index single-crystal faces of L a 6 in commercial cathodes. For these reasons ultrahigh-vacuum studies of single-crystal La& surfaces are of direct practical importance. Studies have shown increased weight loss of La& cathodes at high temperatures in ambient pressures of 02 greater than 1X 10-7Torr.28 Despite its obvious relevance, the details of the surface chemical reactions associated with evaporationof the oxide layer have received relatively little attention and among the studies which have been reported, there are considerable discrepancies. For these reasons we have used temperature-programmeddesorption monitored with a mass spectrometerto gain a more detailed understanding of the chemistry associated with the evaporation of oxide species from LaB6(lOO). We have also investigated the kinetics of La and B evaporation from the clean surface. The structure of the clean LaBs(lO0) surface has been investigated with a variety of techniques including lowenergy electron diffraction (LEED) ultraviolet X-ray photophotoelectron spectroscopy (UPS),2~4~7~21~26 electron spectroscopy (XPS),3*418J9 Auger electron spectroscopy (AES) and atomically resolved scanningtunneling microscopy (STM).15J6These studies are in agreement that the surface is terminated in an unreconstructed square lattice of La atoms with the bulk lattice constant of 4.15 A. Although the surface can yield exceedinglysharp LEED patterns, STM studies show that such LEED patterns are consistent with a surface con,334*8J6920*21p26
14,9,10,12,14117-20124,251~
(26)Nishitani,R.; Oshima,C.; Aono, M.;Ta",T.; Kawai, 5.;Iwasaki, H.; Nakamura, S. Surf. Sci.1982,115, 48. (27) Chambers, S. A.; Swanson, L. W . Surf. Sci.1983,131,385. (28)Davis, P. R.;Schwind, G. A.; Swanson, L. W. J.Vac. Sci. Technol. B 1986,4 (l), 112.
0897-4756/93/2805-1762$04.00/00 1993 American Chemical Society
Oxide Thermal Desorption from the LaBG(100) Surface
taining steps one unit cell in height and with approximately 10% of the La atom sites vacant.l5J6 If such top-layer La atom vacancies are not accompanied by B atom vacancies in the next layer, then the STM results would imply a significant deviation from the ideal L a 6 stoichiometry. From careful measurements of the gas in equilibrium with hot polycrystalline L a 6 samples, Storms and MuellerZ9 concluded that the surface has a congruently vaporizing composition (CVC) of LaBs.~-LaB6.07,depending on bulk composition. Congruent vaporization has been confirmed by others.30~3~An interesting unresolved issue is the microscopicmechanism by which the La and B atoms leave the lattice and enter the gas phase while preserving a CVC. The interaction of L a 6 with 0 2 has been studied with LEED,20,21,26AEs,’7-20,243,27 UpS,2126 XpS,1926 and ternperature-programmed d e s o r p t i ~ n . ~These ~ t ~ ~studies * ~ ~ are in agreement on several points and in disagreement on some others. It is clear that 02 adsorbs dissociatively at room temperature with a high sticking probability and that it raises the work function by 1.4-1.6 eV. Nishitani et al.21 concluded that the sticking probability starts at a low value at low coverage, then rises monotonically to a maximum of 1.0 after a 1.4-langmuir exposure, and then falls monotonically to zero. Goldstein and SzostakZ0 observed a 1 X 1LEED pattern following room-temperature oxygen adsorption which they interpreted as indicating a room-temperature “saturation” coverage of 1 monolayer (ML). Our results show that the surface never truly reaches a saturation coverage but rather the sticking coefficient abruptly falls to a very small value after approximately a 10-langmuir 0 2 exposure. While 1 0 atom/surface unit cell would give a 1 X 1LEED pattern, so would two 2 0 atomdunit cell. Nishitani et aLZ1 observed with UPS that initial 0 atom adsorption caused suppression of a surface state associated with B dangling bonds but that further 0 atom adsorption still gave a 1 X 1 LEED pattern. They concluded that the saturation coverage followingroom-temperature 02 exposure is 2 ML with one 0 atom/unit cell a t a lanthanum site and one 0 atom at a boron site. For a 50-langmuir 02 exposure at room temperature, Chambers and SwansonZ7concluded from angle-resolvedAuger electron emission that 0 atoms occupybothB and Lasites on the LaBe(l00) surface. Auger line shapes and XPS data for a heavily oxidized surface also indicate bonding between oxygen and both boron and lanthanum atom^.'^*^^ The greatest disparities in published work on the 02/ LaB6(100) system concerns oxide thermal desorption. In an early study, Swanson and Dickinsonl* reported oxide desorption between 1600 and 1700 K with B2O3 being the only desorbing species observed. In contrast, Goldstein and Szostak20observed L a 0 as the only desorbing oxide with a desorption peak at 1300 K. Bas et aLZ3observed Lao, BO, and 0 desorption from an oxidized LaBs(ll0) surface, but they did not observe B2O3 or Bz02. In a study somewhat similar to the present one, Davis and Chambersz2 observed desorption of BO, BzOz, BzO3, and L a 0 in the temperature range 1100-1600 K with L a 0 desorption occurring just below 1600 K. They obtained an activation energy of 415 kJ/mol for L a 0 and BO desorption and lower values for B2O3 and BZOZby assuming first-order des(29) Storms,E.; Mueller, B. J . Phys. Chem. 1978,82, 51. (30) Ames, L.; McGrath, L. High Temp. Sci. 1976, 7, 44. (31) Nordine, P. C.; Schiffman, R. A. High Temp. Sci. 1986, 20, 1.
Chem. Mater., Vol. 5, No. 12, 1993 1763
orption kinetics. In contrast, Aono et al.4 obtained an activation energy of 753 kJ/mol for oxide desorption. Our work differs from the previous studies in several ways. Most significantly, we have directly investigated the desorption kinetics by examining the coverage dependence of the thermal desorption peak profiles from a few hundredths of a monolayer to the formation of oxide multilayers.
