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Lide Yao,* Sayani Majumdar, Laura Äkäslompolo, Sampo Inkinen, Qi Hang Qin, and Sebastiaan van Dijken* Transition metal oxides with a perovskite crystal lattice of type ABO3 may possess corresponding oxygen-deficient modulation structures.[1–4] One prototypical example is the brownmillerite crystal structure of type ABO2.5,[5–10] which due to its high ionic conductivity holds the potential for finding applications in solid oxide fuel cells, oxygen-separation membranes, gas sensors and other devices requiring anion diffusion.[11–17] Brownmillerites have been derived from perovskite materials using topotactic reduction,[5] optimized film growth,[6–9] and oxygen getters.[10] Here, we demonstrate that the evolution of the perovskite– brownmillerite phase transition can be fully controlled and monitored in epitaxial La2/3Sr1/3MnO3 (LSMO) films using electron-beam irradiation in a transmission electron microscope (TEM). Real-time TEM imaging with atomic resolution reveals that the structural transition is driven by an incessant ordering of electron-beam-induced oxygen vacancies. Over-irradiation of the brownmillerite phase induces a second transition to a perovskite-like structure with disordered oxygen vacancies and a significantly enhanced out-of-plane lattice compared to the original LSMO film. The demonstrated ability to simultaneously induce and characterize oxygen-deficient structural phases in a continuous and controllable manner opens up new pathways for atomic-scale studies on ionic transport dynamics.[18] The electron-beam-induced structural evolution of epitaxial LSMO thin films on SrTiO3 (001) (STO) single-crystal substrates was systematically investigated using a JEOL 2200FS TEM with double Cs correctors (see Experimental Section for more details). Figure 1 shows TEM images and corresponding local fast Fourier transform (FFT) patterns of a LSMO film during several stages of electron-beam irradiation. In the initial state (Figure 1a), the LSMO exhibits a perovskite structure with an (001)[100]LSMO// (001)[100]STO epitaxial relationship to the underlying STO substrate. Because of the good lattice match (tensile strain of 0.9%), misfit dislocations are absent and the LSMO unit cell is pseudocubic with an average out-of-plane lattice spacing of 3.87 ± 0.01 Å. Exposure to a focused electron beam with a current density of ca. 50 pA/cm2 results in the emergence of a superlattice Dr. L. D. Yao, Dr. S. Majumdar, L. Äkäslompolo, S. Inkinen, Dr. Q. H. Qin, Prof. S. van Dijken NanoSpin, Department of Applied Physics Aalto University School of Science P.O. Box, 15100, FI-00076, Aalto, Finland E-mail: [email protected]; [email protected]
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Electron-Beam-Induced Perovskite–Brownmillerite– Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films
structure. The new structural phase nucleates in the vicinity of the LSMO/STO interface and it steadily grows with increasing exposure time until the entire irradiation area is covered after about 20 minutes (Figure 1b). The periodicity of the electronbeam-induced structure is two unit cells along the c-direction as illustrated by additional reflections in the FFT pattern. Once formed, the superlattice structure is stable both in vacuum and under atmospheric conditions at room temperature. Prolonged electron-beam irradiation beyond the 20 minute period causes a second structural phase transition. The superlattice contrast gradually disappears and a perovskite-like crystal structure is retained after an irradiation time of 50 min (Figure 1c). Longer exposures to the focused electron beam do not further change the structure of the LSMO film. A similar sequence of events is also obtained for other electron-beam conditions with the transitions occurring more rapidly for higher beam intensities. For a current density of ca. 110 pA/cm2, for example, the final perovskite-like phase is obtained after an irradiation time of about 10 min (Supporting Information, Figure S1). To elucidate the crystal structure of the electron-beaminduced phases in more detail, scanning transmission electron microscopy (STEM) was conducted using high-angle annular dark field (HAADF) contrast (Figure 2). In this imaging mode, the intensity of atomic columns is approximately proportional to Z2,[19] where Z is the atomic number (i.e. bright contrast indicates La/Sr columns, fainter contrast denotes Mn), and columns containing oxygen hardly contribute to the image intensity. A modulation of the HAADF intensity is measured in every second MnOx plane of the superlattice structure (Figure 2b), which is characteristic for oxygen vacancy ordering.[8–10] Depletion of oxygen from alternating layers along the c-direction reduces the coordination of Mn cations, causing a vertical displacement of the La/Sr ions. Maps of the out-of-plane lattice spacing and graphs of the local distance between La/Sr rows corroborate this (Figure 2e and 2h). The average lattice spacing of the oxygendeficient and stoichiometric layers are c1 = 4.39 ± 0.01 Å and c2 = 3.79 ± 0.04 Å, respectively. Oxygen vacancy ordering and the concurrent modulation of the out-of-plane lattice parameter confirm the formation of a brownmillerite structure during electron-beam irradiation. In the ideal brownmillerite lattice (ABO2.5), oxygen depletion in every other BOx plane turns the BO6 octahedra into BO4 tetrahedra and the stacking sequence becomes AO-BO2-AO-BO.[5,8,14] Using the out-of-plane lattice spacing as a measure of oxygen content (Supporting Information, Figure S2), the composition of the oxygen-deficient layers in LSMO is estimated as MnO1.22±0.02, indicating predominant
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Figure 1. HRTEM images along the [100] zone axis depicting the evolution of the LSMO film structure during electron-beam irradiation. The current density of the focused electron beam was ca. 50 pA/cm2. The images were recorded at the beginning of the experiment (a) and after irradiation for 20 min (b) and 50 min (c). The insets show the corresponding FFT patterns. The green circle in (b) indicates one of the superlattice reflections.
