Thermal Stability of Oxidized Ultrathin Li Films on TZM for Plasma Facing

Journal of Nuclear Materials 543 (2021) 152587

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Journal of Nuclear Materials

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Thermal stability of oxidized ultrathin Li ﬁlms on TZM for plasma facing components

∗ O. Fasoranti a, E.T. Ostrowski b, B.E. Koel b, a Department of Chemistry, Princeton University NJ, 08540 USA b Department of Chemical and Biological Engineering, Princeton University NJ, 08540 USA

a r t i c l e i n f o a b s t r a c t

Article history: For improved understanding of the behavior of Li plasma facing components (PFCs) in the presence of Received 14 July 2020 oxygen impurities, we report on the thermal stability of ultrathin (up to 1.0 nm) LiO x films on a titanium- Revised 22 September 2020 zirconium-molybdenum (TZM) alloy substrate over the temperature range of 310–1400 K. LiO x films were Accepted 4 October 2020 prepared by either post-oxidation of Li films or Li films deposited on an oxygen-precovered TZM surface. Available online 9 October 2020 Thermal stability of these films was studied using a combination of temperature programmed desorption Keywords: (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS). For post-oxidized Li Lithium films, TPD showed that no Li desorption occurred until temperatures above 620 K, and then Li desorbed molybdenum from the surface via at least three desorption peaks. The O 1s XPS spectra on the post-oxidized films TZM at 310 K indicate the formation of lithium oxide (Li 2 O) and peroxide/hydroxide (Li 2 O 2 /LiOH). The perox- thin films ide/hydroxide converts to oxide after heating to 680 K with no desorption of Li or O 2 , and then this film plasma facing components decomposes to liberate Li into the gas phase while leaving oxygen at the TZM surface. Heating the LiO x films to 1070 K led to a solid-state reaction with surface Mo atoms of the substrate to form a condensed

binary lithium molybdenum oxide (Li x MoO y ) phase, which upon further heating decomposed to liberate gas phase binary lithium molybdenum oxide species. Li deposition on an oxygen-precovered TZM surface formed a complex, highly stable, oxygen rich, Li-O-Mo interfacial oxide. Li desorption from multilayer Li ﬁlms on oxygen-precovered TZM surfaces occurred in a metallic Li multilayer peak and three other oxide- derived peaks. These results support the improved thermal stability of oxidized lithium as compared to metallic Li and have implications for Mo impurity transport into the plasma at high temperatures. ©2020 Elsevier B.V. All rights reserved.

1. Introduction vacuum chamber of these devices. Oxygen can also come from diffusion of impurities from the bulk and grain boundaries of sub- Utilization of solid and liquid lithium (Li) films has been pro- strates supporting the Li films. In either case, oxidation of Li films posed for the first wall and divertor regions in fusion devices. Con- alters the surface composition and can form new compounds with ditioning of plasma facing components (PFCs) with solid Li coat- very different chemical and physical behavior than metallic Li, as ings improved plasma performance and confinement time in ex- shown via offline characterization by surface science experiments periments in TFTR [1] , T-11M [2] , FTU [3] , CDX-U [4] , LTX [5] , EAST [13–15] and the Materials Analysis Particle Probe (MAPP) [16] . In [6] , DIII-D [7] and NSTX-U [8] . Although the reasons for this are the case of hydrogen retention, metallic and oxidized Li films have currently not well understood, these improvements have been at- shown comparable hydrogen retention capabilities, but the mech- tributed to lowered H recycling [ 9 , 10 ], edge density changes [11] , anisms by which hydrogen is chemically retained is different be- and suppressed edge localized modes (ELMs) [12] . Li is also at- tween the two systems [16–18] . In metallic Li, hydrogen is retained tractive as a liquid metal PFC due to its low melting point and as LiH/LiD [17,19] while in Li2 O, hydrogen is retained in part as high chemical reactivity that allows it to efficiently getter hydro- LiOH/LiOD [ 17 , 20 ]. Further understanding retention of hydrogen, gen. However, this high chemical reactivity causes Li to react with erosion, redeposition, and thermal stability of oxidized Li films is background or residual gasses (e.g. H2 O and CO) present in the important for advancing Li PFC applications. Studies of oxidation of bulk Li and Li films have also been motivated by applications in electrochemistry and catalysis. Previ-

∗ ous laboratory experiments have focused on measuring the oxida- Corresponding author at: Princeton University, A311 Engineering Quad, Prince- ton NJ 08544 USA. tion rate and identifying the intermediates and reaction products E-mail address: [email protected] (B.E. Koel). formed [21–26] . Hoenigman and Kiel found that exposure of poly-

crystalline Li films to 100 L O2 produced an O 1 s peak at 530.6 eV Li films were deposited on the TZM substrate by physical evap- binding energy (BE) in X-ray photoelectron spectroscopy (XPS) as- oration from a commercially available alkali metal dispenser (AMD; signed to Li2 O formation [21]. Oxidation of the Li film proceeded SAES Inc.). Before experiments, the Li source was thoroughly out- more slowly with water and produced two O 1s XPS peaks: one at gassed at a temperature below that needed for evaporation. The

530.6 eV BE assigned to Li2 O accounted for 85% of the oxygen at amount of Li deposited on the surface was calibrated by using TPD the surface, and one at 533.5 eV BE assigned to lithium hydroxide measurements. Oxidation was carried out by backﬁlling the UHV

(LiOH) accounted for the rest and arose from H2 O dissociation on chamber with O2 (Ultra-high purity 99.99% O2 ; Matheson) by us- the film. A later study using O2 and H2 O as oxidants established ing a variable leak valve attached to a stainless steel gas handling that a monolayer of Li2 O was formed on the surface regardless of line connected to the O2 lecture bottle. Exposures of 20 L O2 were × −7 oxidant with low amounts of hydroxide at the surface from H2 O carried out using 1 10 Torr O2 for 200 s. oxidation [27] . This oxide monolayer did not passivate the surface, LEIS measurements were done at a lab scattering angle of 104 ° + and oxidation using O2 continued up to a thickness of 1600 A.˚ De- with 1–2 keV He ions incident normal to the surface with a sam- positing Li films on a pre-oxidized Ni(110) substrate, denoted by ple current of 10 nA and an SCA pass energy of 939 eV. All BEs the authors as a Ni(110)-O surface, was found to reduce the oxi- for XPS were referenced to the Mo 3d5/2 peak at 227.7 eV BE [33]. dized Ni substrate [28] . A pass energy of 50 eV for the SCA was used for XPS, and this Herein, results are presented from surface science studies of gave a full width half-maximum (FWHM) of 1.4 eV for the Mo the oxidation of Li films deposited on a titanium-zirconium- 3d5/2 peak at 227.7 eV BE. XPS spectra herein were acquired us- molybdenum (TZM) alloy, a high-Z (high atomic number) substrate ing Mg K α X-rays at 1253.6 eV. Fitting and decomposition of XPS of interest, and the thermal stability of the oxides formed in ul- peaks were done using the CasaXPS program and utilizing a mixed trahigh vacuum (UHV) vessels. TZM is of interest in part due to Gaussian-Lorentzian peak shape and a Shirley background subtrac- its improved mechanical strength at elevated temperatures com- tion for the baseline. pared to pure Mo [29] , high resistance to creep [30] , and proven corrosion resistance against liquid metals, which has led to its use 3. Results and Discussion as a substrate for Li in the limiter region for current fusion devices such as EAST [31] and FTU [32] . Our investigations utilized 3.1. Post-oxidized Li films on TZM a multi-technique approach for surface characterization including XPS, temperature programmed desorption (TPD), Auger electron The thermal stability of post-oxidized ultrathin (1–5 ML; mono- spectroscopy (AES), and low-energy ion scattering (LEIS). These ex- layer) Li films on a TZM substrate was studied by using TPD, XPS, periments extend the prior research by Skinner et al. [13] and pro- and LEIS. For these measurements, either 3.5 ML (0.7 nm) or 5 ML vide information on the effects of post-annealing on the nature of (1.0 nm) Li films were deposited, with the amount of Li calibrated oxidized lithium films and interactions that occur between the film by TPD. Further details on the method of Li film calibration can be and substrate that are relevant for PFCs subject to high tempera- found in our previous report [34] . Oxidation of the Li film was sub- tures. sequently performed by exposing the film at 310 K to 20 L O2 , and these post-oxidized Li films are herein referred to as LiO x films. 2. Experimental Methods

