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Auger spectra of lithium hydride
SURFACE
SCIENCE
AUGER
46 (1974) 345-357 0 North-Holland
SPECTRA
OF LITHI~
Publishing Co.
HYDRIDE*
G. L. POWELL Nuclear Division. Union Carbide Corporation, Oak Ridge, Tennessee 37830, U.S.A. and G. E. McGUIRE,
D. S. EASTON and R. E. CLAUSING
Metalsand Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Received 22 July 1974; revised manuscript
received 3 September
1974
Auger electron emission spectra have been observed for lithium hydride in three conditions: (I) cleaved in vacuum, (2) prepared by the reaction of hydrogen gas with clean lithium metal, and (3) by annealing slightly oxidized lithium hydride single crystals in vacuum. The dominant Auger line (40 + 1 eV) was found to be a KVV transition involving valence electrons from the anion and was indistinguishable from a similar transition for lithium oxide at room temperature. Lithium hydride surfaces lose hydrogen in vacuum causing the formation of a lithium metal phase at room temperature and a significant reduction in surface hydride stoichiometry at 600°C.
1. In~oduc~ion Auger electron emission lines in the energy range of 10 eV to 90 eV have been reported for lithium in lithium compoundsr-s). The Auger spectra of lithium oxide 5), lithium hydroxide 4), and LiH 4, are dominated by a 40+ 1 eV line at room temperature. Zehner et al.‘) have described in detail the lithium metal Auger emission spectra. A 52 eV KVV transition is the primary feature. (A very weak line at 82 eV originating from a doubly ionized lithium K shell is the only other feature in the spectrum initiated from ionized states of lithium metal.) These observations indicate a large (12 eV) “chemical shift” associated with formation of lithium compounds which should be very useful in studying the surface reactions of lithium metal and lithium compounds. This report describes experiments conducted to further characterize the 40 eV Auger electron transitions observed for lithium hydride4) and lithium oxides) at room temperature and to investigate processes that occur at the solid lithium hydride surface over the temperature range from 20°C to 600°C. * Work performed at the Oak Ridge National Laboratory under the sponsorship of Nuclear Division, Union Carbide Corporation, and Los Alamos Scientific Laboratories, Los Alamos, New Mexico, U.S.A,, for the U.S. Atomic Energy Commission. 345
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2. Experimental
The LiH single crystals (99.9% pure) used in these experiments were cleaved from a single crystal prepared by recrystallization from the melt by Holcombe and Johnsong). The lithium metal, 99.97% pure, was Foote Mineral Company reactor-grade, further purified by holding the molten lithium in contact with zirconium foil for LOOhr. The lithium metal was cast into a 0.63 cm diameter stainless steel tube having a 0.025 cm wall thickness. These operations were carried out in a sealed system under argon. Three types of experiments were carried out in this investigation : (Experiment I) LiH single crystals were cleaved in vacuum at room temperature; (Experiment II) lithium metal was cleaned in vacuum by abrasion and ion bombardment and reacted with hydrogen gas at room temperature; and (Experiment HI) an air-exposed LiH crystal was heated from 25 to 600°C while Auger spectra were observed. Experiments I and II were carried out in an ion-pumped vacuum system containing a liquid-nitrogen-cooled titanium-sublimation pump. The residual gas pressure in the system measured by an ionization gauge was 2 x lo-‘* torr consisting of mostly argon and hydrogen as indicated by a residual gas analyzer. Samples could be introduced to the system and removed from the system via an independently ion-pumped interlock manifold while maintaining a high vacuum in the spectrometer chamber. The sample could be manipulated within the analysis chamber so that it could be abraded with a file or, in the case of LiH crystals, cleaved along the (100) plane. Auger electron spectra were taken with a high-resolution, double-pass cylindrical mirror analyzer (Physical Electronics Industries Model 15-256). A co-axial electron gun (1000 eV, 1 pA) was used and the electron beam struck the sample at 15” from normal to the surface. A signal proportional to the first derivative of the electron energy distribution [dN(E)/dE J was obtained by modulating the analyzer sweep voltage with a superimposed 2 kHz ac voltage and using a lock-in amplifier to detect the ac signal in the output current from the analyzer electron multiplier. This signal was recorded on an X-Y recorder as a function of electron energy. A 1 V peak-to-peak amplitude was selected for the modulating voltage. The samples were biased - 150 V with respect to ground potential in order to obtain optimum response from the electron multiplier. The sample could be argon ion-bombarded simultaneously with the Auger analysis. Oxygen-free lithium metal (as indicated by no detectable 510 eV or 40 eV Auger transitions) could be prepared and maintained in this system for periods exceeding I hr. Experiment III was carried out in an ion-pumped vacuum system similar to that described by Holcombe et aL4). A thin crystal (1 cm x 0.3 cm X0.3 cm
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was cleaved in an argon dry box and mounted on a high-purity iron (99 + %) sample holder, which was, in turn, mounted on a universal manipulator. The sample holder and manipulator were then covered with a plastic bag and transferred to the LEED vacuum chamber, which was being purged with flowing nitrogen. These precautions were necessary to prevent the crystal from reacting excessively with water while it was being installed and before a high vacuum could be attained. Even so, the crystal was covered with a thin film consisting of a mixture of lithium oxide, hydroxide, carbonate, and bicarbonate, since lithium hydride reacts readily with water and carbon dioxideie~ii). A sample prepared using similar precautions was cleaved a second time normal to the surface to be analyzed and immediately loaded into a scanning electron microscope. A film approximately 1 urn thick was observed that was identical to that previously identified as LiOHlO). The components of the vacuum system were vacuum baked at 250°C and the sample holder was vacuum baked at 800 “C prior to mounting the LiH sample, The vacuum system containing the LiH sample could not be baked again prior to these measurements since that would perturb the air-exposed LiH surface. After mounting the sample in the vacuum chamber, the