Experimental Section The apparatus used in these studies has been described in detail elsewhere.3z.s Briefly, the experiments were conducted in an ultrahigh-vacuum (UHV)chamber (base pressure 1 1X 10-10 Torr) equipped with low-energy electron diffraction (LEED) optics, an X-ray photoelectron spectroscopy (XPS) system, and a quadrupole mass spectrometer (QMS) for residual gas analysis and temperature-programmed desorption (TPD) experiments. The QMS (UTI 100C) was controlled with our own digital interface box using our own software written specifically for thermal desorption experiments. Further details are contained in ref 33. These experiments were performed using a commercial La& cathode (Model ES-450,Kimball Physics, Wilton, NH) in which a 2-mm-diameter cylindrically shaped single crystal with an exposed (100) face is held in a thin graphite ferrule which in turn is tightly held in a Ta sleeve to which are spot-welded two Ta strips for sample support and heating. We spot-welded a type C thermocouple (5% Re-W/26% Re-W) to the Ta sleeve as close to the LaB~(l00)surface as possible. The same sample mounting used to give a long cathode lifetime makes it ideal for thermal desorption experiments; we experienced no failures due to sample mounting during the hundreds of heating and cooling cycles performed over the several-month period of these experiments. Sample heating was performed by passing current through the Ta leads. A programmable high-current power supply employing a proportional-integral-differential (PID) algorithm was used to achieve linear heating rates between 0.4 and 21 K/s. Thermocouple readings were calibrated using an optical pyrometer corrected for the emissivity of The sample was cleaned by heating to 1950 K for several minutes. Other groups have shown that this procedure leads to a clean surface.ms22Once degassed, the sample could be heated to 1950 K with a rise in background pressure 13 X 10-lO Torr. Although both XPS and retarding field Auger spectra of the sample were obtaineds and showed only La and B, we found desorption of La0 to be the most sensitive indicator of surface oxygen. Desorption of La0 could be observed from an initially clean surface after 20-30 min in a vacuum as low as 5 X 10-” Torr indicating a sensitivity well below 0.01 ML. The surface oxygen is presumably due to chemisorption of background water. In separate experiments we have found that HzO has a high dissociative sticking probability on the LaBe(l00) surface and that it forms the same surface oxides that result from 0 2 exposures.s The good reproducibility of our results is a further indication that there was no significant contamination of the surface.
Results (A) Clean Surface. As a prelude to our oxide studies, we have also investigated the evaporation of the clean surface. Figure 1 shows the dependence of the lOB and 139Lamass spectrometer signals on surface temperature from 1450to 1960K. The loBpeak was monitored because of the overlap of the llB peak with the large l2C peak due to background CO, COZ,and CHd. The two masses were monitored continuously as the temperature was increased (32) Foo, W. C.; Ozcomert, J. S.;Trenary, M. Surf. Sci. 1991,255,245. (33) Ozcomert, J. S. Structure and Reactivity of LaBs(lO0). Diasertation, University of Illinois at Chicago, 1992. (34) Storms, E. K. J. Appl. Phys. 1979,50,4450.
Ozcomert and Trenary
1764 Chem. Mater., Vol. 5, No. 12, 1993 Temperoture 1961
-Eo=
1852
1755
(K)
1667
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Figure 1. Plot of ln(signal) versus 1/T for lOB+ and 139La+ recorded while linearly heating the sample at a rate of 5 K/s.
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Figure 2. B/La desorption flux ratio as a function of sample temperature assuming a value of 6 at high temperatures.
linearly at a rate of 5 K/s. Separate experiments in which mass spectra from 0 to 300 amu were obtained throughout the temperature range used for Figure 1 confirmed that B and La are the only species evaporating from the clean surface.ss The plot shows that at the higher temperatures the B and La data can be fit to parallel straight lines with a slope corresponding to an activation energy of 570 kJ/ mol. At lower temperatures, the La points fall on a second straight line corresponding to an activation energy of 255 kJ/mol. The data of Figure 1 are plotted as the B:La atom flux ratio in Figure 2. The raw data have been normalized by assuming a ratio of 6:l for temperatures greater that 1850 K. The results of Figures 1 and 2 show that the evaporation rate is higher for lathanum relative to boron at lower temperatures. Similar observations were reported by Swanson and Dickinsonls although in their case the ratio changed by only a factor of 2 between 1670 and 2030 K whereas we observe a factor of 12 change from 1710 to 1990 K. We find that this effect depends on how rapidly the sample is cooled after heating to 1900 K. The effects observed in Figure 1 and 2 were obtained by cooling the sample as quickly as possible a t a rate of about 60 Kls. If the sample is cooled at arate of about 5 K/s, the preferential evaporation of La at lower temperatures is much less pronounced. The fact that the evaporation rates depend on cooling rate indicates that rapidly cooling the surface traps structural features present at high temperatures which are different from the fully equilibrated roomtemperature structure. The lower activation energy for
Figure 3. Mass spectrum of positive ions desorbing from the surface obtained with the electron emission current of the mass spectrometer ionizer set to zero.
