316 Björn Pfeiffer et al. elemental peaks for Li+,Mn+,Mn2+,O+,O +
peaks are observed. Additional peaks, not directly related to the analyzed material, are those of hydrogen and gallium resulting from the residual gas in the ultrahigh vacuum (UHV) chamber and the FIB-based specimen preparation, respec- tively. The peak at 71 amu, assigned toMn–O+ also covers the
2 variousMnxOz + y
Figure 3. Logarithmically plotted mass spectra for the two stages of in situ deintercalation of lithium-manganese-oxide at 298K (red and blue curve). The gray curve shows a mass spectrum for a tung- sten sample under similar field evaporation conditions. All spectra are normalized to the same noise level for mass-to-charge ratios <5.
125 μm at the specimen and a wavelength of 355 nm, was operated at a repetition rate of 200 kHz. Depending on the mode of analysis, pulse energieswere varied from25 to 200 nJ. For this study, two different modes of atom probe analysis
were used. Conventional atom probe analysis was performed at 27.5 nJ pulse energy, with a specimen base temperature of 30K and automated voltage adjustment to achieve a detection rate in the range of 0.004–0.01 ions/pulse, corresponding to 8×102 to 2×103 s−
1.For the in situ deintercalation experiments, first a conventional analysis up to a specificvoltage Vmax was per- formed to prepare a well-defined initial specimen state. Then the voltage was set to a constant valueV in the range of 50% up to 80% of Vmax, the sample base temperature was increased up to 298K and a higher pulse energy in the range of 70–200nJ was used, yielding a rate of detected Li-ions in the range of 1×103 to 1×105 s−1 after a short time. The detection rate exhibits strong temporal fluctuations and decreases down ≈30 s−1 after several hours (cf. Supplementary Figure 3 and In Situ Atom Probe Deintercalation at 298K section). As the electric field strength at the apex is reduced compared with conventional atom probe analysis by setting a constant voltage V<Vmax,the lowevaporation field for Li enables field eva- poration of solely Li without affecting theMn–Ohost structure (see In Situ Atom Probe Deintercalation at 298K section).
Supplementary Figure 3
Supplementary Figure 3 can be found online. Please visit
journals.cambridge.org/jid_MAM.
RESULTS AND DISCUSSION
Lithium Mobility Under Conventional Atom Probe Analysis at 30K
A typical mass spectrum for a conventional atom probe analysis of LMO at 30K is shown in Figure 1. Besides the
events from the minor isotope 71Ga+, but since the signal for 69Ga+ is relatively low, this overlap is of minor relevance and not expected to influence the results crucially. By calcu- lating the atomic ratios, a stoichiometry of approximately Li1.2Mn2O2.3 is obtained. This shows that O is detected substoichiometrically, which is a known effect in atom probe tomography of oxides (e.g., Karahka et al., 2015).Nevertheless, the data shows a uniform distribution of all elements and the spatial resolution is high enough to reveal lattice planes in the reconstruction (Maier et al., 2016). To measure eventual Li mobility at 30 K, the automatic
explained because further field evaporation causes tip blunt- ing and thus a decreasing electric field strength at the speci- men apex. However, if Li-ions were mobile in the specimen under the given conditions, the decreasing detection rate would be only expected for the immobile host species Mn and O. For Li, a net flux in the direction of the apex would be expected, either driven by the electric field inside the speci- men or by a gradient in the chemical potential. As elemental Li has a low theoretical field evaporation field strength of 14V/nm, compared to 30V/nm for Mn (Miller et al., 1996: 492; image hump model) and the experimental value of ≈28V/nmforLMO, Li field evaporation at the apex, and thus an increasing Li content, would be expected with time. The temporal evolution of the Li content is analyzed based on the data displayed in Figure 2b. For the content of Li,
base voltage adjustment was stopped during a conventional atom probe analysis at a base voltage of 5.8 kV. The temporal evolution of the resulting normalized detection rate is displayed in Fig. 2a). The dotted lines in Fig. 2b) illustrate the average content of Li, Mn, and O. These graphs show a dif- ferent behavior before and after a characteristic time of 20 h. Initially, the curve forOis decreasing, whereas the curves for Li and Mn are increasing. From 20 h on the curve for O is increasing, whereas the curves for Li and Mn are decreasing. The decreasing detection rate in Figure 2a is naturally
Mn, and O, two different regimes can be observed. In the first 20h the dominant effect is the decreasing electric field strength. Because the detection rate for O+ and O +
observed to decrease more rapidly than that of other atomic or molecular species, the average content of O in this region becomes lower, in accordance with the results of Devaraj et al. (2013). For the same reason, the average content of Li andMn increases. From20h on, this effect plays aminor role and the average content ofOincreases,whereas the average content of Li andMn decreases (dotted lines in Fig. 2). If therewere significant Li transport in the sample, the Li
2 is
content would be expected to increase, thus, the observed decrease cannot be naturally explained this way. On the other hand, the observed increase in the O content is not attributed to mobile O in the sample as O has no significant
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