system was left undisturbed for several days in order to establish a base pressure of approximately 1 x IO-’ torr. For Experiment III, the LiH Auger spectra were obtained using a cylindrical mirror analyzer (Physical Electronics Industries Model 10-234) operated in a manner similar to the analyzer used in Experiments I and II with the exception that a 2 V peak-to-peak amplitude was selected for the modulating voltage. The electron beam (2000 eV, 2 uA) was incident on the crystal at an angle of 70” to the normal of the specimen surface. The Auger electron spectra were recorded by first scanning the spectral region of 0 eV to 100 eV and then scanning two spectral regions approximately 50 eV wide, one containing the carbon Auger emission line at 275 eV and the other containing the oxygen Auger emission line at 510 eV. Approximately five minutes were required to obtain these spectra. Occasionally during the experiment a complete spectrum over the 0 eV to 1700 eV range was obtained. On these occasions, Auger emission lines for other elements were not detected. A multichannel recorder was used to continuously monitor each of the following items: the Auger electron spectra at two sensitivity levels, the electron beam energy, the vacuum gauge, the residual gas analyzer, and the sample temperature. The electron-beam current could not be monitored continuously, but was checked several times during the experiment and found to be constant. The sample could be heated with a tungsten filament affixed to the back of the sample holder. The sample temperature was measured by a chromel-
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alumel thermocouple attached to the sample holder near the sample. The sample holder was operated at ground potential during Experiment III. 3. Results 3.1. EXPERIMENTI Typical Auger spectra of LiH cleaved in vacuum are shown in fig. 1. The spectrum is dominated by a 40+ 1 eV line and no oxygen was detectable by the 510 eV Auger transition. Argon ion bombardment or allowing the cleaved crystal to stand in vacuum overnight at room temperature resulted in the appearance of the lithium metal (52 eV) Auger line and a corresponding decrease in the 40 eV line intensity. Upon establishing the presence of lithium metal on the LiH surface, the sample reacted with 0, as demonstrated by the appearance of the 510 eV line, the growth of the 40 eV line, and the disappearance of the 52 eV lithium metal line. The apparent shift in the 40 eV line shown in fig. 1 may have been due to sample charging. Sample charging was
/’
/ :
I 20
60 Energy
(Volts)
Fig. 1. Auger spectra for cleaved LiH crystal ((100) face) at room temperature. Curve 1: immediately after cleavage. Curve 2: argon ion bombarded (1000 V, - 5 pA/cmz) for 51 min. Curve 3 : argon ion bombarded (1000 V, - 5 pA/cm2) for 62 min.
AUGER
a significant decision
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to prepare
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during the LiH cleaving experiment LiH by the reaction
of lithium
and resulted in the
metal with Hz.
3.2. EXPERIMENT II The Auger spectrum of lithium metal cleaned by abrasion and ion bombardment is shown in fig. 2 (Curve 1). The 27 eV line is due to sodium metal that segregates onto the lithium metal surface after ion bombardment’). The cleaned lithium surface reacted with 0, at pressures as low as 5 x lo-” torr 0, to produce a 40 eV line at the expense of the 52 eV line eventually resulting in a spectrum similar to Curve 4, fig. 2. Spectra of lithium oxide prepared in this manner have been repot-teds). During this reaction of lithium metal with oxygen, the ratio of the 40 eV lithium oxide Auger line intensity to that of the 510 eV oxygen Auger line intensity remained constant. Upon standing
20
40
60
Energy (Volts)
Fig. 2. Auger spectra of the lithium metal sample. Curve 1: as cleaned by abrasion and ion bombardment (1000 V, N 5 uA/cma); note small sodium line (22 eV) and lithium oxide line (40 eV). Curve 2: sample per Curve 1, immediately after 1 hr exposure to 740 torr HP. Curve 3: sample per Curve 2, 35 min after return to high vacuum system. Curve 4: sample per Curve 3, after 70 min exposure to 2 x 10-s torr 02.
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(2 x 10W1’ torr) for three days at room
lithium line did not reappear. The Auger spectrum of cleaned lithium
temperature,
metal was unchanged
the 52 eV by exposure
to H, at pressures up to 1 x 10e5 torr for one hour at room temperature. The cleaned lithium sample was placed in the interlock manifold at room temperature and exposed to H, at 740 torr for 1 hr. The visual appearance of the lithium metal sample was unchanged by this exposure. However, the Auger spectrum displayed an Auger line at 39 eV similar in shape to the lithium oxide Auger line, no Auger at 52 eV (lithium metal) and no 510 eV oxygen Auger line. Upon standing in the analysis chamber at 3 x lo-” torr (mostly Ar and Hz), the 39 eV line intensity decayed to a lower value as the 52 eV lithium line grew in time and both lines exponentially approached the condition shown in fig. 2 (Curve 3) with a time constant of 700 sec. Once the
Time
(seconds)
Fig. 3. Auger analysis of the oxidation of lithium metal on a lithium hydride surface. The origin of the lithium metal was the decomposition of the lithium hydride in vacuum (2 x lo-r0 torr). Oxygen was admitted to the vacuum chamber by a leak valve at t = 0 and maintained at 1 x 10-s torr for I >O. (m) 52 eV lithium metal Auger line; (a) 40 eV lithium hydride and/or lithium oxide Auger line; ( 0 ) 510 eV oxygen Auger line scaled by a zero shift and a scale factor such that the point at t = - 100 set and at t = 3700 set have ordinate positions identical to the corresponding 40 eV Auger line.
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52 eV lithium line was developed, exposure to 1 x lo-* torr 0, resulted in its decay accompanied by the growth of the 39 eV and 510 eV Auger lines (fig. 3). 3.3. EXPERIMENT 111 A typical heat treatment for the air-exposed LiH crystal was to raise the temperature from 20 to 600 “C at a heating rate of 3 “C/min, hold the sample at 600°C for 2 hr, and allow the sample to cool to ambient temperature. The
20
30
40
50
Energy (Volts) Fig. 4. Auger spectra from air-exposed LiH crystal (100) surface during the first anneal (sample number, attenuation, and temperature are reported beside each spectrum). Curve 10: was obtained immediately after sample temperature reached 600°C f+= 5 x lo-? torr). Curve 11: was obtained after the sample had been maintained at 600°C for 2 hr (PH, < 5 x 1O-8 torr).
