Surface Phenomena in the Process of Thin Yttrium Hydride Film

Surface phenomena which occur in the process of thin yttrium hydride film YHx (0 < x < 2.62−2.80) formation were studied through simultaneous measur...
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Surface Phenomena in the Process of Thin Yttrium Hydride Film Formation R. Dus´* and E. Nowicka Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland Received June 6, 2000. In Final Form: October 5, 2000 Surface phenomena which occur in the process of thin yttrium hydride film YHx (0 < x < 2.62-2.80) formation were studied through simultaneous measurements of work function Φ and hydrogen pressure P during H2 interaction “in situ” with thin yttrium films, deposited under ultrahigh vacuum conditions. These studies were correlated with the results of parallel measurements of thin yttrium film electrical resistance and hydrogen pressure and optical observations. The experiments were performed over the pressure and temperature intervals: 10-8-10 Pa and 78-298 K, respectively. Thin YHx films (0 < x < 2.62-2.80) were easily obtained within the temperature interval 176-298 K, while at 78 K the atomic ratio x ) H/Y did not exceed 0.14. At low temperatures, consumption of hydrogen occurred without any measurable work function changes. This suggests generation of a symmetrical H-Y-H adspecies on the surface, reassembling the R′ phase known for the bulk of YHx. This adsorbate is stable at low temperature and inhibits the incorporation of hydrogen into the bulk. The features of Φ(t) and P(t) changes observed above 176 K suggest the formation of YHx via several successive surface processes: (i) generation of the H-Y-H adsorbate of a low degree of short range order; (ii) creation of positively charged β+ hydrogen adspecies which incorporates quickly below the surface; (iii) arising of YH3 precipitates on the surface, accompanied by an increase of work function.

Introduction During the past decade numerous studies have been performed to determine the properties of rare earth metal hydrides including yttrium and scandium, and at present the field is in full development. Several critical reviews have appeared1-5 summarizing the published results of these investigations. It has been established that interaction of H2 with rare earth metals leads to exothermic absorption and the formation of a solid solution or hydrides, depending on the thermodynamic conditions. In the case of yttrium the phase diagram consists of three basic parts: (i) the metallic solid solution of hydrogen in yttrium (hexagonal close-packed (hcp) R phase); (ii) metallic dihydride (face-centered cubic (fcc) β phase) with two H atoms situated in two available tetrahedral sites of the metal lattice, and (iii) trihydride (hcp γ phase) with a bigger unit cell than the original metal, and filling up both tetrahedral sites and one octahedral site. YH3 is not metallic but exhibits semiconducting properties. An interesting feature of the H2-Y system is the formation of a low-temperature R′ phase with an advanced shortrange order. The experiments with diffusive neutron scattering showed5-9 that within the R′ phase the yttrium atom bridged hydrogen pairs along the c direction, which (1) Arons, R. R., Rare Earth Hydrides; Landolt-Bornstein New Series; Hellwege K. H., Ed.; 1982; Vol. 12c, Chapter 63. (2) Arons, R. R., Rare Earth Hydrides; Landolt-Bornstein New Series; Wijn, H. P. J., Ed.; 1991; Vol. 19 d1, Chapter 23. (3) Wiesienger, G., and Hilscher, G. Magnetism of Hydrides. In Handbook of Magnetic Materials; Buschow, K. H. J., Ed.; NorthHolland: Amsterdam, 1991; Vol. 6, Chapter 6. (4) Schlapbach, L. Surface Properties and Activation. In Hydrogen in Intermetallic Compounds. II; Schlapbach, L., Ed.; Topics in Applied Physics; Springer: Berlin, 1992; Vol. 67, Chapter 2. (5) Vajda, P. Hydrogen in Rare Earth Metals. In Handbook on Physics and Chemistry of Rare Earth; Gscheider, K. A., Jr., Eyring, L., Eds.; Elsevier Science B.V.: Amsterdam, 1995; Vol. 20, Chapter 137. (6) Blaschko, O.; Krexner, G.; Daou, J. N.; Vajda, P. Phys. Rev. Lett. 1985, 55, 2876. (7) McKergow, M. W.; Ross, D. K.; Bonnet, J. E.; Anderson, J. S.; Schaerpf, O. J. Phys. C: Solid State Phys. 1987, 20, 1909.