tetrahedral coordination of Mn cations in these layers. The total composition of the brownmillerite phase thus corresponds to La2/3Sr1/3MnO2.61±0.01. The perovskite-like structure that forms after prolonged electron-beam irradiation is no longer modulated (Figure 2c and 2f). The average out-of-plane lattice parameter of this pseudo-cubic LSMO phase is 4.09 ± 0.01 Å, which is significantly enhanced compared to that of the original LSMO film. This lattice expansion can also be rationalized by electron-beam-induced oxygen vacancies. However, contrary to the brownmillerite phase, the vacancies are no longer confined to every second MnOx layer. An estimation of the oxygen content in the MnOx planes of the perovskite-like phase gives x = 1.70 ± 0.02 (Supporting Information, Figure S2). Assuming a completely random distribution of oxygen vacancies, this corresponds to a total composition of La2/3Sr1/3MnO2.55±0.04.
Thus, the second brownmillerite–perovskite phase transition is mainly driven by a disordering of existing oxygen vacancies, rather than a drastic change in oxygen off-stoichiometry during prolonged electron-beam irradiation. Similar ordering-disordering transitions have been observed for bulk brownmillerite samples at elevated temperatures (typically >700 °C),[12,14] in agreement with the high activation energy for oxygen diffusion.[20] In epitaxial thin films, however, both the formation of oxygen vacancies and oxygen diffusion can be considerably enhanced by tensile lattice strain.[21,22] To assess the role of electron-beam-induced heating in our LSMO films, we conducted a series of control experiments using a TEM heating stage. Heating of the initial LSMO perovskite structure did not result in the formation of a brownmillerite structure up to a temperature of at least 500 °C in the absence
Figure 2. a,b,c) STEM images along the [100] zone axis of the original perovskite (a), the brownmillerite (b) and the second perovskite-like LSMO structural phases (c). d,e,f) Maps of the out-of-plane lattice spacing between La/Sr sites. g,h,i) Out-of-plane lattice spacing between rows of La/Sr ions (averaged over 50 sites).