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c
-- --
Ii
Fig. 5. Auger line intensities for carbon (270 eV), oxygen (510 eV), and lithium as a function of temperature during the first anneal; (0) hearing cycle; ( 0) cooling cycle. For lithium: (a) 40-50eV lines, heating cycle; (0) 40-50eV lines, cooling cycle; (m) 52 eV, cooling cycle. heating cycle of the first anneal was carried out in the ion-pumped vacuum system so that gas desorption from the sample could be monitored. Since the sample holder had been exposed to air for approximately 5 min after the vacuum bake at 800°C and could not be baked under ultra-high-vacuum conditions without perturbing the attached sample, the source of the gases evolved cannot be unambiguously assigned to processes occurring only on the LiH sample. Water was evolved over the temperature range of 250 to 400 “C reaching a maximum pressure of 3 x low6 torr at 370°C. Above 400 “C, H, was the main outgassing component reaching a pressure of 5 x 10e7 torr at 600°C. The Hz pressure dropped to 5 x lop8 torr during the 2 hr hold at
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3 Li
\ 0
L-e
0
I
IQ
I
400
200 Temperot”re
-_I 600
PC)
Fig. 6. Auger line intensities for carbon function of temperature during the second For lithium: (0) 40-50 eV lines, heating 52 eV line, cooling cycle;
(270 eV), oxygen (510 eV), and lithium as a anneal: (m) heating cycle; (Cl) cooling cycle. cycle; (0) 40-50 eV lines, cooling cycle; (m) (0) 52 eV line, heating cycle.
600°C. The pressure was maintained cool-down portion of the first anneal
at 5 x low6 tot-r hydrogen during the and the entire second anneal by admit-
ting H, through a leak valve. Two days elapsed between the first and second anneals so the sample was rapidly heated to 600°C in 1 x 1O-6 torr H,, held at 600°C for 1 hr and cooled rapidly prior to the second anneal to minimize the 510 eV oxygen Auger line intensity prior to the second anneal. Fig. 4 shows typical Auger electron spectra obtained during the first anneal. Figs. 5 and 6 show the peak-to-peak intensity variations of several Auger electron lines during the first and second anneals, respectively*. * In Experiment III, the intensity of the low energy Auger lines are significantly attenuated relative to the 510 eV oxygen Auger line intensity when compared to the results of Experiment II. This was primariIy the result of the sample being at ground potential rather than bias - 150 V as in Experiment II. The sample bias increases the electron multiplier sensitivity to the low energy electrons.
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The ratio of the “40 eV” line intensity to that of the oxygen 510 eV line intensity [Z(40 eV)/I(510 eV)] calculated from figs. 5 and 6 is shown in fig. 7. Above 400°C the “40 eV” Auger line split into two separate lines which could not be completely resolved (fig. 4, Spectra 8-l 1). The “40 eV” line intensity for figs. 5 and 6 was taken as the sum of the line intensities between 40 eV and 50 eV. Upon cooling after the first anneal, the intensity of the 40 eV line relative to the 510 eV line increased to 0.75 before decreasing
0.01 0
I
I
200
1
I
400 Temperature (’
I
I
600 C)
Fig. 7. Auger line intensity of the 4&50 eV lithium lines relative to that of the oxygen 510 eV Auger line: ( 0) first anneal, heating cycle; (0) second anneal, heating cycle.
with the appearance and growth of the 52 eV lithium metal Auger line below 100 “C. At temperatures below 300 “C, subsequent anneals gave results similar to those observed in the second anneal independent of the H, pressure up to 1 x lop6 tort-. That is to say, the intensity ratios of the “40 eV line” to the 510 eV line was a reproducible function of temperature independent of H, annealing at 600°C produced the pressure up to 1 x 10e6 torr. Subsequent same results as shown for the second anneal when the H, pressure was maintained at 1 x 10m6 torr but the intensity ratio dropped to 0.2 when the H, pressure was allowed to drop to pressures below 5 x 10e8 torr. During the 600°C anneals with a H, pressure maintained at 1 x 10m6 tort-, the “40 eV lines” changed intensity and shifted toward 46 eV as shown in fig. 4, Curves 2 through 9. At pressures below 5 x lo-* torr the “40 eV lines” changed in intensity and shifted to 49 eV so that the original line has split or been replaced by two or more lines in a manner similar to that shown in fig. 4, Curves 10 and 11.
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4. Discussion The 40+ 1 eV line observed for lithium hydride and lithium oxide at room temperature can be most simply explained as a KVV transition where the K shell vacancy is in a lithium ion and the valence electrons are associated with the anion. It seems unlikeIy that this transition in lithium hydride or lithium oxide arises from the defect structures proposed for lithium fluoride by other experimenters 3*6), since the position and shape of this line at room temperature is essentially independent of how the lithium hydride or lithium oxide was prepared. The Auger spectra of cleaved LiH single crystals and the reaction product of lithium metal with H2 are quite similar. The dominant Auger line for lithium metal reacted with O2 in a clean vacuum system occurs at the same energy as that for lithium metal or LiH reacted with room air {LiOH)lo~ll~ 1%).The linear relationship between the intensity of this K,iVxVx Auger line and the 510 eV oxygen Auger line for the oxidation of lithium metal in vacuum confirms the role of the anion in this Auger process and suggests that both Vx electrons come from either the same oxygen atom or a pair of oxygen atoms which remain in close proximity after adsorption. If the oxygen atoms were randomly dispersed over the lithium metal surface and each oxygen atom contributed one electron to the Auger process, the 40+ 1 eV line intensity should vary as the square of the 510 eV line intensity. Finally, this assignment is consistent with the observation of the 52 eV KVV lithium metal line which is in excellent agreement with values calculated from X-ray data59 8). Oxygen gas reacts with the clean lithium metal probably forming Li,O, which does not tend to decompose on standing in vacuum (N 1 x IO-i0 torr). The lithium hydride surface partially decomposes to lithium metal on standing in vacuum at room temperature. The reaction of f H, + Li -+ LiH does not occur on the lithium metal surface at room temperature at hydrogen pressures of 1 x IO-’ torr. Such a reaction would have been indicated by the 40+ 1 eV LiH line. The existence of lithium metal on the LiH surface is not predicted by the thermodynamic properties for bulk LiH, since at room temperature, the formation of bulk LiH by the reaction of Hz and lithium metal should go to compietion at hydrogen pressures above approximately 10wz4 torr ls). It was also observed that the decomposition of the LiH surface in vacuum does not proceed to the extent that only the lithium metal phase exists. With the assignment of the 40&- 1 eV Auger line to lithium hydride or lithium oxide KVV transitions, the interpretation of the results of annealing LiH (Experiment III) is simplified. In the first anneal, the intensity of the 40 eV Auger line relative to that of the oxygen 510 eV Auger line remained