is perpendicular to the surface. The fundamental cause of the H-Y-H basic units’ formation corresponds to hydrogen-induced coherent stress within the lattice6 or to the creation of a charge density wave as in the ordering of magnetic rare earth impurities in yttrium.7 The reversible transition of yttrium dihydride into trihydride is accompanied by a drastic change of optical properties. This phenomenon can be clearly observed using thin Y films.10,11 In this process, metallic, light-reflecting, thin YH2 film transforms into essentially nonreflecting, transparent, yellowish YHx (2 < x < 3) film. It was found recently that hydrogen exhibits negative valence in YHx (2 < x < 3) compounds,11 in contrast to its positive valence in many transition metal hydrides, such as PdHx or hydrides of group VB metals. The surface phenomena associated with yttrium hydride formation have not been widely studied. There are at least two reasons for the lack of these studies: (i) very high reactivity of yttrium, which leads to difficulties in preparing a clean surface of the sample, and (ii) the falling apart of the bulk samples of yttrium into powder, often observed in the course of trihydride formation. These difficulties can be avoided, however, when thin yttrium films deposited under ultrahigh vacuum (UHV) conditions are applied. Deposition of thin Y film at (1-2) × 10-8 Pa allows a sample with a clean surface to be obtained, and the thin film improves the structural stability of the trihydride.10 The surface and near-surface structure of single crystalline hcp(0001) films, deposited on a W(110) crystal under UHV conditions, during their interaction with H2 was studied (8) Fairclough, J. P. A.; Ross, D. K.; Berk, N. F.; Anderson, J. N.; Daou, J. N.; Vajda, P.; Blaschko, O. Z. Phys. Chem. 1993, 179, 281. (9) Udovic, T. J.; Rush, J. J.; Berk, N. F.; Anderson, J. N.; Daou, J. N.; Vajda, P.; Blaschko, O. Z. Phys. Chem. 1993, 179, 349. (10) Huiberts, J. N.; Griessen, R.; Rector, J. H.; Wijngaarden, R. J.; Dekker, J. P.; de Grott, D. G.; Koeman, N. J. Nature 1996, 380, 231. (11) denBroeder, F. J. A.; van der Molen, S. J.; Kremers, M.; Huiberts, J. N.; Nagengast, D. G.; van Gogh, A. T. M.; Hyuisman, W. H.; Koeman, N. J.; Dam, B.; Rector, J. H.; Plota, S.; Haaksma, M.; Hanzen, R. M. N.; Jungblut, R. M.; Duine, P. A.; Griessen, R. Nature 1998, 394, 656.