3.1.1. TPD of LiO x films Experiments were performed in an UHV system with a 4-axis, A Li (7 amu) TPD trace from a LiO x film formed by oxidation long-transfer manipulator and attached load lock chamber con- of an initial 5 ML Li film is shown in Fig. 1 a. No appreciable Li nected to the main surface analysis chamber. The main chamber multilayer TPD peak was observed, which from prior experiments × −10 × −9 base pressure was 4 10 Torr and rose to 1–2 10 Torr dur- on clean Li films occurs near 600 K [34] , and this indicates that ing experiments. Equipment included: 3-kV Kimball Physics (EFG- the Li film was completely oxidized by the procedure utilized. The 7F-4243) electron gun for AES; PHI (04–303) differentially pumped ion gun for LEIS and Ar + ion sputtering; PHI (10–360) spherical ca- pacitor analyzer (SCA) for AES, LEIS, and XPS; PHI (04–548) X-ray source for either Al K α or Mg K α radiation for XPS; and a UTI 100C quadrupole mass spectrometer (QMS) for residual gas analysis and TPD. The TZM alloy sample (Goodfellow, Huntingdon, England) had a stated bulk composition of Mo:Ti:Zr of 99:0.5:0.1 at.% (no other impurities or their amounts were specified). The TZM sample had dimensions of 1 cm x 1 cm and a thickness of 0.005 in. (0.125 mm) and was mounted by spotwelding two 0.015 in. (0.38 mm) Ta wires to the back of the sample that were attached to larger Ta rods connected to Cu rods using Cu-Be connectors. A DC power sup- ply was used to resistively heat the sample to 1900 K for cleaning and for TPD. A W-5%Re/W-26% Re thermocouple was spotwelded to the TZM sample to monitor the temperature. The sample was cleaned by cycling the use of 1-keV Ar + ion sputtering of the sample at 310 K to reduce O, C, and S contamination to minimum lev- × −7 els, heating at 1500 K in 5 10 Torr O2 for 5–10 min. to remove carbon, and annealing the sample to 1900 K in vacuum to reduce surface oxygen. Before each experiment, the sample was recondi- tioned by heating to 1900 K for 5 s. AES of the cleaned TZM substrate indicated a surface composition of 83.1 at.% Mo, 1.8 at.% Ti, Fig. 1. Li (7 amu) TPD curves after: (a) 20 L O 2 /5 ML Li/TZM at 330 K; (b) 20 L

1.5 at.% O and 13.7 at.% C, with no Zr detected, and was measured O 2 /3.45 ML Li/TZM; (c) 880 K anneal of an oxidized 3.45 ML Li ﬁlm; and (d) 1070 K prior to each experiment. anneal of an oxidized 3.45 ML Li ﬁlm.

2 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

LiO x film was stable until 620 K, after which decomposition lead to of the TZM substrate in altering (either stabilizing or destabilizing) three distinct Li desorption features, with peaks at 860, 990, and lithium compounds in the oxidized film due to intimate involve- 1220 K. The last trace of Li from this LiO x film was not fully des- ment of interfacial Li and O atoms/ions at the TZM substrate sur- orbed from the TZM surface until 1400 K. face, which is primarily composed of Mo as we determined by ISS Fig. 1 shows three additional Li TPD curves: (b) from a post- and AES. oxidized 3.5 ML (0.7 nm) Li film, and from a similar oxidized film Finally, we propose an additional explanation that one or more after pre-annealing for 20 s to (c) 880 K and (d)1070 K, tempera- of the peaks could be due to desorption or decomposition of a tures just before desorption of the higher two Li TPD peaks. The lithium molybdenum oxide (Li x MoO y ) compound. Prior reports ob- peak at 860 K is more prominent in curve (b) compared to that served that Li2 O reacted with Mo and TZM to form Li4 MoO5 , which in curve (a) and this could be associated with the lower coverage decomposed upon heating to liberate Li2 MoO4 in the vapor phase of the oxidized film and the larger influence by the TZM substrate. [ 38 , 39 ]. This reaction occurred measurably within the tempera- Additional TPD experiments (not shown) show that the peaks at ture range of 1073–1223 K in vacuum. In addition, evaporation of

990 and 1220 K grow with increasing Li precoverage, while the lithium molybdate (Li2 MoO4 ) from the solid binary oxide and its 860 K peak saturates for thicker LiO x ﬁlms. It was discovered dur- presence in the vapor phase has been observed previously [ 40 , 41 ]. ing oxidation of lower Li coverages that the feature at 990 K could Reaction of alkali oxides with their metal substrates to form sur- be further resolved and is composed of two desorption peaks at face binary oxides has been observed several times, e.g. a ternary 935 and 989 K. Both the 990 and 1220 K peaks shifted to lower compound of cesium tungsten bronze was formed for O/Cs/W at temperatures as the Li coverage increased while using constant 900 K [42] , and for Li, K, and Na deposited on Ni(110)-O, binary oxygen exposures. When the Li coverage was kept constant at 5 oxides of LiNiO2 , KNiO2 , and Na2 NiO2 were formed [28]. To assess