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constant up to 175 “C where this intensity ratio dropped sharply by a factor of two. Myersll) has demonstrated that at this temperature the reaction 2 LiOH -+ L&O +H,Of achieves a rate that is observable in the time frame of the present experiment. The LiOH also reacts at the LiH interface to form L&O, thus the LiOH decomposes from both the vacuum and hydride interfaces to form Li,O. Myers”‘) results predict that the LiOH decomposition would be very slow below 2OO”C, but complete conversion of a 1 pm film would occur in 5 min at 300°C. The Auger spectra were only sensitive to the initiation of this process since they analyze only a thin layer at the vacuum interface. The factor of two decrease in the intensity ratio of the 40 eV and 510 eV Auger lines occurred concurrently with a factor of two decrease of the O/Li ratio on the sample surface as would be expected for the reaction 2 LiOH --t Li,O +H,O. For temperatures up to 4OO”C, the Li,O apparently remains the stable surface composition. Above 4OO”C, the interdiffusion of hydride and oxide ions is expected along with a decrease in the stoichiometry of the LiHf33 l*). The increase in the intensity of the “40 eV” Auger lines relative to that for the oxygen 510 eV Auger line above 400°C is probably due to the growth of the hydride ion concentration on the sample surface. The generation of hydride vacancies at higher temperatures should be accompanied by an increase in the number of lithium 2s electrons in the LiH,, x< I, which in turn should begin to fifl higher energy levels and eventually the conduction band of Li metal as x approaches zero. The decrease in the value of x with decreasing pressure may explain the shift in energy from 46 eV to 49 eV (fig. 4, Spectra 8- 1 I), when the Hz base pressure of the vacuum system was allowed to decrease. A significant decrease in the hydride stoichiometry of the first monolayer of the sample is consistent with the observed lateral increase in the surface unit mesh relative to the bulk unit mesh observed by low-energy electron diffraction for the (100) face of a LiH single crystal at elevated temperatures4). As stated above, a shift to energies above 46 eV was not observed when a H, base pressure of 1 x 10v6 torr was maintained, but was observed when the base pressure was allowed to fall below 5 x IO-’ torr. The presence of two Auger lines in the 40-50 eV range can be explained in two ways. The low-energy Auger line (40+ 1 eV) could be the lithium oxide Auger line and the higher energy Auger line from transitions involving the valence band of LiH,. On the other hand, the band structure of LiH, could include two bands, one corresponding to the valence electrons of the hydride ion (resulting in the 401 I eV line) and a second partially filled band due to the excess lithium 2s electrons. Thermodynamic measurements reported for bulk LiH, 13, 14) predict x=0.99 at 600°C in equilibrium with a H, pressure
of I torr.
AUGER
Upon
cooling
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357
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the large increase
in the intensity
of
the 40f 1 eV Auger line relative to that of oxygen (510 eV), a reduction in the 510 eV oxygen Auger line intensity compared to its initial value, and the appearance of lithium metal all indicate the presence of hydride ions on the sample surface after the anneal. The 52 eV lithium line was only observed above 150 “C as a very weak line over a narrow temperature range (350-450 “C) during the second anneal (fig. 6).
5. Conclusions The major Auger line in the spectra of lithium hydride and lithium oxide (40 + 1 eV) is due to KVV transitions involving a 1S vacancy in a lithium ion and valence electrons from the anion. A lithium hydride surface tends to lose hydrogen in vacuum to an extent much greater than that estimated from thermodynamic data for bulk lithium hydride: At low temperatures, this results in the segregation of a lithium metal phase on the LiH surface. At high temperatures, the surface structure consists of a single phase of substoichiometric lithium hydride. The excess electrons apparently begin to fill higher bands causing the Auger line for lithium hydride to shift toward that of lithium metal and apparently split into two levels.
References 1) L. A. Harris, J. Appl. Phys. 39 (1968) 1419. 2) T. E. Gallon, I. G. Higginbotham, N. Prutton and H. Tokutaka, 3) 4) 5) 6) 7) 8)
9) 10) 11) 12) 13) 14)
Surface Sci. 21 (1970) 224. F. E. Gallon and J. A. D. Matthew, Phys. Status Solidi 41 (1970) 343. C. E. Holcombe, G. L. Powell, and R. E. Clausing, Surface Sci. 30 (1972), 561. R. E. Clausing, D. S. Easton, G. L. Powell, Surface Sci. 36 (1973) 377. D. G. Lord and T. E. Gallon, Surface Sci. 36 (1973) 606. D. M. Zehner, R. E. Clausing, G. E. McGuire and L. H. Jenkins, Solid State Commun. 13 (1973) 681. W. W. Coghlan and R. E. Clausing, A Catalog of Calculated Auger Transitions for the Elements, Oak Ridge National Laboratory Report ORNL-TM-3576, November, 1971, page 47 (Available through National Technical Information Service, Springfield, VA., USA). C. E. Holcombe, Jr. and D. H. Johnson, J. Crystal Growth 19 (1973) 53. C. E. Holcombe and G. L. Powell, J. Nuclear Mater. 47 (1973) 121. S. M. Myers, J. Appl. Phys. 45 (1974) 4320. W. R. Irvine and J. A. Lund, J. Electrochem. Sot. 110 (1963) 141. W. Mueller, J. P. Blackledge and G. G. Libowitz, Metal Hydrides (Academic Press, New York, 1968) ch. 6, p. 165. C. E. Messer, J. Mellor, J. A. Krol and I. S. Levy, J. Chem. Eng. Data 6 (1961) 328.
SCIENCE
AUGER
46 (1974) 345-357 0 North-Holland
SPECTRA
OF LITHI~
Publishing Co.