10.1021/la0007978 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/2000

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via X-ray photoelectron diffraction.12 The reversible formation of fcc YH2 was clearly observed. It was found that H-induced martensitic transformation of the Y lattice occurred without any loss of crystallinity. Our investigations concerning surface phenomena, which occur in the process of transition metal hydride formation (PdHx,13 TiHy,14 VHz,15 NbHu,16 NiHw17), have shown that two essentially different forms of hydrogen adspecies arise as follows: (1) Atomic, surface stable, negatively polarized (the negative pole of the dipole moment pointing away from the surface) adspecies present at the beginning of adsorption, referred to as β-, which cause an increase in the work function. (2) Positively polarized atomic adspecies induced by increase of coverage, decreasing the work function, referred to as β+. The β+ adspecies are not stable on the surface as are β- but incorporate into the bulk rather quickly. Transition metal hydrides are formed in this way. However, at low temperature some amount of the β+ adspecies remains on the outer surface, strongly decreasing the work function (by 1.3 V for PdH,13 0.55 V for TiH 0.45,14 and 1.7 V for VH0.2515). At temperatures above 90 K the β+ adspecies are mostly situated below the surface image plane (SIP), inducing a negative charge on the outer surface, thus increasing the work function. The question arises whether the above phenomena observed in the course of transition metal hydride formation are also common for rare earth metals or if the special features of these metals, such as the extensive short-range order with symmetrical H-Y-H species present at the surface of the R′ phase of yttrium hydride and a drastic change of the electronic structure during metal-semiconductor transition, lead to different behavior. This work was performed to obtain information in this field. Experimental Section The experiments were performed using an UHV glass system capable of routinely reaching the basis pressure of (1-2) × 10-8 Pa during thin Y film deposition. On the other hand, the apparatus permitted us to work with H2 pressure of the order of 10-8-10 Pa during hydrogen interaction with yttrium. The experimental work was divided into two parts: First we proved that a thin Y film can be converted into YHx (0 < x < 3) as a result of interaction “in situ” with H2 under our experimental conditions. Next, surface phenomena were studied in the course of separate experiments carried out under identical conditions. Thin Y film was deposited on the wall of a spherical cell made of Pyrex glass. The cell was equipped with a tungsten filament (d ) 0.3 mm) with spot-welded strips of yttrium foil (0.025 mm thick, Goodfellow, 99.9%) which served as an evaporator for thin Y film deposition. Two electrodes made of platinum foil, semimelted into the wall of the cell, allowed electrical contact with the film for measurements of its resistance changes in the course of YHx formation. It was possible to evaporate the yttrium completely from the tungsten heater. Thus the weight of the thin Y film was known. The average thickness of the film was ∼75 nm. During thin Y film deposition the cell was immersed in a liquid nitrogen bath. Next, the film was anealed at 350 K for 20 min under UHV conditions. Spectroscopically pure hydrogen, additionally purified in the course of the experiments by diffusion through a palladium thimble, was intrduced into the cell (separated from the pumps) in successive calibrated doses. The H2 pressure was measured continuously by means of an ultrasensitive, short response time Pirani gauge immersed in liquid nitrogen, reading hydrogen pressure within 10-4-10 Pa. Knowing the volume of the cell and (12) Hayoz, J.; Sarbach, S.; Pillo, Th.; Boschung, E.; Naumoviæ, D.; Aebi, A.; Schlapbach, L. Phys. Rev. B 1998, 58, 4270. (13) Dus´, R.; Nowicka, E. Langmuir 2000, 16, 584. (14) Nowicka, E. Vacuum 1996, 47, 193. (15) Dus´, R.; Nowicka, E. Langmuir 1998, 14, 5487. (16) Dus´, R. Surf. Sci. 1975, 5, 2 440. (17) Nowicka, E.; Wolfram, Z.; Dus´, R. Appl. Surf. Sci. 1990, 45, 13.

Langmuir, Vol. 16, No. 26, 2000 10259 the capacity of every H2 dose, the amount of hydrogen consumed can be calculated volumetrically, and the atomic ratio H/Y corresponding to the distinguished steps of the process of YHx formation can be determined. The resistance of the thin film was continuously measured within the interval 10-3-1010 Ω, and the optical features were observed. Usually the resistance of the pure thin Y film after sintering was ∼70 Ω. Surface phenomena in the course of YHx formation were studied by simultaneously measuring the work function Φ and H2 pressure changes, while hydrogen was introduced into the apparatus in successive doses. Precise ∆Φ measurements were performed by means of a continuously recording, sensitive (1 mV), short response time (1 ms), high stability (1 mV/h) static capacitor circuit.18,19 The courses of the ∆Φ(t)i and P(t)i functions were continuously registered following each successive ith hydrogen dose introduction into the static capacitor separated from the pumps. Equilibrium between the gas phase, the surface and the bulk was achieved when d(∆Φ)i/dt ) 0, and dPi/dt ) 0. Having the values of work function changes ∆Φeq.i determined at the equilibrium after each successive ith H2 dose introduction, the work function isotherm Φeq. ) Σ∆Φeq.i ) f(H/Y)T can be constructed.