ML and the O2 exposure was increased, the 990 K peak was found the role of binary oxide decomposition and desorption in our Li to dominate for small O2 exposures, i.e. a Li-rich environment in TPD results, we performed additional studies involving oxidation which the Li film was partially oxidized and the Li multilayer peak of 5 ML Li films on a Mo(100) single crystal surface, as shown in was still present, and the 1220 K peak grew rapidly for large O2 Fig. 2. Multiple signals during TPD were monitored simultaneously exposures, i.e. O-rich environments. To help assess if the peaks ob- corresponding to the parent ions and possible ionization cracking served arise from distinct phases, we annealed the films to two dif- fragment ions that might arise from desorption of Li, pure lithium ferent temperatures. An 880 K anneal removed the 860 K feature oxide phases (O, Li2 O, Li3 O2 , LiO, Li2 O2 , and LiO2 ), pure molybde- and retained the other two desorption states, while an 1070 K an- num oxide phases (Mo, MoO, MoO2 , and MoO3 ), and the binary neal removed all but the peak located at 1160–1220 K. Since these oxide phase Li2 MoO4 (LiMo, Li2 Mo, Li3 O, LiMoO, LiMoO2 , LiMoO3 , pre-annealing experiments did not significantly modify the max- Li2 MoO, Li2 MoO2 , Li3 MoO, and Li3 MoO2 ). The known fragmenta- imum desorption temperatures for the different peaks observed, we conclude that Li desorbs from decomposition of the LiO x film through at least three distinct phases at well-defined temperatures. No coincident desorption in the signals for Li and O or O2 was observed in TPD over the temperature range of 30 0–110 0 K. This indicates that the lithium oxide phases in these films do not have sufficient vapor pressure to be observed desorbing in TPD, but rather decompose during TPD to desorb Li and retain oxygen at the TZM surface up to a temperature of 110 0 K. Such decomposition and subsequent desorption of an alkali metal after oxide formation has been observed for NaO2 films formed on Si(111) [35], although this is not always the case, e.g. coincident signals for K and O were observed in TPD after both were co-adsorbed on Pt(111) [36] . In our studies, a substantial amount of oxygen is lost from the surface at 1220 K, as was observed by XPS (see Section 3.1.2 ). Finally, TPD was used to determine the Li coverage change with temperature, and it was found for the oxidized 5 ML Li film that 0.5 ML Li (10% of the film) desorbed after heating to 880 K, and 3.5 ML Li (70% of the film) desorbed after heating to 1070 K. This analysis as- sumes that the peaks observed are due to Li desorption rather than Li signals from an ionization cracking fragment (and thus requir- ing a different Li QMS sensitivity than that for Li atoms) of some Li-containing desorbing species, e.g. LiO and other compounds discussed in detail later. There are several potential explanations for the origins of the three Li desorption peaks in Fig. 1 . One is that the peaks arise from decomposition of different lithium oxide compounds that co- exist either after oxidation at 310 K or are formed during heating of the films during TPD. This explanation is quite plausible since lithium is known to form multiple compounds with oxygen, i.e. α β Li2 O, Li3 O2, , -LiO, Li2 O2 , and LiO2 , as reported for the Li-O phase diagram [37] . Decomposition of these compounds may occur and desorb Li at different temperatures in reaction rate-limited desorption peaks. In this case, subsequent desorption of Li during heating Fig. 2. TPD signals of multiple masses after oxidation of 5 ML of Li on Mo(100). in TPD reduces the Li:O stoichiometry of the film, and this could Signals followed in amu were: 7 (Li), 23 (LiO), 96 (Mo), 112 (MoO), 128 (MoO 2 ), also lead to formation of new compounds that have different de- 144 (MoO 3 ), 119 (LiMoO), 135 (LiMoO 2 ), 141(LiMoO 3 ), 126 (Li 2 MoO), 142 (Li 2 MoO 2 ), composition temperatures. One other factor is the unknown role 109 (Li2 Mo), 131 (Li3 MoO), and 147 (Li3 MoO2 ).

3 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

+ + + + + tion pattern for Li2 MoO4 is Li , Li2 MoO4 , MoO2 , MoO3 , Li2 O , and LiO + in a ratio of 100:10:2.5:2.3:2.0:0.5 [40] . We were not able to monitor signals above 150 amu due to a limitation in our QMS. Based on our results, and those highlighted by the curves shown in Fig. 2 , the Li peak observed on TZM at 1159–1220 K has a small contribution from a cracking fragment due to desorption of Li2 MoO4 , which can be deduced to occur near 1196 K, with an onset at 1092 K, as indicated in Fig. 2 . Other signals with peaks in TPD near 1196 K (some not shown) assigned to Mo,

Li2 Mo, MoO, MoO2 , MoO3 , LiMoO3 , Li2 MoO, Li2 MoO2 , Li3 MoO, and

Li3 MoO2 cracking fractions were observed. Another peak at 1182 K was observed for signals corresponding to LiMoO and LiMoO2 . On Mo(100), only a signal at 21 amu (LiO) was observed in TPD when monitoring any of the Li x O y masses as listed above. This LiO TPD peak, which occurs at 1176 K, was coincident in temperature and shape with the high temperature Li TPD peak from the oxidized ﬁlm on Mo(100) (bottom curve, Fig. 2 ). This Li TPD curve shows a similar shape and desorption peak temperature as that from TZM ( Fig. 1 a), which leads to the conclusion that the same oxide phases were formed on these two substrates. In Fig. 2 , relative intensities of the Li + /LiO + signals is 300, which is higher than the value of 200 found by Yamdagni et al. [40] . This suggests that another oxide phase (likely Li2 O) different from the binary oxide contributes to the desorbing Li + fragment [39] . This is reasonable since solid

Li4 MoO5 is known to decompose to gaseous Li2 MoO4 and solid

Li2 O. We propose then that Li2 O formed from decomposition of the binary oxide phase and is not evolved into the gas phase, but further decomposes to desorb Li and LiO. We did not observe Li2 O desorption in TPD at the temperatures studied, although this was expected from a prior report [43] . Comparison of the areas of the TPD signals for + + + + Li :MoO2 :MoO3 :LiO were determined to be 100:0.7:0.4:0.3, which shows lower molybdenum oxide fragment intensities compared to that found by Yamdagni et al [40] . Furthermore, prior studies of the oxidation of single crystals of Mo(100) [44] and

Mo(110) [45] resulted in Mo, MoO, MoO2 and MoO3 fragment peaks that had matching line shapes and desorption peaks at ≥1400 K, which is at a higher temperature than the 1206 K molybdenum oxide derived fragments observed in our results. Both of these observations indicate that these TPD signals in our studies arise due to decomposition and desorption of the binary oxide rather than simply formation of a pure molybdenum oxide phase. In other studies that formed bulk lithium molybdenum + oxides, a Li2 MoO4 signal was observed at 1500 K [38], which is higher than in our results and is likely due to the difference in pressures studied. Another contribution to the difference in temperatures could be due to destabilization of the ultrathin binary oxide ﬁlm by the TZM substrate.