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of a focused electron beam. Thermal activation of the second brownmillerite–perovskite phase transition was achieved at a lower temperature of about 125 °C, which is comparable to the estimated temperature in the center of the irradiation area during electron-beam exposure.[23] Thus, while the brownmillerite structure is directly induced by the electron beam, the subsequent formation of a perovskite lattice with disordered oxygen vacancies is assisted by indirect sample heating inside the TEM. A similar perovskite-like structure with enhanced out-of-plane lattice parameter has been stabilized previously in LaMnO3 using epitaxial growth under low oxygen pressure.[24] Outward diffusion of cations (La and Sr) during electronbeam irradiation could also affect the structure of LSMO. Energy-dispersive X-ray spectroscopy (EDX) measurements of our samples, however, do not indicate a change in La and Sr stoichiometry during electron-beam exposure. Finally, we note that the in-plane lattice parameter of LSMO equals that of the STO substrate (a = 3.91 Å) for all structural phases. Schematic illustrations of the electron-beam-induced phases of LSMO are shown in Figure S3 in the Supporting Information. The time-evolution of the structural phase transitions in LSMO was monitored by the acquisition of high-resolution transmission electron microscopy (HRTEM) images during electron-beam irradiation. From such a series of images, the outof-plane lattice spacing was extracted as a function of irradiation time at different sample locations. A typical result for a current density of ca. 110 pA/cm2 is shown in Figure 3. In this experiment, the brownmillerite structure nucleates in the vicinity of position P1. At P1, first a rapid increase of the out-of-plane lattice spacing in every other layer is observed (c1), which is followed by a more gradual decrease of this parameter towards the perovskite-like structure. Although the brownmillerite structure is already obtained after an irradiation time of 2 min, the data evidences a steady increase of the lattice modulation (i.e. the parameters c1 and c2 change incessantly during the formation and ordering of oxygen vacancies). The second phase transition involving the disordering of oxygen vacancies also evolves via a continuous variation of the out-of-plane lattice. The difference between c1 and c2 gradually decreases with irradiation time until the perovskite-like structure is fully established after about 10 min. The evolution of the out-of-plane lattice spacing at P2 is similar but slightly delayed with respect to P1, demonstrating fast lateral growth of the brownmillerite phase. Oxygen vacancy formation during electron-beam irradiation is further confirmed by electron energy loss spectroscopy (EELS). The O–K edge fine structure in EELS spectra provides
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Figure 3. a,b,c) HRTEM image along the [100] zone axis of the LSMO film (a) and time-evolution of the out-of-plane lattice spacing at locations P1 (b) and P2 (c) during electron-beam irradiation with a current density of ca. 110 pA/cm2. The parameters c1 and c2 indicate the average lattice spacing of alternating layers in the LSMO structure. A full series of HRTEM images is shown in Figure S1 in the Supporting Information.
information on excitations from O 1s electrons to 2p bands. Three main features can be identified for LSMO:[25–27] a pre-peak at about 530 eV, the first main peak around 535 eV which reflects hybridization with La 5d and Sr 3d bands, and a second main peak at about 540 eV containing contributions from Mn 4sp bands. The pre-peak is strongly associated with the filling of the Mn 3d band and thus the Mn oxidation state (valence). The intensity of the pre-peak and the energy separation with the adjacent main peak have been found to decrease with a lowering of Mn valence.[26,27] Figure 4a compares EELS
Figure 4. a,b) STEM-EELS spectra of the O–K edge (a) and the Mn-L2 and Mn-L3 edges (b) for the three structural phases of LSMO. The dashed lines in (a) indicate the position of the O–K edge pre-peak.
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Figure 5. a,b,c) HRTEM images of (a) the original perovskite (a), the brownmillerite (b) and the second perovskite-like LSMO structural phases (c) obtained using NCSI (Cs = −30 µm) conditions. The images were recorded along the [110] zone axis. The insets show corresponding structural models for this projection (La/Sr (yellow), Mn (green), and O (red in (a) and (b), grey in (c)). d,e,f) Line profiles of two neighbouring MnOx layers (indicated by A and B in the HRTEM images).