Experimental Results and Discussion 1. Changes of Optical Transparency and Resistance of Thin Yttrium Films in the Process of Its Interaction with H2 at 298 and at 78 K. Figure 1 shows photographs of the “empty” cell (without Y film, Figure 1A), the cell with the light-reflecting pure thin Y film (Figure 1B), and the cell with the transparent yellowish thin YH2.62 film kept under H2 pressure 1Pa (Figure 1C). The observed changes of optical properties agree with those reported for β f γ transition within YHx (2 < x < 3).10 Freezing of the cell containing YH2.62-2.80 to 78 K resulted in an increase of the film’s resistance, indicating its negative temperature coefficient of electrical resistivity, characteristic of semiconducting materials. Next the cell was reheated to 298 K and evacuated below 10-3 Pa. This led to a gradual loss of the film’s optical transparency and to a decrease of its resistance. The positive temperature coefficient of electrical resistivity, characteristic of metals, was now determined while freezing the cell to 78 K. H2 redosing at 298 K restored all the features of YH2.62 described above. We can conclude that pure thin film deposited under UHV conditions exhibits switchable optical properties at 298 K as a result of its interaction “in situ” with H2 within the pressure interval 10-3-10 Pa. The cell with the transparent thin YH2.62 film maintained at H2 pressure ∼1 Pa was immersed in a liquid nitrogen bath and evacuated below 10-4 Pa. At 78 K the evacuation did not alter the transparency. Figure 2 shows the dependence of the relative equilibrium resistance (Req/R0 ) of thin yttrium film, determined at 298 K, as a function of hydrogen concentration (H/Y). Here Req ) ∑Req.i is the sum of the resistance Req.i registered after the ith hydrogen dose introduction into the cell with thin Y film when an equilibrium indicated by the requirements dR(t)i/dt ) 0 and dP(t)i/dt ) 0 was reached. The courses of the resistance changes ∆R(t)i measured as a result of the ith H2 dose introduction, characteristic of the distinguished steps of YHx formation, are presented in the inserts. The shape of the determined function (Req/R0 ) ) f(H/Y)298K agrees with that reported in the literature for thin YHx (0 < x < 3) film.10 H2 interaction with thin Y film carried out at 78 K in the course of a separate experiment resulted in a low hydrogen uptake corresponding to (H/Y) ∼ 0.14, while the relative resistance increased by ∼10%. The dependence (18) Nowicka, E.; Dus´, R. Surf. Sci. 1984, 144, 665. (19) Bachtin, A. Vacuum 1985, 12, 519.

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Figure 1. Change of the optical transparency of thin Y film in the process of thin yttrium hydride film formation at 298 K: (a, left) empty cell (without thin Y film); (b, center) cell with reflective pure thin yttrium film; (c, right) cell with transparent thin YH2.62 film.

Figure 3. Dependence of the equilibrium relative resistance Req/R0 of thin YHx (0 < x < 0.14) on the average (H/Y) ratio at 78 K. R0 corresponds to the resistance of pure Y film. The concentration na of the hypothetical deposit of a surface layer consisting of H-Y-H units is also shown on the abscissa axis. The course of the time-dependent resistance changes ∆ri, caused by the introduction of a single successive ith H2 dose into the cell for the distinguished (H/Y) ratio, is shown in the insert.

Figure 2. Dependence of the relative equilibrium resistance Req/R0 of thin YHx film on the average (H/Y) ratio at 298 K. R0 corresponds to the resistance of pure thin Y film. Courses of the time-dependent resistance changes ∆Ri, caused by the introduction of single successive ith H2 doses into the cell at the distinguished (H/Y) ratio, are shown in the inserts.

of (Req/R0) on (H/Y)78K is shown in Figure 3, while the typical course of ∆R(t)i is presented in the insert. It can be seen that the equilibrium was reached very quickly, pointing to the change of resistance due to a surface process. We suggest that under our experimental conditions, H2 interaction with thin Y film at 78 K leads to the formation of a layer of hydride on the surface, which strongly inhibits incorporation of hydrogen into the bulk.