3.1.2. XPS of annealed LiO x films XPS was utilized to characterize the chemical nature of the oxidized 5 ML Li film and TZM substrate at 310 K and changes that occurred after heating. XPS spectra for the Mo 3d, Li 1s, and O 1s regions are shown in Fig. 3 a. The Li 1s peak is partially over- Fig. 3. XPS spectra for annealed LiO x films on TZM over a temperature range of lapped by the Mo 4s peak of the substrate. In the bottom curve 30 0–180 0 K. (a) Spectra for the Mo 3d, Li 1s and O 1s core levels. (b) Decomposition of Fig. 3 a for the oxidized Li film at 310 K, the Li 1s peak is at of the Mo 3d spectrum after a 1070 K anneal of the LiO x film. 55.6 eV BE, which agrees with a prior report of 55.8 eV for oxidized Li prepared from vapor deposited films on tantalum foil [21] . The corresponding O 1s spectra show peaks at 529.5 and 532.1 eV at 534.5 eV [46] , while a peak at 533.5 eV has been assigned to

BE, which we tentatively assign to lithium monoxide (Li2 O) and OH species [21]. Residual gas analysis shows a small H2 O pressure lithium peroxide/hydroxide (Li2 O2 /LiOH), respectively [46,47]. Clear in the chamber that could react with Li to form small amounts of ࢞ = assignment of the peak at 532.1 eV to a single species is challeng- LiOH. Under standard state conditions, formation of Li2 O2 ( G - ࢞ = ing due to results showing that LiOH and Li2 O2 have overlapping 136.5 kcal/mol) and Li2 O ( G -134 kcal/mol) are both thermody- XPS peaks near that BE [48] . In studies of the interaction of Li with namically favorable [46] . The O 1 s spectra in Fig. 3 a do not show a condensed oxygen, Qiu et al. assigned O 1s peaks from Li2 O at signiﬁcant peak at 534.5 eV, which rules out the presence of LiO2 .

530–530.5 eV, Li2 O2 at 532.5–533 eV, and the superoxide (LiO2 ) No additional XPS experiments were done to obtain information

4 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

on the nature of the oxide layer at the surface and subsurface in- to Li2 O is unknown in this case. After heating to 880 K, the Li terfaces, though Li2 O is known to form over Li2 O2 when these two 1s intensity decreases to 80% of its original value, then to 43% of oxides are present [49] . The Mo 3d spectra contain the spin-orbit its original value after heating to 1070 K, and is eliminated after split 3d5/2 and 3d3/2 components and can be fit well using only heating to 1400 K. The increase in the Li 1s peak must arise from single peaks from metallic Mo(0). This indicates that Mo was not a structural change in the film, but the decrease between 680 to extensively oxidized, but these spectra do not probe sensitively the 880 K and higher temperatures is due to desorption of Li from the interfacial Mo atoms. This indicates that most of the oxygen forms TZM substrate. No change in the Li 1s peak position occurred dur- an LiO x film rather than incorporating into the Mo to form Mo ox- ing heating. We note that the Li 1s peak intensities are difficult ides. to use to assign Li:O stoichiometries since a large increase in the Heating the sample causes the Li 1s peak to decrease in size Li 1s intensity was observed previously [13] during oxidation of significantly by 1070 K and to disappear by 1400 K, consistent with solid Li. This increase could arise from changes in Li density, mean Li desorption observed in TPD. However, the O 1s peak intensity free path for inelastic scattering, or intensities of satellite features (peak area) is constant at temperatures up to 1070 K, confirming (shake up and shake off processes) that affect the main line inten- that the LiO x film on TZM primarily decomposes upon heating as sity. opposed to evaporating or desorbing oxygen. Heating to 1400 K Considering the changes in the O 1s spectra as the LiO x films causes the O 1s intensity to drop to 46% of the original value, indi- are heated, Li2 O and Li2 O2 /LiOH were the only phases identified cating that oxygen leaves the TZM surface at 1090–1400 K. TPD re- in the LiOx film since an O 1s peak corresponding to LiO2 was sults indicate that this is due to LiO desorption and the decomposi- not observed. We note that conversion of Li2 O2 to Li2 O species has tion of Li4 MoO5 into volatile species that desorb and yield oxygen- been observed in bulk studies of Li2 O2 [49] and similarly in other + + + containing mass fragments such as LiO , MoO3 , Li2 MoO2 , etc. as metal oxide systems such as MgO films on Mo(100) [54], and this already discussed. could account for the changes in relative intensities in the spec- As seen in Fig. 3 a, no significant changes occur in the Mo 3d tra. However, the temperatures for these changes could be strongly spectrum for heating to 680 or 880 K. However, upon anneal- affected by interactions with the substrate. The O 1s binding en- ing to 1070 K, the Mo 3d spectrum changes and, as shown in ergy of Li2 O increases during annealing due to a decrease in crys- Fig. 3 b more clearly, can be decomposed into three components, tallinity and/or changes in structural ordering of the film [55–57] . with Mo 3d5/2 peaks at 227.7, 229.5, and 232.9 eV BE, assigned The O 1s peaks after heating to 1070 K or higher were not decom- to Mo(0), Mo(IV), and Mo(VI). The detection of Mo(VI) could arise posed into components since they are likely to arise from multiple from Li4 MoO5 (as discussed above) or MoO3 , and the larger Mo(IV) oxygen species and we have no clear structure within the peak to peaks could be due to other binary oxides or MoO2 . The oxidized guide us. As stated above, there is a lithium molybdenum oxide

Mo components comprise 15.8% of the Mo 3d intensity. At this species, which could be Li4 MoO5 as well as other complex interfa- temperature, a chemical reaction occurs between the LiO x ﬁlm and cial Li x Mo y O z species. The O 1s decrease from heating from 1400– the Mo substrate to oxidize the Mo interfacial layers to form ei- 1800 K is not from desorption of O2 since this was not observed ther Mo oxides [50] or binary oxides of lithium and molybdenum, in TPD. After annealing to 1400 K or above, smaller Mo(IV) and such as Li4 MoO5 [38,51], that would give rise to a Mo(VI) sig- Mo(VI) peaks in the Mo 3d spectra shows that little oxygen is as- nal. Several other binary oxide phases could be formed. The solid sociated with molybdenum oxides, but rather the oxygen retained state quasi-binary phase diagram of lithium oxide-molybdenum is bound to the grain boundaries, diffused into the subsurface re-

(VI) oxides shows four intermediate phases of Li4 MoO5 , Li2 MoO4 , gion of the substrate, or is associated with TiO2 or ZrO2 . Ti and Zr

Li4 Mo5 O17 , and Li2 Mo4 O13 [52], while the Li-O-Mo(IV) phase dia- XPS peaks were not followed during the annealing experiments in gram is composed of Li2 MoO3 , Li4 Mo5 O12 , and Li6 Mo2 O7 [53]. The order to limit XPS data collection times. As stated in Section 2, AES Mo(IV) and Mo(VI) peaks decrease substantially after heating fur- spectra determined that the clean surface contained only 1.8 at.% ther to 1400 K, consistent with our TPD data indicating desorption Ti and no detectable Zr. At temperatures above 900 K, LEIS (dis- of species such as MoO3 or Li2 MoO4 at 1070-1400 K. It seems clear cussed in Section 3.1.3) detected Ti but no appreciable level of Zr that binary lithium molybdenum oxide compounds, and not only in the top surface layer. As such, we attribute the O 1s signal from molybdenum oxide phases, exist in these ﬁlms and this is sup- 140 0–180 0 K largely to O chemisorbed on Mo and associated with ported by the XPS data. The TPD results in 3.1.1 show desorption of TiO2 , with little contribution from ZrO2 . We note that Ti and Zr + + + species containing both Li and Mo, and MoO , MoO2 , and MoO3 bind with oxygen and carbon in the bulk and at grain boundaries, signals near 1200 K. Prior studies of the oxidation of a Mo(100) and these oxides and carbides play an important role in the bulk single crystal observed in TPD desorption of MoO3 and MoO2 at mechanical behavior of TZM [29,30,58].