spectra of the O–K edge for the initial LSMO film, the brownmillerite structure, and the over-irradiated perovskite-like phase of LSMO. The shift of the pre-peak to higher energy losses indicates that the Mn oxidation state is reduced via the creation of oxygen vacancies during the first perovskite–brownmillerite transition. The nearly identical EELS spectra of the two electron-beam-induced structures confirm that the second phase transition is driven by a disordering of oxygen vacancies without a significant change in oxygen stoichiometry. The same conclusion can be drawn from a comparison of the Mn L3/L2 peak ratio (Figure 4b), which is another measure of the Mn valence.[26,27] The increase of the L3/L2 ratio during the first structural phase transition evidences a lowering of the Mn oxidation state, yet the smaller enhancement of the L3/L2 ratio after the second phase transition indicates a slowdown in the formation of oxygen vacancies upon prolonged electron-beam irradiation. Additional information on the distribution of oxygen and oxygen vacancies in the three structural phases of LSMO was obtained by HRTEM under negative Cs imaging (NCSI) conditions.[28] Figure 5 shows HRTEM images for Cs = −30 µm along the [110] zone axis. The images were recorded at the same sample area after sequential electron-beam irradiation periods. Oxygen atoms are visible in all MnOx layers of the original perovskite LSMO structure (Figure 5a), as clearly demonstrated by the line profiles at A and B (Figure 5d), indicating octahedral oxygen coordination of Mn cations. The NCSI contrast from the oxygen columns is reduced in every second MnOx layer of the brownmillerite structure (Figure 5b and 5e). In the [110] projection, oxygen in the MnOx planes almost aligns with Mn if the layer consists of MnO4 tetrahedra (see structural model in 2792
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the inset of Figure 5b). The absence of oxygen contrast in every other MnOx layer thus manifests a depletion of oxygen and predominant tetrahedral coordination of Mn. The modulation of imaging contrast from oxygen columns disappears when the LSMO crystal transforms into the oxygen-deficient perovskitelike structure with enhanced out-of-plane lattice parameter (Figure 5c and 5f). In this case, the oxygen content is randomly distributed, which is facilitated by oxygen diffusion from MnO6 octrahedra to MnO4 tetrahedra during the second structural phase transition. In summary, we have demonstrated full control over the formation of ordered and disordered oxygen-deficient structural phases in La2/3Sr1/3MnO3 films using electron-beam irradiation in a transmission electron microscope. The ability to continuously change and monitor the lattice structure, oxygen content and degree of vacancy ordering opens up promising new routes for the exploration of transition metal oxides and other complex materials. In particular, we anticipate that our results will stimulate research on ionic conduction using in situ TEM techniques for simultaneous electrical transport and structural characterization during electron-beam exposure.
Experimental Section Film Growth: Epitaxial LSMO films were grown on single-crystalline STO (001) substrates using pulsed laser deposition (PLD). Prior to film growth, the substrates were first etched in buffered HF for 30 s and subsequently annealed in oxygen atmosphere at 950 °C for 1 h. This process resulted in fully TiO2-terminated surfaces as confirmed by atomic force microscopy (AFM) images showing regular terraces with straight and one-unit-cell-high step edges.[30] The LSMO films were
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Academy of Finland (Grant Nos. 260361 and 252301) and by the European Research Council (ERC2012-StG 307502). TEM analysis was conducted at the Aalto University Nanomicroscopy Center (Aalto-NMC). S.M. acknowledges financial support from the Jenny and Antti Wihuri Foundation and L.Ä. was supported by the Väisälä Foundation.
Received: November 15, 2013 Revised: December 17, 2013 Published online: February 19, 2014 [1] M. T. Anderson, J. T. Vaughey, K. R. Poeppelmeier, Chem. Mater. 1993, 5, 151. [2] J. P. Hodges, S. Short, J. D. Jorgensen, X. Xiong, B. Dabrowski, S. M. Mini, C. W. Kimball, J. Solid State Chem. 2000, 151, 190. [3] S. Stølen, E. Bakken, C. E. Mohn, Phys. Chem. Chem. Phys. 2006, 8, 429.