This layer could correspond to the short-range ordered R′ phase consisting of symmetrical H-Y-H species.8,9 This phenomenon essentially distinguishes the low-temperature behavior of the H2-Y system from the features of hydrogen-transition metal systems, which exhibit a rapid and abundant consumption of hydrogen under similar conditions.13-17 According to the above suggestion, knowing the amount of hydrogen consumed, the geometrical area of the film, and taking the density of yttrium atoms on the thin film surface as the average for the planes (111), (100), and (211)20 equal to 9.2 × 1014 Y atoms/cm2, one can roughly estimate the roughness factor of thin Y film as ∼ 4.7, and the total population of yttrium atoms (20) Brennan, D.; Hayward, D. O.; Trapnell, B. M. W. Proc. R. Soc. London, Ser. A 1960, 58, 81.

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Figure 4. Work function and H2 pressure changes during thin yttrium hydride film formation at 78 K. Arrows with numbers indicate successive hydrogen dose introduction into the static capacitor. The course of ∆Φ(t) during isothermal desorption caused by evacuation is also shown.

on the thin Y film surface as ∼6.5 × 1017. In this method the roughness factor represents the ratio of the real area of the thin film to its geometrical area. 2. Work Function Changes ∆Φ in the Process of H2 Interaction with Thin Yttrium Films. 2.1. Hydrogen Interaction with Thin Y Film at 78 K. The introduction of several successive H2 doses, each of capacity corresponding to ∼10% of the population of Y atoms on the thin film surface, into the static capacitor did not result in any measurable change of work function, while hydrogen consumption was clearly detected from pressure decrease ∆P(t)i measurements after every ith dose (Figure 4). The introduction of several successive bigger H2 doses, increasing the pressure above 1 Pa, resulted in a work function decrease by ∼35 mV. Isothermal evacuation increased Φ by the same value, and H2 redosing restored it again. There are only two explanations for hydrogen consumption which is not accompanied by a change of work function: (i) formation of a nonpolar adspecies on the surface, and (ii) extremely rapid positioning of hydrogen adatoms deep in the bulk of the metal (below the subsurface), within the space of the metal which is completely separated from the surface by electron screening. However, we reject this second explanation for the experiments carried out at 78 K using a short response time (1 ms) detection circuit for ∆Φ measurements. On the other hand, the conditions for nonpolar adspecies creation can be fulfilled for a hydrogen-yttrium system when symmetrical short range order units H-Y-H, with axis perpendicular to the surface8,9 are formed as suggested above. Strong inhibition of further adsorption suggests that one H atom is on the top of the Y surface and one is between the first and second Y atoms layer. The experiment shows that this layer is stable on the surface at 78 K and is not altered by isothermal evacuation. At H2 pressure of the order of 1 Pa, reversible adsorption of hydrogen occurs. Examination of the work function decrease caused by isothermal evacuation showed that its rate fits the first-order kinetics equation. This corresponds to desorption of the molecular adspecies, assuming

Figure 5. Work function and H2 pressure changes during the distinguished steps of thin yttrium hydride film formation at 298 K. Φ(t)i and P(t)i changes registered as a result of a single ith hydrogen dose introduction at the determined concentration (H/Y) are represented in the successive sections A, B, and C. Arrows indicate hydrogen dose introduction. The arrow numbered (1) represents three successive small doses rapidly consumed without any measurable work function changes. ∆Φ(t) in the course of the isothermal desorption caused by evacuation of the static capacitor containing thin YH2.62 film is shown in section D.