HYDRIDE*
G. L. POWELL Nuclear Division. Union Carbide Corporation, Oak Ridge, Tennessee 37830, U.S.A. and G. E. McGUIRE,
D. S. EASTON and R. E. CLAUSING
Metalsand Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Received 22 July 1974; revised manuscript
received 3 September
1974
Auger electron emission spectra have been observed for lithium hydride in three conditions: (I) cleaved in vacuum, (2) prepared by the reaction of hydrogen gas with clean lithium metal, and (3) by annealing slightly oxidized lithium hydride single crystals in vacuum. The dominant Auger line (40 + 1 eV) was found to be a KVV transition involving valence electrons from the anion and was indistinguishable from a similar transition for lithium oxide at room temperature. Lithium hydride surfaces lose hydrogen in vacuum causing the formation of a lithium metal phase at room temperature and a significant reduction in surface hydride stoichiometry at 600°C.
1. In~oduc~ion Auger electron emission lines in the energy range of 10 eV to 90 eV have been reported for lithium in lithium compoundsr-s). The Auger spectra of lithium oxide 5), lithium hydroxide 4), and LiH 4, are dominated by a 40+ 1 eV line at room temperature. Zehner et al.‘) have described in detail the lithium metal Auger emission spectra. A 52 eV KVV transition is the primary feature. (A very weak line at 82 eV originating from a doubly ionized lithium K shell is the only other feature in the spectrum initiated from ionized states of lithium metal.) These observations indicate a large (12 eV) “chemical shift” associated with formation of lithium compounds which should be very useful in studying the surface reactions of lithium metal and lithium compounds. This report describes experiments conducted to further characterize the 40 eV Auger electron transitions observed for lithium hydride4) and lithium oxides) at room temperature and to investigate processes that occur at the solid lithium hydride surface over the temperature range from 20°C to 600°C. * Work performed at the Oak Ridge National Laboratory under the sponsorship of Nuclear Division, Union Carbide Corporation, and Los Alamos Scientific Laboratories, Los Alamos, New Mexico, U.S.A,, for the U.S. Atomic Energy Commission. 345
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2. Experimental
The LiH single crystals (99.9% pure) used in these experiments were cleaved from a single crystal prepared by recrystallization from the melt by Holcombe and Johnsong). The lithium metal, 99.97% pure, was Foote Mineral Company reactor-grade, further purified by holding the molten lithium in contact with zirconium foil for LOOhr. The lithium metal was cast into a 0.63 cm diameter stainless steel tube having a 0.025 cm wall thickness. These operations were carried out in a sealed system under argon. Three types of experiments were carried out in this investigation : (Experiment I) LiH single crystals were cleaved in vacuum at room temperature; (Experiment II) lithium metal was cleaned in vacuum by abrasion and ion bombardment and reacted with hydrogen gas at room temperature; and (Experiment HI) an air-exposed LiH crystal was heated from 25 to 600°C while Auger spectra were observed. Experiments I and II were carried out in an ion-pumped vacuum system containing a liquid-nitrogen-cooled titanium-sublimation pump. The residual gas pressure in the system measured by an ionization gauge was 2 x lo-‘* torr consisting of mostly argon and hydrogen as indicated by a residual gas analyzer. Samples could be introduced to the system and removed from the system via an independently ion-pumped interlock manifold while maintaining a high vacuum in the spectrometer chamber. The sample could be manipulated within the analysis chamber so that it could be abraded with a file or, in the case of LiH crystals, cleaved along the (100) plane. Auger electron spectra were taken with a high-resolution, double-pass cylindrical mirror analyzer (Physical Electronics Industries Model 15-256). A co-axial electron gun (1000 eV, 1 pA) was used and the electron beam struck the sample at 15” from normal to the surface. A signal proportional to the first derivative of the electron energy distribution [dN(E)/dE J was obtained by modulating the analyzer sweep voltage with a superimposed 2 kHz ac voltage and using a lock-in amplifier to detect the ac signal in the output current from the analyzer electron multiplier. This signal was recorded on an X-Y recorder as a function of electron energy. A 1 V peak-to-peak amplitude was selected for the modulating voltage. The samples were biased - 150 V with respect to ground potential in order to obtain optimum response from the electron multiplier. The sample could be argon ion-bombarded simultaneously with the Auger analysis. Oxygen-free lithium metal (as indicated by no detectable 510 eV or 40 eV Auger transitions) could be prepared and maintained in this system for periods exceeding I hr. Experiment III was carried out in an ion-pumped vacuum system similar to that described by Holcombe et aL4). A thin crystal (1 cm x 0.3 cm X0.3 cm
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347
was cleaved in an argon dry box and mounted on a high-purity iron (99 + %) sample holder, which was, in turn, mounted on a universal manipulator. The sample holder and manipulator were then covered with a plastic bag and transferred to the LEED vacuum chamber, which was being purged with flowing nitrogen. These precautions were necessary to prevent the crystal from reacting excessively with water while it was being installed and before a high vacuum could be attained. Even so, the crystal was covered with a thin film consisting of a mixture of lithium oxide, hydroxide, carbonate, and bicarbonate, since lithium hydride reacts readily with water and carbon dioxideie~ii). A sample prepared using similar precautions was cleaved a second time normal to the surface to be analyzed and immediately loaded into a scanning electron microscope. A film approximately 1 urn thick was observed that was identical to that previously identified as LiOHlO). The components of the vacuum system were vacuum baked at 250°C and the sample holder was vacuum baked at 800 “C prior to mounting the LiH sample, The vacuum system containing the LiH sample could not be baked again prior to these measurements since that would perturb the air-exposed LiH surface. After mounting the sample in the vacuum chamber, the system was left undisturbed for several days in order to establish a base pressure of approximately 1 x IO-’ torr. For Experiment III, the LiH Auger spectra were obtained using a cylindrical mirror analyzer (Physical Electronics Industries Model 10-234) operated in a manner similar to the analyzer used in Experiments I and II with the exception that a 2 V peak-to-peak amplitude was selected for the modulating voltage. The electron beam (2000 eV, 2 uA) was incident on the crystal at an angle of 70” to the normal of the specimen surface. The Auger electron spectra were recorded by first scanning the spectral region of 0 eV to 100 eV and then scanning two spectral regions approximately 50 eV wide, one containing the carbon Auger emission line at 275 eV and the other containing the oxygen Auger emission line at 510 eV. Approximately five minutes were required to obtain these spectra. Occasionally during the experiment a complete spectrum over the 0 eV to 1700 eV range was obtained. On these occasions, Auger emission lines for other elements were not detected. A multichannel recorder was used to continuously monitor each of the following items: the Auger electron spectra at two sensitivity levels, the electron beam energy, the vacuum gauge, the residual gas analyzer, and the sample temperature. The electron-beam current could not be monitored continuously, but was checked several times during the experiment and found to be constant. The sample could be heated with a tungsten filament affixed to the back of the sample holder. The sample temperature was measured by a chromel-
348
G. L. POWELL
ET AL.