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[4] H. Yang, Z. H. Chi, F. Y. Li, C. Q. Jin, R. C. Yu, Phys. Rev. B 2006, 73, 024114. [5] T. G. Parsons, H. D’Hondt, J. Hadermann, M. A. Hayward, Chem. Mater. 2009, 21, 5527. [6] P. Boullay, V. Dorcet, O. Pérez, C. Grygiel, W. Prellier, B. Mercey, M. Hervieu, Phys. Rev. B 2009, 79, 184108. [7] Y. M. Kim, J. He, M. D. Biegalski, H. Ambaye, V. Lauter, H. M. Christen, S. T. Pantelides, S. J. Pennycook, S. V. Kalinin, A. Y. Borisevich, Nat. Mater. 2012, 11, 888. [8] A. Y. Borisevich, A. N. Morozovska, Young-Min Kim, D. Leonard, M. P. Oxley, M. D. Biegalski, E. A. Eliseev, S. V. Kalinin, Phys. Rev. Lett. 2012, 109, 065702. [9] H. Jeen, W. S. Choi, J. W. Freeland, H. Ohta, C. U. Jung, H. N. Lee, Adv. Mater. 2013, 25, 3651. [10] J. D. Ferguson, Y. Kim, L. Fitting Kourkoutis, A. Vodnick, A. R. Woll, D. A. Muller , J. D. Brock, Adv. Mater. 2011, 23, 1226. [11] K. R. Kendall, C. Navas, J. K. Thomas, H.-C. zur Loye, Solid State Ionics 1995, 82, 215. [12] L. Qiu, T. H. Lee, L.-M. Liu, Y. L. Yang, A. J. Jacobson, Solid State Ionics 1995, 76, 321. [13] K. Kakinuma, H. Yamamura, H. Haneda, T. Atake, Solid State Ionics 2001, 140, 301. [14] J. B. Goodenough, Annu. Rev. Mater. Res. 2003, 33, 91. [15] J. Sunarso, S. Baumann, J. M. Serra, W. A. Meulenberg, S. Liu, Y. S. Lin, J. C. Diniz da Costa, J. Membr. Sci. 2008, 320, 13. [16] Q. Li, L. P. Sun, X. Zeng, H. Zhao, L. H. Huo, J. C. Grenier, J. M. Bassat, F. Mauvy, J. Power Sources 2013, 238, 11. [17] T. Motohashi, Y. Hirano, Y. Masubuchi, K. Oshima, T. Setoyama, S. Kikkawa, Chem. Mater. 2013, 25, 372. [18] S. V. Kalinin, N. A. Spaldin, Science 2013, 341, 858. [19] M. Varela, A. R. Lupini, K. van Benthem, A. Y. Borisevich, M. F. Chisholm, N. Shibata, E. Abe, S. J. Pennycook, Annu. Rev. Mater. Res. 2005, 35, 539. [20] R. A. De Souza, J. A. Kilner, Solid State Ionics 1998, 106, 175. [21] U. Aschauer, R. Pfenninger, S. M. Selbach, T. Grande, N. A. Spaldin, Phys. Rev. B 2013, 88, 054111. [22] M. Kubicek, Z. Cai, W. Ma, B. Yildiz, H. Hutter, J. Fleig, ACS Nano 2013, 7, 3276. [23] L. Reimer, Transmission Electron Microscopy, Springer-Verlag, Berlin, Germany 1997. [24] Z. T. Xu, K. J. Jin, L. Gu, Y. L. Jin, C. Ge, C. Wang, H. Z. Guo, H. B. Lu, R. Q. Zhao, G. Z. Yang, Small 2012, 8, 1279. [25] H. Kurata, C. Colliex, Phys. Rev. B. 1993, 48, 2102. [26] M. Varela, M. P. Oxley, W. Luo, J. Tao, M. Watanabe, A. R. Lupini, S. T. Pantelides, S. J. Pennycook, Phys. Rev. B. 2009, 79, 085117. [27] Z. Li, M. Bosman, Z. Yang, P. Ren, L. Wang, L. Cao, X. Yu, C. Ke, M. B. H. Breese, A. Rusydi, W. Zhu, Z. Dong, Y. L. Foo, Adv. Funct. Mater. 2012, 22, 4312. [28] C. L. Jia, M. Lentzen, K. Urban, Science 2003, 299, 870. [29] A. M. Sánchez, L. Äkäslompolo, Q. H. Qin, S. van Dijken, Cryst. Growth Des. 2012, 12, 954. [30] Z. L. Wang, J. S. Yin, Y. D. Jiang, Micron 2000, 31, 571.
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grown at a substrate temperature of 700 °C, an oxygen partial pressure of 0.5 mbar, a pulse repetition rate of 4 Hz and a laser fluence of 2.5 J/cm2. After deposition, the films were cooled down in about 30 min under 0.5 mbar oxygen. High-Resolution TEM Characterization: High-resolution microstructural analysis and electron energy loss spectroscopy (EELS) were performed on a JEOL 2200FS TEM with double Cs correctors, operated at 200 keV. The cross-sectional TEM specimens were prepared by standard mechanical methods using a MultiPrep polishing machine (Allied High-Tech) and subsequent argon ion milling. For Z-contrast imaging, a high-angle annular dark field (HAADF) detector (inner diameter 0.3 mm and outer diameter 8 mm) was used. In this configuration, the camera length was set to 50 cm, so that the inner and outer angles were about 100 mrad and 170 mrad. For HRTEM imaging under NCSI conditions, the Cs value was set to −30 µm. The noise in all TEM images was reduced using a Wiener filter in the DigitalMicrograph software. The thickness of the TEM specimens was roughly estimated to be less than 15 nm by measuring the intensity ratio of the plasmon loss and the zero-loss peaks in EELS. For the analysis of EELS core-loss peaks, background subtraction was performed using a power-law fit. The Pearson method[29] was used for quantitative calculations of the white line intensity of the Mn valence state. Maps of the lattice spacing were calculated with a Matlab script applying 2D Gaussian fits to determine the peak coordinates. EDX measurements were carried out to analyze the composition of all LSMO phases.