the linear dependence of Φ on the concentration of the weakly bound deposit of hydrogen. 2.2 Hydrogen Interaction with Thin Yttrium Film at 298, 205, and 176 K. The introduction of three successive H2 doses of a typical capacity (each of 10% of the population of yttrium atoms on the surface of the thin film) into the static capacitor maintained at 298 K did not result in a measurable change of work function, while pressure measurements indicated a rapid consumption of hydrogen (Figure 5A). We correlate this phenomenon with the generation of the symmetrical H-Y-H species on the surface, similarly as at low temperature. However, at 298 K the degree of short-range order is low, the layer is unstable, and a lot of Y atoms are present on the outer surface. A big dose of H2, increasing pressure by 1 Pa, caused adsorption on these sites accompanied by a significant change of work function (Figure 5A). A small decrease of Φ followed by its strong increase was registered. This corresponds to the formation of a positively polarized hydrogen adspecies, referred to as β+, within the nonpolar H-Y-H layer, followed by their incorporation below the surface. The nature of the β+ adspecies differs essentially from that of the molecular adsorbate which arose at 78 K. We suggest that according to Grimley’s model,21 the driving force for β+ adspecies creation is the coverage-induced split between even and odd induced localized electron states within the hydrogen

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Figure 6. Work function and H2 pressure changes during the distinguished steps of thin yttrium hydride film formation at 205 K. Φ(t)i and P(t)i changes registered as a result of a single ith hydrogen dose introduction at the determined concentration (H/Y) are represented in the successive sections A, B, C, and D. Arrows represent hydrogen dose introduction. Creation of the positively polarized state of hydrogen adsorbate β+, which is a precursor to yttrium hydride formation in the bulk, is clearly seen in sections A and B. The effect of isothermal evacuation of the static capacitor (no change of work function) is marked in section D.

adsorbate. At a critical coverage the lowest states of the adsorbate merge into the metal conduction band. Then donation of electrons from hydrogen adatoms into unoccupied states of the conduction band occurs, leading to the generation of the positively polarized adsorbate. The β+ adspecies positioned on the outer surface decrease the work function, while if situated below the SIP, they induce a negative charge on the outer surface, increasing Φ according to Smoluchowski’s model.22 It is not certain whether the SIP is situated within the first or the second atomic layer of thin Y film. In this paper we shall call the “subsurface” the space around the yttrium surface in which positioned β+ species induce a negative charge on the outer surface, thus increasing the work function. The β+ adspecies present in the interstitial sites within the dipper atomic layers of yttrium do not influence Φ because of metal electron screening. Positioning within the subsurface, rather than on the outer surface, is clearly more preferable for β+ adspecies. Further doses of H2 resulted in a strong, rapid increase of work function, followed by its slow decrease (Figure 5B). We correlate this phenomenon with generation of YH3 precipitates with negatively charged hydrogen on the outer surface. It has been proved

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that hydrogen in YH3-δ exhibits a negative valence11 and hence leads to the increase of Φ. The concentration of hydrogen on the surface strongly predominates that in the bulk; thus the system readjusts to the new equilibrium via negatively charged hydrogen incorporation below the surface. The mobility of negatively charged hydrogen species characteristic of YHx (2 < x < 3) within a dihydride matrix was recently proved.11 As a result, the ∆Φ transient is observed (Figure 5B,C). If our interpretation of the work function changes is correct, then at lower temperature the presence of β+ adspecies on the outer surface should be more pronounced, and their incorporation below the surface should be slower. Also, YH3 precipitates on the surface, increasing the work function, should be more stable. Thus the ∆Φ transient should disappear at high average concentration (H/Y). Studies of YHx formation at 205 and 176 K confirmed our expectations. The work function changes registered at 205 K are shown in Figure 6. The features of ∆Φ registered at 176 K were similar. The presence of β+ adspecies, forming a precursor surface state for the process of yttrium hydride formation, is then clearly demonstrated, and the concept concerning their incorporation into the subsurface and further into the bulk is supported (Figure 6A-C). At (H/Y) ∼ 2.8, the ∆Φ transient is not visible any more (Figure 6D). One can see in Figure 5D that evacuation of the static capacitor containing thin YH2.68 film at 298 K resulted in a decrease of the work function, while at 205 K (Figure 6) the change of Φ was unmeasurable within the time of observation (∼10 min), indicating a significant stability of the hydride at this temperature. If the rate of ∆Φ caused by the evacuation is determined by decomposition of YH3, then it should be proportional to the difference between the work function at a specified time during the evacuation Φ(t) and at the equilibrium Φeq achieved at low H2 pressure: ∆Φ(t) ) [Φ(t) - Φeq], assuming the linear dependence of the work function on YH3 concentration on the surface. Expressing the dependence of ∆Φ(t) on time in the course of evacuation in the integrated form we have