alumel thermocouple attached to the sample holder near the sample. The sample holder was operated at ground potential during Experiment III. 3. Results 3.1. EXPERIMENTI Typical Auger spectra of LiH cleaved in vacuum are shown in fig. 1. The spectrum is dominated by a 40+ 1 eV line and no oxygen was detectable by the 510 eV Auger transition. Argon ion bombardment or allowing the cleaved crystal to stand in vacuum overnight at room temperature resulted in the appearance of the lithium metal (52 eV) Auger line and a corresponding decrease in the 40 eV line intensity. Upon establishing the presence of lithium metal on the LiH surface, the sample reacted with 0, as demonstrated by the appearance of the 510 eV line, the growth of the 40 eV line, and the disappearance of the 52 eV lithium metal line. The apparent shift in the 40 eV line shown in fig. 1 may have been due to sample charging. Sample charging was
/’
/ :
I 20
60 Energy
(Volts)
Fig. 1. Auger spectra for cleaved LiH crystal ((100) face) at room temperature. Curve 1: immediately after cleavage. Curve 2: argon ion bombarded (1000 V, - 5 pA/cmz) for 51 min. Curve 3 : argon ion bombarded (1000 V, - 5 pA/cm2) for 62 min.
AUGER
a significant decision
problem
to prepare
SPECTRA
OF LITHIUM
349
HYDRIDE
during the LiH cleaving experiment LiH by the reaction
of lithium
and resulted in the
metal with Hz.
3.2. EXPERIMENT II The Auger spectrum of lithium metal cleaned by abrasion and ion bombardment is shown in fig. 2 (Curve 1). The 27 eV line is due to sodium metal that segregates onto the lithium metal surface after ion bombardment’). The cleaned lithium surface reacted with 0, at pressures as low as 5 x lo-” torr 0, to produce a 40 eV line at the expense of the 52 eV line eventually resulting in a spectrum similar to Curve 4, fig. 2. Spectra of lithium oxide prepared in this manner have been repot-teds). During this reaction of lithium metal with oxygen, the ratio of the 40 eV lithium oxide Auger line intensity to that of the 510 eV oxygen Auger line intensity remained constant. Upon standing
20
40
60
Energy (Volts)
Fig. 2. Auger spectra of the lithium metal sample. Curve 1: as cleaned by abrasion and ion bombardment (1000 V, N 5 uA/cma); note small sodium line (22 eV) and lithium oxide line (40 eV). Curve 2: sample per Curve 1, immediately after 1 hr exposure to 740 torr HP. Curve 3: sample per Curve 2, 35 min after return to high vacuum system. Curve 4: sample per Curve 3, after 70 min exposure to 2 x 10-s torr 02.
350
in vacuum
G. L. POWELL
ET AL.
(2 x 10W1’ torr) for three days at room
lithium line did not reappear. The Auger spectrum of cleaned lithium
temperature,
metal was unchanged
the 52 eV by exposure
to H, at pressures up to 1 x 10e5 torr for one hour at room temperature. The cleaned lithium sample was placed in the interlock manifold at room temperature and exposed to H, at 740 torr for 1 hr. The visual appearance of the lithium metal sample was unchanged by this exposure. However, the Auger spectrum displayed an Auger line at 39 eV similar in shape to the lithium oxide Auger line, no Auger at 52 eV (lithium metal) and no 510 eV oxygen Auger line. Upon standing in the analysis chamber at 3 x lo-” torr (mostly Ar and Hz), the 39 eV line intensity decayed to a lower value as the 52 eV lithium line grew in time and both lines exponentially approached the condition shown in fig. 2 (Curve 3) with a time constant of 700 sec. Once the
Time
(seconds)
Fig. 3. Auger analysis of the oxidation of lithium metal on a lithium hydride surface. The origin of the lithium metal was the decomposition of the lithium hydride in vacuum (2 x lo-r0 torr). Oxygen was admitted to the vacuum chamber by a leak valve at t = 0 and maintained at 1 x 10-s torr for I >O. (m) 52 eV lithium metal Auger line; (a) 40 eV lithium hydride and/or lithium oxide Auger line; ( 0 ) 510 eV oxygen Auger line scaled by a zero shift and a scale factor such that the point at t = - 100 set and at t = 3700 set have ordinate positions identical to the corresponding 40 eV Auger line.
AUGER
SPECTRA
OF LITHIUM
HYDRIDE
351
52 eV lithium line was developed, exposure to 1 x lo-* torr 0, resulted in its decay accompanied by the growth of the 39 eV and 510 eV Auger lines (fig. 3). 3.3. EXPERIMENT 111 A typical heat treatment for the air-exposed LiH crystal was to raise the temperature from 20 to 600 “C at a heating rate of 3 “C/min, hold the sample at 600°C for 2 hr, and allow the sample to cool to ambient temperature. The
20
30
40
50
Energy (Volts) Fig. 4. Auger spectra from air-exposed LiH crystal (100) surface during the first anneal (sample number, attenuation, and temperature are reported beside each spectrum). Curve 10: was obtained immediately after sample temperature reached 600°C f+= 5 x lo-? torr). Curve 11: was obtained after the sample had been maintained at 600°C for 2 hr (PH, < 5 x 1O-8 torr).