1400–1600 K, where MoO3 monomers and dimers were desorbed The Li/O ratio as the LiOx film is annealed to the temperatures from thick oxide films at 1400 K and from a monolayer at 1600 K shown in Fig. 3 was calculated using the peak area intensity of [45] . Our XPS studies indicate that the lithium molybdate phase(s) Li and O, normalized to atomic sensitivity factors [22] . The results are more volatile and also decompose at lower temperatures than show the film goes from a Li-rich stoichiometry to an O-rich sto- these molybdenum oxides. ichiometry due to desorption of the Li. The ratio at 310 K is 1.36 The Mo 3d signal intensity increases significantly (to 95.8% of which is consistent with the O 1s spectra that indicate a 3:1 ratio the clean surface value) after heating to 1400 K due to desorp- of Li2 O to Li2 O2 /LiOH. The Li/O ratio initially rises as the film is an- tion of most of the oxidized film, but even heating to 1800 K did nealed to 680 K due to interconversion of the peroxide/hydroxide not remove all oxygen. Annealing to 1800 K leaves a Li-free TZM to the oxide. After annealing to 880 K, the Li/O ratio drops to ~1. substrate, with metallic Mo 3d peaks and a reduced O 1s peak at At this temperature, a fraction of the Li has desorbed leaving ad- 530.1 eV BE. This O 1s intensity is the same as that measured from sorbed oxygen. In the 1070 K case the Li/O ratio is 0.61 which is a freshly cleaned TZM substrate, which is present due to some oxy- an environment that is O-rich indicative of a complex binary oxide gen in the subsurface region or located in the grain boundaries. in which Li and Mo exist in an oxidized state. Finally, the Li 1s intensity increased by 20% from its original value upon heating from 310 to 680 K, and the conversion of 3.1.3. LEIS of LiO x films

Li2 O2 /LiOH to Li2 O in the O 1 s spectra occurs over the same range. Low energy ion scattering (LEIS) spectra of the clean TZM sub- + No desorption of molecular oxygen (O2 signal) was observed in strate and Li and LiO x ﬁlms on TZM are shown in Fig. 4 . The bot- TPD. The mechanism by which the peroxide/hydroxide is reduced tom curve shows Mo (1810 eV), Ti (1643 eV), and O (1090 eV) LEIS

5 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

companied by Li desorption (see Fig. 1 ). At 90 0–10 0 0 K, a Ti LEIS peak appears from the TZM substrate, presumably due to oxygen-

induced segregation of Ti and formation of TiO2 . There was no similar growth in the Zr LEIS peak in the range of 90 0–10 0 0 K or at higher temperatures. The Mo substrate signal was observed at 10 0 0 K after more of the LiO x ﬁlm decomposed and Li desorbed. Further oxygen-induced segregation of Ti to the surface occurred after heating to 1700 K. The Ti/Mo ratio in LEIS was 0.23 for clean TZM, and this increased to 1.02 after the LiO x /TZM surface was heated to 1700 K. The decrease in the O LEIS signal at 1700 K is

likely due to diffusion of O atoms into the bulk since no O2 desorption was observed from TPD [61] . The presence of an O LEIS peak at 1800 K (not shown) indicates that O is still present at the surface after annealing to high temperatures and has not all diffused into the bulk. Oxygen atoms are likely trapped at the surface

in grain boundaries or as TiO2 . Based on our results so far, the following model is proposed for reactions in the post-oxidized Li ﬁlm on a TZM substrate. Metallic Li deposited on TZM is oxidized at 310 K by 20 L of + → O2 to form Li2 O by the reaction: 4Li (s) O2 (g) 2Li2 O (s). + Lithium peroxide can be formed via two reactions: 4Li (s) 2O2 → + → (g) 2Li2 O2 (s), or 2Li2 O (s) O2 (g) 2Li2 O2 (s). LiOH can be

formed by reaction of H2 O from the background gas with Li (2Li + → + + → H2 O LiOH LiH), Li2 O (Li2 O H2 O 2LiOH), or Li2 O2 (2Li2 O2 + → + 2H2 O 4LiOH O2 ). As the surface temperature increases, in the range of 310 to 1070 K, all of these Li oxides, composed pri-

marily of Li2 O, decompose at several distinct temperatures that we can observe by the accompanying Li desorption (see Fig. 1 ) and leave interfacial oxygen at the surface that reacts to form

TiO2 and lithium molybdenum oxides, including Li4 MoO5 , near

1070 K. Li4 MoO5 decomposes into lithium molybdate (Li2 MoO4 )

which desorbs near 1200 K and lithium oxide (Li2 O) [39] which further decomposes and desorbs Li and LiO.

3.2. Li adsorption on oxygen-precovered TZM (O/TZM)

Fig. 4. (a) LEIS spectra for monitoring the thermal stability of the LiO x films formed from a 20 L O2 exposure on a 5 ML Li film on TZM. In these experiments, TZM at 310 K was exposed to 20 L of O2 prior to deposition of Li films, and the pre-oxidized TZM substrate is herein referred to as O/TZM. XPS results (provided below) show peaks characteristic of the TZM surface. Zr may be present but the that this forms a chemisorbed oxygen layer and does not lead to Zr peak, calculated to be at 1793 eV, is not resolvable from the extensive oxidation of the TZM. This is consistent with prior re- Mo peak in these spectra. Depositing 5 ML of Li on the surface ports demonstrating that molecular oxygen dissociates on both Ti eliminates the Mo and Ti peaks, causes a characteristic change in and Mo to form an overlayer of O adatoms after a 20 L O2 expo- the background signal, and causes the appearance of a Li signal at sure at ambient temperatures [ 44 , 62 ]. The Ti/Mo peak ratio in LEIS 390 eV. The deposited Li film was pure with only a very small O (not shown) of the TZM surface increased from 0.14 (for the “clean LEIS signal observed. The absence of any Mo or Ti substrate sig- surface”) to 0.25 after oxygen exposure, which is associated with nals shows that Li wet the TZM surface and left no uncovered TZM more Ti present at the TZM surface after oxidation. The coverage regions. Subsequent oxidation of this Li film at 310 K by a 20 L of O atoms in the chemisorbed oxygen layer at the TZM surface