the low-temperature La evaporation suggests that it may be due to La atoms on top of the La-terminated (100) plane. This is supported by oxidation experiments which show a larger L a 0 desorption peak for a given 0 2 exposure for a rapidly cooled sample. It should be noted that our earlier atomically resolved STM study16J6 of the clean La& surface was for a slowly cooled sample. It would obviously be of interest to explore this effect further with STM. The data of Figures 1 and 2 were obtained using 70-eV electron impact ionization. The mass spectrum shown in Figure 3 in which only La+ and B+ are observed was obtained with the surface a t 1900K and with the emission current of the mass spectrometer ionizer set to zero. This shows that La+ and B+ ions desorb directly from the surface. The fact that none of the background gases in the vacuum chamber, such as CO or H20, are seen excludes the possibility that some alternative ionization source, such as an ion gauge or the electron emission from the La& itself, forms La+ and B+ from the desorbing neutrals. The directly desorbed ion signal is about 2orders of magnitude less than the neutral signal. By monitoring the La+ and B+ signals as a function of temperature it should be possible, in principle, to obtain the activation energy for ion desorption. However, to obtain accurate values, great care must be taken to eliminate all extraneous electric fields a t the surface. In our experimental arrangement the ionizer contains both positive and negative voltages with respect to the sample and is quite close to it. The result is a small but poorly defined electric field at the sample. However, by placing a grounded grid in front of the sample, it would be possible to obtain ion desorption data in a field-free region and thus obtain accurate values of the ion desorption activation energy. The value of such a measurement is described in the Discussion. (B)Oxidized Surface. Figure 4a shows the Lao, B202, and BO mass spectrometer signals as a function of La& temperature following a 1-langmuir (1 langmuir = 1 X 10-6 Torr s) exposure to 0 2 with the sample at room temperature. The L a 0 shows a relatively sharp peak with a maximum at 1430 K and a slight shoulder on the hightemperature side of the main peak. The BO peaks shows a maximum at 1445 K but is considerably broader. The llBO peak at m/e = 27 is not completely resolved from the much larger peak at mle = 28 due to background CO which
Chem. Mater., Vol. 5, No. 12, 1993 1765
Oxide Thermal Desorption from the LaBs(100) Surface
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increases as the sample is heated. The B202 signal is considerably smaller and displays a broad complex peak with a maximum at 1320 K which then falls abruptly to zero between 1450 and 1470 K. Figure 4b shows the desorption of the three oxides following a 50-langmuir exposure with the sample a t lo00 K which leads to higher oxide coveragesand corresponding changes in peak position. The full coverage dependence of each of the three oxides is described below. Other than a 50 K upward shift in temperature, the L a 0 and BO peaks are similar to the 1-langmuir room-temperature exposure, while the relative amount of B202 is considerably higher and shows a markedly different peak shape. The results in Figure 4b were obtained under essentially the same conditions as Figure 1 of Davis and Chambers.22 However, they observed a small B2O3 peak for exposures 1 1 0 langmuir at 1000 K while we did not detect B2O3 under any conditions. Even where they do observe it, the B2O3 peak is far weaker than any other. While their desorption peak shapes are similar to ours, their desorption peak temperatures for BO and L a 0 appear to be about 1 W 1 5 0 K higher but their B202 peaks appear to be at about the same temperature. Also, they used a highly nonlinear heating rate which increased their B202 signal relative to the L a 0 and BO signals. Since our study focused mainly on the exposure dependence for low exposures at room temperature, while theirs was more concerned with studies under ambient 0 2 pressures on the order of 1 X lo4 Torr, the two studies are complementary and explore different aspects of the problem. Although other possible species were monitored, only the three oxides shown in Figure 4 were observed. We have established the cracking patterns for both B2O3 and B202 for our mass spectrometer in previous studies.3k3' For both species the parent ion dominates the spectrum.
1200
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Temperature ( K )
Figure 5. La0 thermal desorption as a function of 02 exposure at room temperature for a linear heating rate of 5 K/s.
We can thus exclude the possibility that the observed BO and B202 are simply fragments produced during ionization of desorbing B2O3. The results of Figure 4 indicate that all surface oxides are removed after heating to 1500 K. We have obtained thermal desorption data for each of the three oxidesof Figure 4 as a function of oxygen exposure at both room temperature and lo00 K. The collision rate for 0 2 at 300 K is such that we expect to obtain 10 atom/ unit cell of the surface after 0.8 langmuir, assuming a dissociativesticking probability of unity. Each of the three oxides desorbs over a different temperature range, although the desorption rate falls to zero for all three oxides at the same temperature near 1480 K. In describing the desorption process we assume the Polyani-Wigner formBpa (see eq 1 in the Discussion) for the dependence of the desorption rate on coverage and temperature. This expression gives characteristic features in thermal desorption for zero, first, and second-order desorption. We have the highest sensitivity for L a 0 as it desorbs as a single isotope over a relatively narrow temperature range and is well isolated from the boron oxides and from all background gases. Thus we can observe L a 0 desorption even following exposures as low as 0.01 langmuir. On the basis of the low initial sticking probabilities reported by Nishitani et al.,21 such a low exposure must correspond to only a few thousandths of a monolayer. The high sensitivity for L a 0 also suggests why it, but not the boron oxides, was observed by Goldstein and S ~ o s t a k .Figure ~ 5 shows the L a 0 desorption as a function of oxygen exposure. For the first five exposures (0.01-0.2 langmuir) a single peak is observed which undergoes a slight shift from about 1430 to 1450 K. A constant desorption peak (35) Foo, W. C.; Ozcomert,J. S.; Trenary, M. Surf. Sci. 1992,262,88. (36) Wang, Y.;Fan,J.; Trenary, M. Chem. Mater. 1993,5, 192. (37) Wang, Y.;Trenary, M. Chem. Mater. 1993,5, 199. (38) Redhead, P.A. Vacuum 1962,12, 203. (39) King, D.A. Surf. Sci. 1978,47, 384.