352
G. L. POWELL
ET AL.
c
-- --
Ii
Fig. 5. Auger line intensities for carbon (270 eV), oxygen (510 eV), and lithium as a function of temperature during the first anneal; (0) hearing cycle; ( 0) cooling cycle. For lithium: (a) 40-50eV lines, heating cycle; (0) 40-50eV lines, cooling cycle; (m) 52 eV, cooling cycle. heating cycle of the first anneal was carried out in the ion-pumped vacuum system so that gas desorption from the sample could be monitored. Since the sample holder had been exposed to air for approximately 5 min after the vacuum bake at 800°C and could not be baked under ultra-high-vacuum conditions without perturbing the attached sample, the source of the gases evolved cannot be unambiguously assigned to processes occurring only on the LiH sample. Water was evolved over the temperature range of 250 to 400 “C reaching a maximum pressure of 3 x low6 torr at 370°C. Above 400 “C, H, was the main outgassing component reaching a pressure of 5 x 10e7 torr at 600°C. The Hz pressure dropped to 5 x lop8 torr during the 2 hr hold at
AUGER
SPECTRA
OF LITHIUM
353
HYDRIDE
3 Li
\ 0
L-e
0
I
IQ
I
400
200 Temperot”re
-_I 600
PC)
Fig. 6. Auger line intensities for carbon function of temperature during the second For lithium: (0) 40-50 eV lines, heating 52 eV line, cooling cycle;
(270 eV), oxygen (510 eV), and lithium as a anneal: (m) heating cycle; (Cl) cooling cycle. cycle; (0) 40-50 eV lines, cooling cycle; (m) (0) 52 eV line, heating cycle.
600°C. The pressure was maintained cool-down portion of the first anneal
at 5 x low6 tot-r hydrogen during the and the entire second anneal by admit-
ting H, through a leak valve. Two days elapsed between the first and second anneals so the sample was rapidly heated to 600°C in 1 x 1O-6 torr H,, held at 600°C for 1 hr and cooled rapidly prior to the second anneal to minimize the 510 eV oxygen Auger line intensity prior to the second anneal. Fig. 4 shows typical Auger electron spectra obtained during the first anneal. Figs. 5 and 6 show the peak-to-peak intensity variations of several Auger electron lines during the first and second anneals, respectively*. * In Experiment III, the intensity of the low energy Auger lines are significantly attenuated relative to the 510 eV oxygen Auger line intensity when compared to the results of Experiment II. This was primariIy the result of the sample being at ground potential rather than bias - 150 V as in Experiment II. The sample bias increases the electron multiplier sensitivity to the low energy electrons.
354
G. L.
POWELL
ET AL.
The ratio of the “40 eV” line intensity to that of the oxygen 510 eV line intensity [Z(40 eV)/I(510 eV)] calculated from figs. 5 and 6 is shown in fig. 7. Above 400°C the “40 eV” Auger line split into two separate lines which could not be completely resolved (fig. 4, Spectra 8-l 1). The “40 eV” line intensity for figs. 5 and 6 was taken as the sum of the line intensities between 40 eV and 50 eV. Upon cooling after the first anneal, the intensity of the 40 eV line relative to the 510 eV line increased to 0.75 before decreasing
0.01 0
I
I
200
1
I
400 Temperature (’
I
I
600 C)
Fig. 7. Auger line intensity of the 4&50 eV lithium lines relative to that of the oxygen 510 eV Auger line: ( 0) first anneal, heating cycle; (0) second anneal, heating cycle.
with the appearance and growth of the 52 eV lithium metal Auger line below 100 “C. At temperatures below 300 “C, subsequent anneals gave results similar to those observed in the second anneal independent of the H, pressure up to 1 x lop6 tort-. That is to say, the intensity ratios of the “40 eV line” to the 510 eV line was a reproducible function of temperature independent of H, annealing at 600°C produced the pressure up to 1 x 10e6 torr. Subsequent same results as shown for the second anneal when the H, pressure was maintained at 1 x 10m6 torr but the intensity ratio dropped to 0.2 when the H, pressure was allowed to drop to pressures below 5 x 10e8 torr. During the 600°C anneals with a H, pressure maintained at 1 x 10m6 tort-, the “40 eV lines” changed intensity and shifted toward 46 eV as shown in fig. 4, Curves 2 through 9. At pressures below 5 x lo-* torr the “40 eV lines” changed in intensity and shifted to 49 eV so that the original line has split or been replaced by two or more lines in a manner similar to that shown in fig. 4, Curves 10 and 11.
AUGER
SPECTRA
OF LlTHIUM
HYDRIDE
355
4. Discussion The 40+ 1 eV line observed for lithium hydride and lithium oxide at room temperature can be most simply explained as a KVV transition where the K shell vacancy is in a lithium ion and the valence electrons are associated with the anion. It seems unlikeIy that this transition in lithium hydride or lithium oxide arises from the defect structures proposed for lithium fluoride by other experimenters 3*6), since the position and shape of this line at room temperature is essentially independent of how the lithium hydride or lithium oxide was prepared. The Auger spectra of cleaved LiH single crystals and the reaction product of lithium metal with H2 are quite similar. The dominant Auger line for lithium metal reacted with O2 in a clean vacuum system occurs at the same energy as that for lithium metal or LiH reacted with room air {LiOH)lo~ll~ 1%).The linear relationship between the intensity of this K,iVxVx Auger line and the 510 eV oxygen Auger line for the oxidation of lithium metal in vacuum confirms the role of the anion in this Auger process and suggests that both Vx electrons come from either the same oxygen atom or a pair of oxygen atoms which remain in close proximity after adsorption. If the oxygen atoms were randomly dispersed over the lithium metal surface and each oxygen atom contributed one electron to the Auger process, the 40+ 1 eV line intensity should vary as the square of the 510 eV line intensity. Finally, this assignment is consistent with the observation of the 52 eV KVV lithium metal line which is in excellent agreement with values calculated from X-ray data59 8). Oxygen gas reacts with the clean lithium metal probably forming Li,O, which does not tend to decompose on standing in vacuum (N 1 x IO-i0 torr). The lithium hydride surface partially decomposes to lithium metal on standing in vacuum at room temperature. The reaction of f H, + Li -+ LiH does not occur on the lithium metal surface at room temperature at hydrogen pressures of 1 x IO-’ torr. Such a reaction would have been indicated by the 40+ 1 eV LiH line. The existence of lithium metal on the LiH surface is not predicted by the thermodynamic properties for bulk LiH, since at room temperature, the formation of bulk LiH by the reaction of Hz and lithium metal should go to compietion at hydrogen pressures above approximately 10wz4 torr ls). It was also observed that the decomposition of the LiH surface in vacuum does not proceed to the extent that only the lithium metal phase exists. With the assignment of the 40&- 1 eV Auger line to lithium hydride or lithium oxide KVV transitions, the interpretation of the results of annealing LiH (Experiment III) is simplified. In the first anneal, the intensity of the 40 eV Auger line relative to that of the oxygen 510 eV Auger line remained
356
G.LP0WEI.L
ET AL.