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Lide Yao,* Sayani Majumdar, Laura Äkäslompolo, Sampo Inkinen, Qi Hang Qin, and Sebastiaan van Dijken* Transition metal oxides with a perovskite crystal lattice of type ABO3 may possess corresponding oxygen-deficient modulation structures.[1–4] One prototypical example is the brownmillerite crystal structure of type ABO2.5,[5–10] which due to its high ionic conductivity holds the potential for finding applications in solid oxide fuel cells, oxygen-separation membranes, gas sensors and other devices requiring anion diffusion.[11–17] Brownmillerites have been derived from perovskite materials using topotactic reduction,[5] optimized film growth,[6–9] and oxygen getters.[10] Here, we demonstrate that the evolution of the perovskite– brownmillerite phase transition can be fully controlled and monitored in epitaxial La2/3Sr1/3MnO3 (LSMO) films using electron-beam irradiation in a transmission electron microscope (TEM). Real-time TEM imaging with atomic resolution reveals that the structural transition is driven by an incessant ordering of electron-beam-induced oxygen vacancies. Over-irradiation of the brownmillerite phase induces a second transition to a perovskite-like structure with disordered oxygen vacancies and a significantly enhanced out-of-plane lattice compared to the original LSMO film. The demonstrated ability to simultaneously induce and characterize oxygen-deficient structural phases in a continuous and controllable manner opens up new pathways for atomic-scale studies on ionic transport dynamics.[18] The electron-beam-induced structural evolution of epitaxial LSMO thin films on SrTiO3 (001) (STO) single-crystal substrates was systematically investigated using a JEOL 2200FS TEM with double Cs correctors (see Experimental Section for more details). Figure 1 shows TEM images and corresponding local fast Fourier transform (FFT) patterns of a LSMO film during several stages of electron-beam irradiation. In the initial state (Figure 1a), the LSMO exhibits a perovskite structure with an (001)[100]LSMO// (001)[100]STO epitaxial relationship to the underlying STO substrate. Because of the good lattice match (tensile strain of 0.9%), misfit dislocations are absent and the LSMO unit cell is pseudocubic with an average out-of-plane lattice spacing of 3.87 ± 0.01 Å. Exposure to a focused electron beam with a current density of ca. 50 pA/cm2 results in the emergence of a superlattice Dr. L. D. Yao, Dr. S. Majumdar, L. Äkäslompolo, S. Inkinen, Dr. Q. H. Qin, Prof. S. van Dijken NanoSpin, Department of Applied Physics Aalto University School of Science P.O. Box, 15100, FI-00076, Aalto, Finland E-mail: [email protected]; [email protected]
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Electron-Beam-Induced Perovskite–Brownmillerite– Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films
structure. The new structural phase nucleates in the vicinity of the LSMO/STO interface and it steadily grows with increasing exposure time until the entire irradiation area is covered after about 20 minutes (Figure 1b). The periodicity of the electronbeam-induced structure is two unit cells along the c-direction as illustrated by additional reflections in the FFT pattern. Once formed, the superlattice structure is stable both in vacuum and under atmospheric conditions at room temperature. Prolonged electron-beam irradiation beyond the 20 minute period causes a second structural phase transition. The superlattice contrast gradually disappears and a perovskite-like crystal structure is retained after an irradiation time of 50 min (Figure 1c). Longer exposures to the focused electron beam do not further change the structure of the LSMO film. A similar sequence of events is also obtained for other electron-beam conditions with the transitions occurring more rapidly for higher beam intensities. For a current density of ca. 110 pA/cm2, for example, the final perovskite-like phase is obtained after an irradiation time of about 10 min (Supporting Information, Figure S1). To elucidate the crystal structure of the electron-beaminduced phases in more detail, scanning transmission electron microscopy (STEM) was conducted using high-angle annular dark field (HAADF) contrast (Figure 2). In this imaging mode, the intensity of atomic columns is approximately proportional to Z2,[19] where Z is the atomic number (i.e. bright contrast indicates La/Sr columns, fainter contrast denotes Mn), and columns containing oxygen hardly contribute to the image intensity. A modulation of the HAADF intensity is measured in every second MnOx plane of the superlattice structure (Figure 2b), which is characteristic for oxygen vacancy ordering.