1766 Chem. Mater., Vol. 5, No.12, 1993
Ozcomert and Trenary
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Figure 6. BO thermal desorption as a function of 02 exposure at room temperature for a linear heating rate of 5 K/s. temperature is characteristic of first-order desorption, while a shift in peak temperature to lower values with increasing coverage is characteristic of second-order desorption. For higher exposures, a lower temperature component develops which continues to grow indefinitely. The appearance of a more or less common leading edge for 0 2 exposures 10.3 langmuir is indicative of zero-order desorption kinetics. Zero-order desorption kinetics are seen from multilayers or from the sublimation of pure solids. It is somewhat surprising that the zero-order peak begins to develop at 0 2 exposures as low as 0.3 langmuir. Since the sticking probability2I is 51, this indicates that the oxide giving rise to the zero-order peak develops before reaching a coverage of 1 ML. For zero-order desorption the desorption rate is equal to the rate constant which can be assumed to be of the Arrhenius form. Since each point on the common leading edge of the desorption curve is proportional to the desorption rate, a plot of ln(rate) versus 1 / T should yield a straight line with a slope equal to EJR. We have prepared such plots from the L a 0 desorption curves for 12 different 0 2 exposures ranging from 0.5 to 1000 langmuirs at room temperature. In all cases the data falls on straight lines whose slopes give an activation energy of 775 f 30 kJ/mol. A comparison of this value with results obtained in previous experiments on La83 oxidation and with studies of lanthanum oxide sublimation is presented in the discussion section. Figure 6 shows llBO TPD results as a function of 0 2 exposure. For the lowest 0 2 exposure, the temperature of maximum BO desorption, Tm, occurs at 1485 K and undergoes only a small shift to 1495 K at the highest 0 2 exposures. This is indicative of first-order desorption. At 0 2 exposures 23 langmuirs a second broad BO desorption peak is observed at 1430 K which, like the 1485 peak, has a T m which is roughly constant with increasing exposure. As noted earlier, a constant sticking probability of unity
would lead to 1adsorbed oxygen atom/surface unit cell for an exposure of 0.8 langmuir. Whereas this lower temperature BO TPD peak does not develop until an 0 2 exposure of 3 langmuiw, the zero-order L a 0 peak of Figure 5 starts to appear at only 0.3 langmuir. This indicates that the transition from a single peak to two peaks in both the BO and L a 0 TPD curves does not correspond to a simple change from monolayer to multilayer surface oxide. The dependence of B202 desorption on 0 2 exposure shown in Figure 7 is markedly different from that of L a 0 and BO. At the lowest exposures the B202 peak rises gradually from about 1300 K then falls off abruptly at 1460 K. The 3- and 10-langmuir results show a distinctly new peak at 1250 K which persists for the highest exposures. For exposures 150 langmuirs a third peak grows in at a constant temperature of 1170 K. The B202 desorption clearly goes through three distinct stages with each stage being completed before the next one begins. The fact that B202 desorption takes place at much lower temperatures than for L a 0 desorption suggests that there is a temperature range between 1200 and 1400 K where the surface La:B ratio is higher than it is either for temperatures below the onset of desorption at =lo50 K or for temperatures higher than 1520 K where all the surface oxygen has been removed. For first-order desorption the activation energy, E,, and preexponential, v, can be obtained38from the dependence of peak desorption temperatures, Tp,on heating rates, j3. A plot of ln(j3/Tp2)versus l / T p yields a straight line with slope equal to EJR and intercept equal to ln(uR/E& We applied this method to the BO peak that appears at 1490 K in Figure 6 for a 5 langmuir 0 2 exposure and to the B202 peak that appears at 1170 K in Figure 7 for a 50 langmuir exposure at a heating rate of 5 K/s. By variation of the heating rate from 0.4to 21 K/s these peaks shift by about 170 K. This then yields first-order rate constants, k = v exp[-EJRTl, of (6.3 X lo9s-l) exp[-(303 kJ/mol)/RTl for BO and (1.3 X lo9 s-l) exp[-(233 kJ/mol)/RTJ for B202. If we simply assume v = 1013s-l as is commonly done and as Davis and Cambers22did, we obtain E. = 398 kJ/mol for BO (compared with their 415 kJ/mol) and E. = 311 kJ/mol for B202 (compared with their 318 kJ/mol). Figure 8 shows the Lao, BO, and B202 TPD peak areas versus 0 2 exposure with the L a 6 crystal at both room have temperature and at 1000 K. Previous studies20*22 indicated that higher oxide coveragescan be achieved with
Chem. Mater., Vol. 5,No. 12,1993 1767
Oxide Thermal Desorption from the LaB6(100) Surface
+ -
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Figure 9. Plot of amounts of B removed as BO and B202relative to amount of La removed as La0 following 02 exposure at (a) room temperature and (b) lo00 K. Assumptions involved in establishingrelativemass spectrometersensitivities for the oxides are described in the text.
Oxygen Exposure (La ngmuirs)
Figure 8. Plot of thermal desorption peak areas for Lao, BO, and BzO2 as a function of 02exposure at both room temperature and 1000 K. The vertical scale is percent of maximum area observed for each oxide.