constant up to 175 “C where this intensity ratio dropped sharply by a factor of two. Myersll) has demonstrated that at this temperature the reaction 2 LiOH -+ L&O +H,Of achieves a rate that is observable in the time frame of the present experiment. The LiOH also reacts at the LiH interface to form L&O, thus the LiOH decomposes from both the vacuum and hydride interfaces to form Li,O. Myers”‘) results predict that the LiOH decomposition would be very slow below 2OO”C, but complete conversion of a 1 pm film would occur in 5 min at 300°C. The Auger spectra were only sensitive to the initiation of this process since they analyze only a thin layer at the vacuum interface. The factor of two decrease in the intensity ratio of the 40 eV and 510 eV Auger lines occurred concurrently with a factor of two decrease of the O/Li ratio on the sample surface as would be expected for the reaction 2 LiOH --t Li,O +H,O. For temperatures up to 4OO”C, the Li,O apparently remains the stable surface composition. Above 4OO”C, the interdiffusion of hydride and oxide ions is expected along with a decrease in the stoichiometry of the LiHf33 l*). The increase in the intensity of the “40 eV” Auger lines relative to that for the oxygen 510 eV Auger line above 400°C is probably due to the growth of the hydride ion concentration on the sample surface. The generation of hydride vacancies at higher temperatures should be accompanied by an increase in the number of lithium 2s electrons in the LiH,, x< I, which in turn should begin to fifl higher energy levels and eventually the conduction band of Li metal as x approaches zero. The decrease in the value of x with decreasing pressure may explain the shift in energy from 46 eV to 49 eV (fig. 4, Spectra 8- 1 I), when the Hz base pressure of the vacuum system was allowed to decrease. A significant decrease in the hydride stoichiometry of the first monolayer of the sample is consistent with the observed lateral increase in the surface unit mesh relative to the bulk unit mesh observed by low-energy electron diffraction for the (100) face of a LiH single crystal at elevated temperatures4). As stated above, a shift to energies above 46 eV was not observed when a H, base pressure of 1 x 10v6 torr was maintained, but was observed when the base pressure was allowed to fall below 5 x IO-’ torr. The presence of two Auger lines in the 40-50 eV range can be explained in two ways. The low-energy Auger line (40+ 1 eV) could be the lithium oxide Auger line and the higher energy Auger line from transitions involving the valence band of LiH,. On the other hand, the band structure of LiH, could include two bands, one corresponding to the valence electrons of the hydride ion (resulting in the 401 I eV line) and a second partially filled band due to the excess lithium 2s electrons. Thermodynamic measurements reported for bulk LiH, 13, 14) predict x=0.99 at 600°C in equilibrium with a H, pressure
of I torr.
AUGER
Upon
cooling
SPECTRA
OF LITHIUM
to room temperature,
357
HYDRIDE
the large increase
in the intensity
of
the 40f 1 eV Auger line relative to that of oxygen (510 eV), a reduction in the 510 eV oxygen Auger line intensity compared to its initial value, and the appearance of lithium metal all indicate the presence of hydride ions on the sample surface after the anneal. The 52 eV lithium line was only observed above 150 “C as a very weak line over a narrow temperature range (350-450 “C) during the second anneal (fig. 6).
5. Conclusions The major Auger line in the spectra of lithium hydride and lithium oxide (40 + 1 eV) is due to KVV transitions involving a 1S vacancy in a lithium ion and valence electrons from the anion. A lithium hydride surface tends to lose hydrogen in vacuum to an extent much greater than that estimated from thermodynamic data for bulk lithium hydride: At low temperatures, this results in the segregation of a lithium metal phase on the LiH surface. At high temperatures, the surface structure consists of a single phase of substoichiometric lithium hydride. The excess electrons apparently begin to fill higher bands causing the Auger line for lithium hydride to shift toward that of lithium metal and apparently split into two levels.
References 1) L. A. Harris, J. Appl. Phys. 39 (1968) 1419. 2) T. E. Gallon, I. G. Higginbotham, N. Prutton and H. Tokutaka, 3) 4) 5) 6) 7) 8)
9) 10) 11) 12) 13) 14)
Surface Sci. 21 (1970) 224. F. E. Gallon and J. A. D. Matthew, Phys. Status Solidi 41 (1970) 343. C. E. Holcombe, G. L. Powell, and R. E. Clausing, Surface Sci. 30 (1972), 561. R. E. Clausing, D. S. Easton, G. L. Powell, Surface Sci. 36 (1973) 377. D. G. Lord and T. E. Gallon, Surface Sci. 36 (1973) 606. D. M. Zehner, R. E. Clausing, G. E. McGuire and L. H. Jenkins, Solid State Commun. 13 (1973) 681. W. W. Coghlan and R. E. Clausing, A Catalog of Calculated Auger Transitions for the Elements, Oak Ridge National Laboratory Report ORNL-TM-3576, November, 1971, page 47 (Available through National Technical Information Service, Springfield, VA., USA). C. E. Holcombe, Jr. and D. H. Johnson, J. Crystal Growth 19 (1973) 53. C. E. Holcombe and G. L. Powell, J. Nuclear Mater. 47 (1973) 121. S. M. Myers, J. Appl. Phys. 45 (1974) 4320. W. R. Irvine and J. A. Lund, J. Electrochem. Sot. 110 (1963) 141. W. Mueller, J. P. Blackledge and G. G. Libowitz, Metal Hydrides (Academic Press, New York, 1968) ch. 6, p. 165. C. E. Messer, J. Mellor, J. A. Krol and I. S. Levy, J. Chem. Eng. Data 6 (1961) 328.