O2 exposure produced the curve labeled as “LiOx ”, in which the Li is likely near 1.1–1.5 the density of the Mo atoms in the surface signal was eliminated, and an O peak appeared. The LiO x film also layer of TZM, based on previous reports of the saturation cover- completely wet the TZM surface, and furthermore no de-wetting age of chemisorbed oxygen on single crystal molybdenum surfaces of the LiO x film occurred prior to heating to 10 0 0 K and evapora- [ 61 , 63 ]. tion of Li. Only a small Li signal after oxidation is expected from the crystal structure of solid Li2 O, which is an antifluorite struc- 3.2.1. TPD results ture with a low Miller-index surface composed of O atoms with Pure Li films of varying thickness were deposited on the O/TZM Li atoms buried in the second layer [59] . Another explanation for surface. Li TPD curves for these films are shown in Fig. 5 . The Li changes in the Li signal could be matrix effects leading to changes coverages reported in Fig. 5 were calculated based on the Li dose in the neutralization probability (i.e. changes in the neutralization time, which is linear with Li coverage and was calibrated previ- during scattering from a given atom as a function of its chemi- ously for the Li evaporator using Li TPD from a metallic Li film. Li cal environment) as observed during oxidation of Ce atoms [60] . coverages derived from Li dose time should be the same as those The O LEIS intensity increases with annealing temperature, slowly from the areas under the Li TPD curves, however we observed dif- increasing by a factor of 1.4 from 30 0–90 0 K, prior to significant ferent values. This situation arises when the Li mass spectrometer Li desorption, and then rapidly increasing by a factor of 2.3 upon signal is produced from desorption of Li compounds in addition to heating from 900 to 1500 K, with a small decrease after heating Li vapor. The ionization and cracking of these complex Li species further to 1700 K. As before, these intensity changes could arise produce a Li ion signal, but with different and unknown sensitivi- from morphological or composition changes in film structure ac- ties. Therefore, the Li TPD curve area is no longer necessarily pro-

6 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

contamination of TZM substrates. The differences in desorption on- sets become smaller as the Li films become thicker since eventu- ally there is not enough oxygen to influence all of the Li in the film. For the 0.5 ML Li case, the desorption onset increases from 560 K on TZM to 920 K for O/TZM. The 1 ML Li film has a desorption onset of 490 K that is shifted to 722 K on O/TZM. The 2 ML Li film on TZM starts to desorb at 477 K while this is shifted to 572 K on O/TZM, and no change in the peaks was observed for the 4 ML Li film. Using the desorption peak temperature of the first peak above that of the Li multilayer, we observe that the Li film thermal stability is increased by 422 K when oxygen is on the surface for a 0.5 ML Li film, and this changed to 233 K for 1 ML Li. For the thicker films, surface oxygen contamination of the TZM substrate did not affect the thermal stability of the bulk of the Li film, and Li primarily desorbed in the unaltered metallic Li multilayer peak. However, the Li monolayer that ultimately remained upon heating this film desorbed at 141 K higher temperature from O/TZM for the initial 2 ML Li case, and little change is observed for the 4 ML case on O/TZM.

Fig. 5. Li TPD curves after depositing different Li amounts on an O/TZM surface. 3.2.2. XPS results θ Li coverages ( Li ) given in bold on the ﬁgure are based on a calibration using the XPS studies were done to examine chemical modiﬁcations that

Li evaporator. Li coverages deduced from the Li TPD areas using a 1 ML Li TPD occur when Li is deposited on an-oxygen covered TZM surface at reference are provided here in parentheses: θ (ML) = 0.25 (0.08), 0.5 (0.23), 1.0 Li (0.66), 2.0 (1.01), and 3.0 (1.92) (TPD not shown for 4.0 (2.27) and 5 (3.04)). The 310 K, and these results are shown in Fig. 7. The Mo 3d spectra temperatures of the TPD peaks are given directly above the dashed lines indicating shown in the left panel are not significantly changed after 20 L the peak maxima. O2 exposure and showed only metallic components indicating that Mo was not extensively oxidized. As shown in the O 1s spectra, the cleaned TZM sample retained some oxygen, assigned either to portional to the Li coverage on the surface. Thus, we have provided O chemisorbed on Mo or Ti or oxidized Ti on the surface or in θ hereon both the Li coverage ( Li in ML) obtained from the Li dose grain boundaries. The O 1s peak increases in area and shifts 0.2 eV time and also values from the measured integrated Li TPD areas to lower BE from the higher coverage of chemisorbed oxygen at from O/TZM referenced to the 1 ML Li TPD area in parentheses. the surface after 20 L O2 exposure. These observations are con- θ = The lowest Li coverage, Li 0.25 (0.08), led to the formation sistent with previous studies showing that very high (2000 L) O2 of a peak at 1196 K. The temperature of this peak is the same exposures on Mo at very high temperatures of 90 0–120 0 K were as the highest temperature peak observed in the Li desorption required to form oxides such as MoO, MoO2 , and MoO3 [44]. spectra of post oxidized lithium films, making it likely that this XPS spectra following deposition of 0.5 and 5 ML Li on O/TZM peak arises from the Li signal from desorption of a binary ox- at 310 K is shown in the top two sets of curves in Fig. 7 . Li depo- ide phase (Li x MoO y ). At this low coverage, the interfacial Li is sition caused no new peaks or peak shifts in the Mo 3d spectra. in an O-rich environment that facilitates interactions with Ti and The Li 1s peak for both the 0.5 and 5 ML films has a BE indicative Mo atoms, apparently forming complex binary oxide phases dur- of an oxidized Li state. The O 1s peak exhibits a + 0.4 eV positive ing heating. With increasing amounts of Li deposited, a new Li shift in the BE when 0.5 ML of Li is deposited on the O/TZM. This TPD peak appeared at 1157 K, which is presumably also due to a shift arises from formation of a submonolayer of interfacial lithium Li signal from a second binary oxide decomposition product, one oxide containing either or both Li-O-Mo or Li-O-Ti bonding. For the which has a higher vapor pressure. The interfacial oxide formed 5 ML Li film, the O 1s peak has the same binding energy as that on the surface from 0.25–0.5 ML Li deposition is extremely stable of the 0.5 ML Li film, indicating also the presence of an interfacial and does not form any of the Li-decomposition products observed oxide. The calculated Li/O ratio for the 5 ML film is 2.5, indicating at lower temperatures ranges of 80 0–10 0 0 K. After deposition of a Li-rich environment, while that for the 0.5 ML film is 0.94, which θ = Li 1.0(0.66), an additional peak appeared at 923 K. After depo- is O-rich. θ = sition of Li 2.0(1.01), another peak at 793 K appeared, and the In separate experiments, 0.5, 1 (not shown), and 5 ML Li films θ = 923 K peak decreased in size. Deposition of Li 3.0(1.92) led to deposited on O/TZM at 310 K were annealed to various tempera- the formation of the metallic Li multilayer and three high temper- tures and probed by XPS as displayed in Fig. 8 . In Fig. 8 a, following ature Li peaks at 812, 934, and 1157 K. The peak at 1157 K satu- deposition of 0.5 ML Li on O/TZM at 310 K, annealing to 650 K and rates with increasing thicknesses of Li films on O/TZM. A metal- 850 K leads to a reduction in the O 1s peak binding energy by lic Li multilayer is formed after large depositions of Li since there 0.2–0.4 eV possibly due to changes in either the interfacial oxide is insufficient oxygen available to oxidize all the deposited Li. Al- structure and bonding or surface morphology. TPD measurements though the result shows that different lithium oxide phases form in Fig. 6 show that no Li desorbs at these temperatures and so this depending on the initial coverage of Li deposited on O/TZM and shift cannot be attributed to Li loss. After a 1400 K anneal, Li is these have different vapor pressures and thermal stabilities, the fully desorbed, and oxygen is left on the oxidized substrate. temperatures that these compounds are formed is not revealed by Fig. 8 b shows that annealing the 5 ML Li on O/TZM sample to the TPD measurements, and so one needs additional temperature- 500 K caused the O 1s peak to shift 0.2 eV higher BE. Annealing dependent spectroscopic information. to 680 and 900 K causes substantial desorption of Li from the film Fig. 6 directly compares Li TPD curves from clean TZM with and shifts of the O 1s peak to 0.7–0.9 eV lower BE. This annealing those from O/TZM for the same Li film coverages. Using the Li des- creates an interfacial oxide characterized by an O 1s peak similar orption onset and peak temperatures from the pure Li and oxidized to that for Li2 O. Further annealing to 1050 K causes the O 1s peak Li films, we obtain useful information on the modification of the to shift by + 0.4 eV, which is likely due to the formation of lithium thermal stability of Li films due to the presence of surface oxygen molybdenum oxides on the surface. This is supported by the iden-