the sample a t 1000 K than a t room temperature. The plots are similar in their overall appearance but also show important differences. Each of the plots shows an initial rapid uptake of oxygen followed by a much slower uptake. After a l-langmuir exposure the La0 area has reached approximately 50 % of the area attainedafter a 50-langmuir exposure. Furthermore, the amount of La0 desorbed for a given exposure is only weakly dependent on the surface temperature. Below 1langmuir slightly more La0 desorbs following room-temperature exposure, whereas higher exposures reveals little difference. The BO plots are somewhat similar to those for Lao. At 1000 K, a l-langmuir exposure yields 80 % of the BO desorbed after a 50-langmuir exposure while a t room temperature 1 langmuir yields 60% of the area resulting from a 50langmuir exposure. Unlike the La0 peak areas, the amount of BO that desorbs for exposures 210 langmuir is 10-20% higher for exposures a t room temperatures than at lo00 K. The B202 TPD area rises more gradually with increasing 0 2 exposures. After 1langmuir a t lo00 K the B202 area has reached only 10% of the area obtained from a 50-langmuir exposure and at room temperature l-langmuir yields 20% of the area reached after 50 langmuirs of 0 2 . Furthermore, the amount of B202 that desorbs following a 50-langmuir exposure at 1000K is almost twice the amount produced following 50 langmuirs a t room temperature. The increased amount of B202 desorption for exposure at lo00 K occurs almost exclusively in the 1170 K component of the B202 peak with the 1250 and
1460 K components basically the same for the lo00 K and room-temperature exposures. An important question is whether B and La are removed as oxides in a 6 1 ratio. An estimate of the relative amounts of B and La removed from the surface during oxide desorption can be obtained by making a few reasonable assumptions. First, we assume, as before, that a t high temperatures there is congruent vaporization from the clean surface so that the B flux into the spectrometer is 6 times that of the La atom flux. The raw data used to prepare Figure 2 then imply that our mass spectrometer has a higher sensitivity for La relative to B of 1.7. Second, we assume that the mass spectrometer sensitivity for B is the same as for B202 and BO while the La0 sensitivity is the same as for La. The observed BO and B202 peak areas are then corrected for the isotopic abundance5 of llB and 1OB in the desorbing species. This allows us to plot the amount of boron desorbing as BO and B202 and the total amount of boron desorbing as oxides relative to the amount of lanthanum desorbing as La0 as a function of 02exposure at both room temperature and lo00 K. The results are shown in Figure 9. These plots clearly reveal that relatively little boron is removed in the form of B202 compared to BO. We also see that the B/La ratio in the desorbing oxides is alwaysless than 6 and that it varies with exposure. At both lo00 K and room temperature we see that for low exposures we remove the minimum amount of B relative to La. At 1000 K the ratio increases to a maximum of just over 41 at an exposure of 1langmuir. At room temperature there is an abrupt increase to just over 3, which then slowly increases to about 5. At both temperatures almost all of the B is removed as BO for exposures less than 1langmuir. A t lo00 K the amount of B removed as B202 shows a small but steady increase but it is not enough to offset the lack
1768 Chem. Mater., Vol. 5, No. 12,1993 of BO increase as the L a 0 increases with increasing 02 exposure. It is possible that our assumption about the relative values of the mass spectrometer sensitivities for the oxides are not valid so that the absolute value of the ratio is in error. However, this would not alter the fact that the ratio changes with exposure. The implications of these observations for the surface stoichiometry are discussed below.
Ozcomert and Trenary
coefficient less than unity thus would not be surprising. Storms and Muellerm report an evaporation coefficient for B from La& of 0.13 at 1700 K and 0.16 a t 2100 K . Nordine and Shiffman31 compared their equilibrium measurements to the free evaporation studies of Storms and M ~ e l l e and r ~ ~concluded that the evaporation coefficient is 0.49. An accurate measurement of the evaporation coefficient would be an essential first step in determining the detailed mechanism by which La and B Discussion atoms leave the La& lattice and enter the gas phase. However, even the present limited evidence for a non(A) General Considerations. The technique of therunity evaporation coefficient indicates that one would not mal desorption employed in these studies is widely used expect the apparent enthalpy of evaporation measured to study gases adsorbed on metal surfaces a t submonolayer through free evaporation studies, such as reported here, coverages.m*39Typically the gas desorbs without removing to agree with the thermodynamic enthalpy of vaporization. metal or otherwise altering the state the surface was in Since oxygen is removed from the La& surface in the prior to gas adsorption. The prototypical example is the form of lanthanum and boron oxides, the oxidation reaction adsorptionfdesorption of CO onlfrom transition-metal is best thought of as formation of a new solid compound surfaces. The rate of desorption, expressed as the rate of on the L&6 surface rather than as simple chemisorption. coverage change with respect to time, dOfdt,can be related The thermal desorption of these oxides thus represents a to the coverage change with respect to temperature, T, for case somewhere between ordinary thermal desorption and a linear heating rate of j3 K/s, by free evaporation of a pure solid. As in ordinary thermal -dO/dT = (Onv//3) exp[-E,/RTI (1) desorption the surface concentration of the desorbing species changes with time and temperature, which is where E, is the activation energy for desorption, v is the particularly significant a t low oxide coverages. However, preexponential and n is the order of desorption. The in this case the desorbing species are apparently produced coverage is usually expressed in number of molecules/ by the reaction of the surface oxide and the L a 6 substrate. cm2. For nondissociative adsorption the molecules typFor a thick oxide layer we may be simply observing the ically desorb with first-order kinetics. free evaporation of pure oxides without any influence of Related to thermal desorption from surfaces is the the underlying substrate. A general treatment of thermal evaporation kinetics of solids. The measurement of desorption in cases such as this apparently has not been evaporating fluxes into vacuum is often referred to as free developed yet, despite the relevance of such processes to or Langmuir evaporation. An important issue in free many important technological