ln[Φ(t) - Φeq] ) ln[Φ(t)0) - Φeq] - k1t

(1)

where k1 is a rate constant. Figure 7 shows that our expectations are fulfilled. ∆Φ(t) dependence on time, registered during isothermal desorption of the molecular hydrogen adsorbed on the H-Y-H surface layer at 78 K, should be described by the same type of equation, however with a different rate constant. One can notice that desorption of the molecular adspecies from the outer surface of YH0.14, induced by lowering of H2 pressure at 78 K, is much faster than YH3 decomposition occurring at 298 K. The rate constants are k1(78 K) ) 0.112 s-1 and k1(298 K) ) 0.031 s-1. It has been observed that the molecular deposit of hydrogen was also reversibly adsorbed on the outer surface of the thin YH2.80 film when the temperature of the static capacitor was reduced from 205 to 78 K. All features of ∆Φ(t) were exactly the same as those described above for adsorption on YH0.14. The work function isotherm determined for thin YHx (0 < x < 2.68) film at 298 K is shown in Figure 8. It differs significantly from those known for transition metal hydrides such as PdHx, TiHy, and VHz.13-15 Conclusions

(21) Grimley, T. B. In Chemisorption; Garner, W. E., Ed.; Butterworth: London, 1957. (22) Smoluchowski, R. Phys. Rev. 1941, 60, 661.

1. Thin yttrium film deposited under UHV conditions interacting “in situ” with H2 easily forms hydrides YHx

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Figure 7. Examination of work function changes caused by isothermal evacuation of the static capacitor with thin YH0.14 film at 78 K (O) and with thin YH2.68 film at 298 K (×), according to the first-order kinetic equation for desorption.

(0 < x < 2.62-2.80) within the temperature interval 176298 K at hydrogen pressure of the order of 1 Pa. However at 78 K under these conditions the uptake of hydrogen is small, and the average atomic concentration (H/Y) does not exceed 0.14. 2. The measurements of ∆Φ(t), ∆R(t), and ∆P(t) suggest that at 78 K, a stable adsorbate of extensive short-range order, consisting of H-Y-H units with the axis perpendicular to the surface, is formed on the surface of thin Y film. This strongly inhibits incorporation of hydrogen from the gas phase into the bulk. At H2 pressure above 1 Pa, molecular hydrogen is reversibly adsorbed on the surface layer H-Y-H. 3. Within the temperature interval l76-298 K, formation of YHx in the bulk occurs via the following successive surface processes: (i) Arising of an unstable, low degree of short range order layer consisting of symmetrical H-Y-H adspecies, which do not change the work function of thin Y film.

Figure 8. Work function isotherm obtained at 298 K for the process of thin YH2.68 film formation. H2 equilibrium pressure is marked for several points. The dotted line represents the effect of the isothermal decreasing of hydrogen pressure to 10-4 Pa.

Becouse of unstability of the adsorbate a large amount of Y atoms is present on the outer surface. (ii) Creation of positively charged atomic β+ adspecies on yttrium atoms present on the surface among H-Y-H units. The β+ adatoms quickly incorporate below the surface, increasing hydrogen concentration in the bulk. (iii) Formation of negatively charged hydrogen adspecies associated with arising of YH3 precipitates on the surface. The negatively charged H adspecies incorporate below the surface, forming trihydride in the bulk. LA0007978