7 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

Fig. 6. Comparisons of Li TPD curves from Li ﬁlms deposited on TZM and O/TZM surfaces. The labeled amounts of deposited Li in the ﬁgures are referenced to the deposition of 1 ML of Li on clean TZM. Information on the O/TZM surface is given in the text.

Fig. 7. Mo 3d, Li 1s, and O 1s XPS spectra from TZM and O/TZM surfaces, along with spectra after deposition of 0.5 and 5 ML Li ﬁlms on an O/TZM surface at 310 K. The

XPS spectrum for the 50–75 eV BE region for the 20 L O 2 /TZM sample (not shown) was unchanged from that of clean TZM.

8 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587

greater than 900 K, indicating that the binary oxides are formed within a temperature range of 900–1050 K. The results for Li ﬁlms deposited on oxygen pre-covered TZM suggest the formation of an interfacial lithium oxide containing either or both Li-O-Mo or Li- O-Ti bonding at 310 K. Subsequent annealing causes decomposition and desorption of Li in TPD (T p = 812 and 934 K) followed by Li x MoO y decomposition and desorption in TPD (T p = 1157 K) as- → + signed in part to: Li4 MoO5 (s) Li2 MoO4 (g) Li2 O (s), where the

Li2 O formed immediately decomposes to Li and LiO.

3.3. Implications for lithium-coated plasma facing components

The solid or liquid Li coatings that are used in fusion devices

can be oxidized by reactions with residual gasses (H2 O and CO) in the vacuum vessels or dissolved oxygen in the grain boundaries of the metal substrates. The post-oxidation experiments reported here in Section 3.1 are most closely related to the former case and experiments with Li films deposited on the oxygen-precovered TZM surfaces are relevant to the second case. The effects of oxygen on the thermal stability of Li films by post-oxidation and deposition on an O/TZM substrate are compared in Fig. 9 . In the absence of much oxygen, as shown in the bottom Li TPD curve, multilayer metallic Li films on TZM are stable until 520 K where the Li in the multilayer begins to desorb in the multilayer peak at 606 K. The Li monolayer on TZM is much more tightly bound, and Li desorption is not complete until 1200 K where any oxygen impurities cause small lower temperature peaks. As shown in the middle curve, Li films subjected to extensive post-oxidation to

form a Li2 O/Li2 O2 /LiOH mixed oxide ﬁlm are more thermally stable than without oxygen, and Li does not begin to desorb until

Fig. 8. O 1s XPS spectra from TZM and O/TZM surfaces, along with spectra after deposition of (a) 0.5 ML and (b) 5 ML Li on an O/TZM surface and annealing of the Li films. tical BEs of the O 1s peaks for the 1050 K anneal for Li on O/TZM and the 1070 K anneal for the post-oxidized Li films. The O 1s peak area does not vary during annealing from 310– 10 0 0 K for the 0.5 and 1 ML (not shown) Li films on O/TZM, indicating no loss of oxygen until decomposition of the binary oxides and desorption of the volatile, oxygen-containing products, which agrees with the post-oxidation experiments. The O 1s area for 5 ML Li films initially decreases after annealing at 500 K and starts to increase until a maximum value at 900 K due to desorption of multilayer Li and diffusion of the interfacial oxygen into the Li. We note that the O 1s spectra for 0.5–1 ML Li films deposited on O/TZM do not clearly indicate Li2 O2 or Li2 O phases at 310 K or during annealing. For the 1 (not shown) and 5 ML Li films, the O 1s Fig. 9. Li TPD curves from 6.47 ML Li/TZM (bottom curve), 20 L O 2 /5 ML Li/TZM peak due to the binary oxides was not observed until temperatures (middle curve), and 3 ML Li/O/TZM (top curve).

9 O. Fasoranti, E.T. Ostrowski and B.E. Koel Journal of Nuclear Materials 543 (2021) 152587 over 700 K with peaks at temperatures of 860, 990, and 1220 K. LiO x films formed by either post-oxidation of deposited Li films or The 860 K peak could come from particular interfacial compound by the deposition of Li films on an oxygen-precovered titanium- structures/clusters and the 990 K peak could come from desorption zirconium-molybdenum (TZM) alloy surface was studied by using and decomposition of lithium-deficient oxide phases, Li2- αO, that TPD, XPS, and LEIS under UHV conditions. The 5 ML LiOx films are formed after Li desorption. Post-oxidation increases the onset prepared by post-oxidation of Li were stable until temperatures of Li desorption by 200 K, and significant Li desorption and mate- of 620 K, after which they desorbed in three main peaks in TPD. rial loss does not occur until 250 K higher temperatures. Moreover, Desorption of Li from the surface is attributed to the decomposi- a large fraction of the Li coating remains on the surface until bi- tion and desorption of lithium oxide, LiO x , (860 and 990 K) and nary oxide decomposition leads to Li loss in a peak at 1220 K. The then the decomposition of solid lithium molybdates in the Mo(IV) top TPD curve in Fig. 9 is for a Li film deposited on an oxygen- and Mo (VI) state with subsequent evaporation of binary oxides, precovered O/TZM surface. In this case, there are four desorption including Li2 MoO4 , and fragments of Li2 O (1220 K). One of these peaks that are at 572, 812, 938, and 1155 K. The amount of oxygen solid lithium molybdates could be Li4 MoO5 . The onset for Li des- available for oxidation of the Li film is very limited in this case orption from a LiO x film is 100 K higher than that from a metal- (1.1–1.5 ML O) and is insufficient to oxidize the entire 3 ML Li film lic Li film, demonstrating the higher stability of the oxides. XPS that was deposited, and so ~0.5 ML Li desorbs in a peak at 572 K. showed the formation of a mixed oxide phase of lithium monoxide