problems. We thus make evaporation is the relationship between measured actiuse of ideas from both the thermal desorption and solid vation energies and the corresponding equilibrium therevaporation literature. modynamic quantities. This relationship is thoroughly and clearly treated by RosenblatP and by S t e a r ~ yA. ~ ~ (B) Clean Surface. The details of the vaporization of measurement of the free evaporation of a solid corresponds L a 6 a t high temperatures is of direct relevance to the use to zero-order thermal desorption as described by eq 1since of La& as a cathode and has therefore been the subject the number of desorbing species per unit area does not of numerous s t ~ d i e s The . ~issues ~ ~of ~interest ~ ~ ~ ~ ~ ~ change until the solid has completely evaporated. A plot include the identity of the evaporating species, the kinetics of In(-dO/dt) versus 1 / T would then yield a straight line of evaporation including activation energies, whether La with a slope of EJR. As noted by Rosenblatt,40 an and B are removed at rates which preserve the bulk activation energy measured in such a way is related to an stoichiometry (congruent vaporization), the relationship apparent enthalpy of vaporization by between the activation energy for free evaporation compared with the equilibrium vaporization enthalpy, the AH*,= E, + '/$T (2) relationship between evaporation process and crystalloThis apparent enthalpy of vaporization can then be graphic face, and the effects of the thermal history of a compared with the thermodynamic enthalpy of sublimaparticular sample. One point of agreement among all tion, AHv". In general, one would expect AH*,to be vaporization studies, including ours, is that clean La.& different from AH," with the extent of the difference evaporates only to atomic La and B, i.e., no La,, or B, described by the vaporization coefficient, CY,Only . when with n,m > 1, or La,B,species are observed. Other aspects CY, = 1 are AH*,and AH," identical. The microscopic of the evaporation, and in particular the activation = 1is that the rate-limiting step for interpretation of CY, energies, show a great deal of variability among the vaporization is desorption of the vapor species from the published values. There is poor reproducibility even surface of the solid. Unit evaporation coefficients are among studies following good surface science protocol on commonly observed when the structure of the vapor-phase the same single-crystal face for La& samples of similar species is the same as in the solid. In the case of L&6, bulk composition. This suggests that there are important there is obviously a great deal of structural rearrangement variables for La83 which have not been adequately involved in the evaporation process. An evaporation controlled in most studies. It is therefore important to identify these variables. (40)Roeenblatt, G. N. In Treatise on Solid State Chemistry Hannay, N. B., Ed.;Plenum Press: New York, 1976; Vol. 6A, Ser. I, pp 166-240. A primary consideration in evaporation studies of LaB6, (41) Searcy, A. W. In Chemical andMechanicalBehauior oflnorganic or of any binary compound, is the composition at which Materials; Searcy, A. W.,Ragone, D. V., Umberto, C., Eds.: Wiley-Interscience: New York, 1968; pp 107-133. congruent vaporization occurs. The phase diagram of the
Oxide Thermal Desorption from the LaBG(100) Surface
La/B system was first proposed by Johnson and Daane42 Storms with subsequent minor modifications by and M ~ e l l e rand , ~ ~McKelvy.4 According to the phase diagram, the La& phase exists over a fairly narrow compositionrange from the ideal stoichiometry to a slightly La-deficient stoichiometry a t 1600 K. At temperatures above 2000 K the degree of La deficiency is much larger. However, in disagreement with the phase diagram, Larich single crystals are obtained by the arc float-zone te~hnique.~5As noted earlier, Storms and Mueller29 concluded that in a free evaporation experiment, such as reported here, the surface will attain a CVC of La6.04 to La6.07. Furthermore, only at the CVC do the La and B show the same activation energy for evaporation. Because both boron and lanthanum have higher vapor pressures at a given temperature than La&, heating a La/B mixture to high temperatures will lead to evaporation of the component in excess of the La& phase. However, even within the single-phase region the La:B ratio can be less than or greater than the CVC. When the vaporization rate is greater than the bulk-to-surface diffusion rate, the surface region will reach a composition very close to the CVC. At lower temperatures, diffusion rates will be higher than evaporation rates and the surface composition will show a greater dependence on bulk composition. For this reason, when the bulk is not exactly at the CVC, the surface composition will actually depend somewhat on temperature. Thus, our results in Figure 1 that show parallel Arrhenius plots for La and B above 1850 K imply that the surface has reached the CVC of LaB6.04-6.07. At the lower temperatures in Figure 1,a lower activation energy for La is observed which can be attributed to a surface composition that is La rich relative to the CVC. Although the results of Figure 1were obtained as the crystal was heated linearly a t 5 K/s, we would have presumably observed an increase in &(La) as a function of time for constant temperatures in the range 1750-1850 K. Similar effects have been observed in the past. Swanson and Dickinsonlg observed an increase of the B/La ratio with increasing sample temperature for an La&( 100) surface of unspecified bulk stoichiometry. Swanson and McNeelf report variation in La and B activation energies for a given La&(100) sample with length of heating time between 1600 and 1800 K. Davis et al.14 reported activation energies for La and B evaporation from LaB5.74(100) of 425 and 704 kJ/mol and from LaB6.,~,(lOo)of 550 and 704 kJ/mol. For LaB&.74(llO)they found E h = 425 and Eg = 618 kJ/mol. The dependence of activation energies on crystal face makes it unlikely to find agreement between studies on single crystals and studies on polycrystalline powders. Our observation that the activation energy for the lower temperature La evaporation depends on cooling rate is presumably due to the relative magnitudes of the evaporation rates for La and B, the bulk-to-surface diffusion rates which may be different for the two elements, and the cooling rate. (42) Johnson, R.W.;Daane, A. H. J . Phys. Chem. 1961,65,909. (43) Spear, K. E. In Phase Diagrams: Materials Science and Technology; Alper, A. M., Ed.; Academic Press: New York, 1976; pp 92-159. (44) McKelvy, M. J.; Eying, L.; Storms, E. J. Phys. Chem. 1984,88, 1785 -.
(45) Davis, P. R.;Swanson, L. W.; Hutta, J. J.; Jones, D. L. J. Mater. Sci. 1986, 21, 825. (46) Nakamura, S.;Yee, S. N.; Tsong, T. T.; McLane, S. B., Jr. Surf. Sci. 1979, 87, 656.
Chem. Mater., Vol. 5, No. 12, 1993 1769
Nordine and Shiffman obtained a value of 602.9 f 5.9 kJ/mol for the heat of vaporization, AHv0(298),for La& by the third-law method by measuring absolute pressures of B and La in equilibrium with levitated La& spheres heated up to 2530 K.31 Our measured activation energy gives the apparent enthalpy of vaporization from eq 2 of AH*,(1900) = 578 kJ/mol, where 1900 K is in the middle of the temperature range of the measurements. Using heat capacities from ref 31 (and references therein), we obtain a AH*,(298) of 586 kJ/mol. Although this is 3% lower than the equilibrium value,31the second-lawmethod used here is generally less accurate than the third-law meth0d.4~ It is not possible to know if the difference between our number and that of Nordine and Shiffman is due to the difference between an equilibrium and a free evaporation measurement, differences between a low index face of a single crystal and a polycrystalline sphere, or to experimental error. We should note that apparent enthalpies of vaporization can be greater than, equal to, or less than true equilibrium e n t h a l p i e ~so ~ ?that ~ ~ the fact that our number is slightly lower is not necessarily at odds with an aV C 1. Despite the numerous previous studies of L a 6 evaporation, the direct desorption of both La+ and B+ has not been previously reported. However, Storms and MuelleP do mention, without comment, that they observed La+ evaporation in their Langmuir evaporation studies. Nakamura et aLG observed La3+,B+, and B2+ ions in field evaporation studies from a L a 6 tip in a field ion microscope. However, this is quite distinct from fieldfree thermal desorption of ions. Field-free desorption of ions is a well-known phenomena and is frequently encountered when the ionization energy of the parent atom of the desorbing ion is less than the work function of the substrate. For example, Chambers et al.24v25studied Cs+ desorption from a LaBs(1OO) surface for which the work function had been raised through the adsorption of oxygen. They showed that the activation energy for Cs+desorption followed the expected4 equation for ion desorption in the absence of external electric fields:
(3) where EIand E Nare the activation energies of neutral and ion desorption respectively, $ is the work function of the surface and I,, is the ionization energy of the atom. This equation can generally be used to describe both the case when E I C E N such as Cs+ desorption from W as well as for E1 > EN.^^ The latter case applies to La+ and B+ desorption as the ionization energies of La and B are 5.6 and 8.3 eV, respectively, while the work function of the LaBs(1OO) surface is only 2.5 eV. The above equation indicates that accurate measurements of activation energies of ion desorption combined with measurements for neutral desorption and the well-known ionization potentials of La and B could provide an alternative way of determining the work function of the surface. As the majority of past surface studies of L a 6 have focused on work function measurements, a new way to measure this quantity would be of considerable value. (C)Oxidized Surface. Each of the three oxides, Lao, BO, and BzOz, displays distinctly different thermal (47) Chase, J. M.; Davies, C.; Downer, J.; Fruip, D.; McDonald, R.; Syverud, A. J. Phys. Chem. Ref. Data 1986,14 (Suppl. No. l),270-272. Part I, Al-Co; Vol. 14, JANAF thermochemical tables, pp 270-272. (48) Ionov, N. I. B o g . Surf. Sci. 1972, 1 (3), 238.
1770 Chem. Mater., Vol. 5, No. 12, 1993
Ozcomert and Trenary
desorption peak profiles and 0 2 exposure dependences. from L a 0 fragmentation that obviously follows the same This indicates that the rate-determining step for desorpdesorption profile as Lao. Although the L a 0 that we observe could be due simply to the disproportionation of tion is different for each oxide. However, each oxide desorption rate falls to zero a t about the same temperature Lazo3, it could also be due to reduction by the substrate. From the work of Chupka et al.,S1one obtains for the of 1480 K. This suggests that there is some degree of equilibration among the oxides rather than sequential reaction desorption. I t is thus probably not correct to think of the '/,La(s) + 1/3La203(s) LaO(g) (5) surface near the desorption temperature as composed of particular oxides or of the oxygen as occupying lanthanum a m 2 9 g 0 of 564kJ/mol. Alternatively, we obtain from the or boron sites, although such considerations may apply at standard enthalpies of formation of reactants and room temperature. In discussing the desorption behavior products46~4~2 a AH29g0 of 523 kJ/mol for the reaction as a function of 02 exposures, we consider three oxide coverage regimes; the very low submonolayer coverages, '/3La@3(S) + 1/3B(s) 2/3LaO(g)+ '/,BO(g) (6) the region near one monolayer and the final stage of the This reaction could explain the appearance of L a 0 and oxidation above -20 langmuirs where the sticking coefBO at about the same temperature. However, if this ficient has fallen to a low value. reaction were the rate-limiting step in the desorption of At the very lowest oxygen coverages both the L a 0 and BO(g), we would expect the BO(g) desorption to follow BO desorption peaks grow in as single peaks with slight the same zero-order kinetics as observed for L a 0 with the shifts to higher temperatures. A strictly constant dessame activation energy. Thus, if the L a 0 is produced by orption peak temperature is observed when the desorption the above reaction, there must be some other rate-limiting rate is proportional to coverage and the rate constant step for desorption of BO(g). follows an Arrhenius form with a preexponential and The vaporization of pure BzO3(s) under nonreducing activation energy that are independent of coverage. Firstconditions produces B203(g).53 However, under reducing order desorption would imply that the desorption of L a 0 conditions B202(g) is the dominant p r o d ~ c t . ~ We ~~~" and BO molecules occurs as isolated events with the have found that thin films of B2O3 directly deposited on probability of desorption for a given molecule at a a boron surface evaporate mainly as B202(g) in the range particular temperature being simply equal to the first1100-1200 K,36-37 essentially the same temperature range order rate constant at that temperature. The net desas observed here for the low-temperature BzOz(g) peak. orption rate is then just equal to the number of molecules Inghram et al." found a AH~~OOO of 393 kJ/mol for the per unit area times their probability of desorbing. We reaction would expect first-order desorption if the rate-limiting step for the appearance of LaO(g) and BO(g) were simply 1/3B(s)+ '/3B203(1) Bz02(g) (7) desorption of the molecules adsorbed on top of the surface. which is significantly higher than the value of 233 kJ/mol First-order desorption would also be observed if the rateobtained here for the activation energy of B202(g) deslimiting step were the combination of mobile La and B