TPD peaks at 812 and 938 K for the 3 ML Li/O/TZM curve are be- Li2 O and lithium peroxide/hydroxide Li2 O2 /LiOH upon oxidation of lieved to have the same origin as the 860 and 990 K peaks that are the Li film at 310 K. Li2 O2 /LiOH are thermally unstable and began present in the 20 L O2 /5 ML Li/ TZM system. The 1155 K peak is to decompose to Li2 O when annealed to 680 K. XPS also showed formed at a lower coverage of Li and saturates with increasing Li that at 1070 K, Mo in the TZM substrate was oxidized to form thicknesses for Li/O/TZM indicating that it is an interfacial binary Mo(IV) and Mo(VI) species. Molybdenum oxides typically can only Li x Mo y O z state whose formation is strongly dependent on the oxy- be formed at conditions of high temperature ( > 10 0 0 K) and high gen concentration at the interface. This peak overlaps with another oxygen exposures ( > 80 L) indicating that Li promotes molybde- state causing the 1220 K peak that was observed in post-oxidation num oxidation through a solid-state reaction. LEIS experiments of experiments, which continues to grow with increasing Li thickness the oxidized Li films confirm the wetting of TZM by LiO x films at and oxygen exposure. This peak arises from formation of lithium room temperature. In addition, the deposition of sub-monolayers molybdenum binary oxide(s) that are formed at high temperatures of Li on an oxygen-precovered TZM substrate was also investigated for increasing Li and oxygen amounts and decompose in TPD to that may be relevant to dryout conditions of PFCs. Deposition of desorb gas phase oxide species that produce signals at more than Li films at 0.5 ML on O/TZM at 310 K led to the formation of an a half-dozen amu values as shown in Fig. 2 . interfacial lithium oxide species that decomposes and desorbs Li A 3 ML Li film is about 6 A˚ (0.6 nm) and the coatings used in a single broad peak at 1126 K in TPD that closely matched that in NSTX-U and LTX are much thicker (hundreds of nanometers to of the post-oxidized films. Overall these results highlight the in- micron scales) than this. At sufficiently high temperatures, oxygen creased thermal stability of oxidized lithium over that of lithium from the bulk or grain boundaries of molybdenum in a CPS system metal and the importance of accounting for Mo impurity transport could diffuse to the surface and react with the Li films to form into the plasma via formation of volatile lithium molybdate com- lithium oxides. This would lead to a situation where there is a liq- pounds at high temperatures. uid Li overlayer with solid Li2 O layer or Li4 MoO5 formed at the substrate interface due to reaction with interfacial oxygen. More Declaration of Competing Interest extensive oxidation by post-oxidation via residual gasses leads to higher thermal stability of Li, but the increase in stability is ac- None. companied by a much higher melting point of the LiO x compounds

= = CRediT authorship contribution statement (e.g. Li2 O mp 1711 K) than pure Li (mp 453 K), which has important implications for flowing liquid Li systems. O. Fasoranti: Conceptualization, Methodology, Investigation, One primary motivation for studying the thermal stability of Validation, Formal analysis, Writing - original draft, Writing - re- these lithium oxide films is to provide a fundamental understand- view & editing. E.T. Ostrowski: Formal analysis, Writing - review ing of how deuterium retention will change due to chemical state & editing. B.E. Koel: Conceptualization, Writing - review & editing, and structural changes that occurs on LiO x films as a function of Supervision, Funding acquisition. temperature. Although these results are for ultrathin Li films, if it is assumed that the thick Li coatings used in the divertor region Acknowledgement of fusion devices have oxides with similar thermal stabilities, there are several implications for fusion devices. For a Li film oxidized This material is based upon work supported by the U.S. De- + by residual gasses at 310 K, energetic D ions will interact with partment of Energy, Office of Science/Fusion Energy Sciences under a film that may have a surface composed of a mixture of lithium Award Number DE-SC0012890. OF acknowledges partial support by oxide, peroxide, hydroxide compounds. While hydrogen retention the Program in Plasma Science and Technology at Princeton Uni- in metallic Li and Li2 O is believed to be comparable, the hydrogen versity. retention behavior of LiOH and Li2 O2 is not yet understood. Also, if high temperatures of > 10 0 0 K occur due to high heat flux, ELMs, References or disruptions, one could expect that binary oxides, e.g. lithium molybdenum oxides, could be formed that could liberate Mo into [1] D.K. Mansfield, K.W. Hill, J.D. Strachan, M.G. Bell, S.D. Scott, R. Budny, E.S. Mar- mar , J.A. Snipes , J.L. Terry , S. Batha , R.E. Bell , M. Bitter , C.E. Bush , Z. Chang , the gas phase and radiatively cool the plasma. D.S. Darrow , D. Ernst , E. Fredrickson , B. Grek , H.W. Herrmann , A. Janos , D.L. Jassby , F.C. Jobes , D.W. Johnson , L.C. Johnson , F.M. Levinton , D.R. Mikkelsen ,

4. Conclusion D. Mueller, D.K. Owens, H. Park, A.T. Ramsey, A.L. Roquemore, C.H. Skinner, T. Stevenson , B.C. Stratton , E. Synakowski , G. Taylor , A. von Halle , S. von Goeler , K.L. Wong , S.J. Zweben , T. Group , Enhancement of Tokamak Fusion Test Reactor Experiments were performed to improve our fundamental un- performance by lithium conditioning, Phys. Plasmas 3 (1998) 1892–1897 . derstanding of how lithium-oxide ﬁlms will behave at elevated [2] S.V Mirnov , E.A. Azizov , V.A. Evtikhin , V.B. Lazarev , I.E. Lyublinski , A.V Vertkov , D.Y. Prokhorov , Experiments with lithium limiter on T-11 M tokamak and ap- temperatures that may arise from high heat loads on PFCs dur- plications of the lithium capillary-pore system in future fusion reactor devices, ing fusion experiments. The thermal behavior of ultrathin ( ≤5 ML) Plasma Phys. Control. Fusion 48 (2006) 821–837 .

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