orption. Since the appearance of BzOz(g) is generally with 0 atoms on the surface and if the number of such La observed as a product of the reduction of B203(s,l), a and B atoms were independent of oxygen coverage. reasonable hypothesis is that the BzOz(g) desorption that Although some B202 desorption is observed at the lowest we observe is the result of reduction of surface B2O3. This coverages, it is an insignificant amount. is also consistent with the results of Davis and Chambers,22 For the final stage of oxidation evidence exists for the presence of La203 and B2O3 on the ~ u r f a c e .The ~ ~ ~ ~ ~who observed a large increase in B202 relative to BO for oxidation under an ambient pressure of 02,suggesting the question then arises as to what would be observed if a formation of more surface B203 than was possible under mixture of B203 and La203 were heated in vacuum. The our conditions. If we take as a given that B203 is present vaporization of pure LazOs(s) proceeds by the reaction on our surface, then the small amount of B202 observed relative to BO and L a 0 suggests that the B203 undergoes 1/3~az03(s)2 / 3 ~ a ~ + ( g'/,O(g) ) (4) an unusual reduction reaction to BO(g). The latter with a AH of 605 k J / m 0 1 . ~ ~This 1 is 22% lower than the possibility could be tested by studying the reactions of activation energy for L a 0 desorption that we obtain. As Bz03 directly deposited on the La& surface. noted earlier, the activation energies of free evaporation The net reaction for the removal of the oxide from the can often be considerably larger than the corresponding surface should be consistent with (a) the presence of both equilibrium enthalpies. Note that for the purpose of Bz03 and La203 on the surface, (b) the dominance of LaOcomparing AH'S of reaction with activation energies (g) and BO(g) as the desorbing oxide species, and (c) a net measured by monitoring one product of the reaction, it is stoichiometry that removes B and La in the form of oxides important to write the reaction as producing 1 atm of gas in a 6:l ratio. Such a reaction is the following: (or more precisely, 0.1MPa). This is equivalent to making sure the stoichiometric coefficients of all gas-phase prod' / z l ~ ~ 6 (+s'/21~a203(s) ) + 2/7B203(1) ucts add to 1. The direct desorption of 0 atoms from the 1/7LaO(g)+ 6/7B0(g) (8) surface would be difficult to observe in our experiments as there is a large O+ signal due to fragmentation of which has a A H 2 9 g 0 of 446kJ/mol. The fact that activation background CO, C02, and H2O. There is also an 0 signal energies for the desorption of LaO(g) and BO(g) are not the same indicates that although the above equation might
-
-
-
-
-
(49) Goldatein, H. W.; Walsh, P. N.; White, D. J. Phys. Chem. 1961,
65,1400. (50) Smodes, S.; Drowart, J.; Verhaegen. J. Chem. Phys. 1965,43 (2),
732.
(61)Chupka, W. A.; Inghram, M. G.; Porter, R. F. J. Chem. Phys. 1966,24, 792.
(52) Topor, L.; Kleppa, 0. J. J. Chem. Thermodyn. 1984, 16, 993. (53) Soulen, J. R.; Margrave, J. L. J . Am. Chem. SOC.1966, 78,2911. (54) Inghram, M. G.; Porter, R. F.; Chupka, W. A. J. Chem. Phy8. 1956,25, 498.
Oxide Thermal Desorption from the LaBG(100)Surface
represent the net chemical change, the rate-limiting steps for the desorption of LaO(g) and BO(g) must be considerably different from the above. In the above net reaction (eq 8) LaO(g) and BO(g) are the only products which implies that 7/2 mol of 0 2 is consumed for every mole of LaB6 removed through oxide vaporization. This reaction does not account for the small amount of B202 observed nor is it consistent with the results of Figure 9, which indicate less than a 6:l B:La removal rate in the form of oxides. The fact that the ratio varies with 0 2 exposure as shown in Figure 9 is further evidence that reaction 8 is far too simple to describe even the net reaction. We now consider oxide desorption from intermediate oxide coverages corresponding to exposures from 0.3 to about 10 langmuirs. For this exposure range we observe the emergence of a zero-order L a 0 desorption peak followed by a second broad and lower temperature BO peak. The zero-order L a 0 peak that starts at 0.3langmuir clearly has the same leading edge as observed for even the highest oxygen exposures. Thus the rate-limiting step for this peak is the same at 0.3 langmuir as for the final oxidation stage. Since we attribute the final stage of Lao(8) desorption as due to either the reduction of LazOds) by the underlying substrate or to disproportionation of LazO&) (eq 4), this implies the presence of LazOds) on the surface at high temperatures even at submonolayer coverages. From LEED and UPS studies20921 it appears that 1ML of 0 atoms form a 1 X 1structure with the 0 atoms placed on top of a boron atom in the center of a 4-fold hollow of La atoms. Thus there must be significant surface rearrangement to form local patches of La203 before the onset of oxide desorption. Desorption of L a 0 then may occur as these local patches are reduced by nearby B atoms which are no longer part of the La& structure. This would then lead to L a 0 and BO desorption as in eq 3. Some B203 may also form under these conditions which
Chem. Mater., Vol. 5, No. 12, 1993 1771
is reduced to BzOz, giving rise to the small peak observed in this intermediate coverage range. In interpreting our results, we assume that following oxide desorption the surface stoichiometry is restored to its original value. Otherwise, it would not be possible to achieve the sort of reproducibility and systematic development of TPD features as a function of exposure that we observe. Preservation of the surface stoichiometry could, however,be achieve in three distinctly different ways. First, the B:La ratio of the oxides may be exactly the same as the surface composition. Second, the oxide desorption may lead to a nonstoichiometric surface composition, but the element in excess preferentially evaporates to restore the original surface composition. Third, the CVC could be restored following oxide desorption by diffusion from the bulk of the element in deficit at the surface. The results of Figure 9 exclude the first possibility and imply that oxide desorption would leave the surface La deficient. This in turn implies that mechanisms 2 and/or 3 replenishes the surface region with more lanthanum relative to boron.
Acknowledgment. This work was supported by agrant (AFOSR-92-J-0179)from the Air Force Office of Scientific Research. The authors thank Kimball Physics for supplying the LaB6 sample and for furnishing unpublished technical reports on the cathode performance. Note Added in Proof. After submitting this paper, we became aware of two recent p a p e r ~ on ~ ~the p ~structure of the clean and oxygen-covered LaBs(1OO) surface determined from surface vibrational spectra measured with high-resolution electron energy loss spectroscopy. (55) Nagao, T.; Kitamura, K.; Iizuka, Y.;Oehima, C. Surf. Sci. 1993, 290,436. (56) Nagao, T.; Kitamura, T.; Iizuka, M.; Umeuchi, M.; Oehima, C. Surf. Sei. 1993, 287/288, 391.