[8–10] Depletion of oxygen from alternating layers along the c-direction reduces the coordination of Mn cations, causing a vertical displacement of the La/Sr ions. Maps of the out-of-plane lattice spacing and graphs of the local distance between La/Sr rows corroborate this (Figure 2e and 2h). The average lattice spacing of the oxygendeficient and stoichiometric layers are c1 = 4.39 ± 0.01 Å and c2 = 3.79 ± 0.04 Å, respectively. Oxygen vacancy ordering and the concurrent modulation of the out-of-plane lattice parameter confirm the formation of a brownmillerite structure during electron-beam irradiation. In the ideal brownmillerite lattice (ABO2.5), oxygen depletion in every other BOx plane turns the BO6 octahedra into BO4 tetrahedra and the stacking sequence becomes AO-BO2-AO-BO.[5,8,14] Using the out-of-plane lattice spacing as a measure of oxygen content (Supporting Information, Figure S2), the composition of the oxygen-deficient layers in LSMO is estimated as MnO1.22±0.02, indicating predominant
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Figure 1. HRTEM images along the [100] zone axis depicting the evolution of the LSMO film structure during electron-beam irradiation. The current density of the focused electron beam was ca. 50 pA/cm2. The images were recorded at the beginning of the experiment (a) and after irradiation for 20 min (b) and 50 min (c). The insets show the corresponding FFT patterns. The green circle in (b) indicates one of the superlattice reflections.
tetrahedral coordination of Mn cations in these layers. The total composition of the brownmillerite phase thus corresponds to La2/3Sr1/3MnO2.61±0.01. The perovskite-like structure that forms after prolonged electron-beam irradiation is no longer modulated (Figure 2c and 2f). The average out-of-plane lattice parameter of this pseudo-cubic LSMO phase is 4.09 ± 0.01 Å, which is significantly enhanced compared to that of the original LSMO film. This lattice expansion can also be rationalized by electron-beam-induced oxygen vacancies. However, contrary to the brownmillerite phase, the vacancies are no longer confined to every second MnOx layer. An estimation of the oxygen content in the MnOx planes of the perovskite-like phase gives x = 1.70 ± 0.02 (Supporting Information, Figure S2). Assuming a completely random distribution of oxygen vacancies, this corresponds to a total composition of La2/3Sr1/3MnO2.55±0.04.
Thus, the second brownmillerite–perovskite phase transition is mainly driven by a disordering of existing oxygen vacancies, rather than a drastic change in oxygen off-stoichiometry during prolonged electron-beam irradiation. Similar ordering-disordering transitions have been observed for bulk brownmillerite samples at elevated temperatures (typically >700 °C),[12,14] in agreement with the high activation energy for oxygen diffusion.[20] In epitaxial thin films, however, both the formation of oxygen vacancies and oxygen diffusion can be considerably enhanced by tensile lattice strain.[21,22] To assess the role of electron-beam-induced heating in our LSMO films, we conducted a series of control experiments using a TEM heating stage. Heating of the initial LSMO perovskite structure did not result in the formation of a brownmillerite structure up to a temperature of at least 500 °C in the absence
Figure 2. a,b,c) STEM images along the [100] zone axis of the original perovskite (a), the brownmillerite (b) and the second perovskite-like LSMO structural phases (c). d,e,f) Maps of the out-of-plane lattice spacing between La/Sr sites. g,h,i) Out-of-plane lattice spacing between rows of La/Sr ions (averaged over 50 sites).
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of a focused electron beam. Thermal activation of the second brownmillerite–perovskite phase transition was achieved at a lower temperature of about 125 °C, which is comparable to the estimated temperature in the center of the irradiation area during electron-beam exposure.[23] Thus, while the brownmillerite structure is directly induced by the electron beam, the subsequent formation of a perovskite lattice with disordered oxygen vacancies is assisted by indirect sample heating inside the TEM. A similar perovskite-like structure with enhanced out-of-plane lattice parameter has been stabilized previously in LaMnO3 using epitaxial growth under low oxygen pressure.[24] Outward diffusion of cations (La and Sr) during electronbeam irradiation could also affect the structure of LSMO. Energy-dispersive X-ray spectroscopy (EDX) measurements of our samples, however, do not indicate a change in La and Sr stoichiometry during electron-beam exposure. Finally, we note that the in-plane lattice parameter of LSMO equals that of the STO substrate (a = 3.91 Å) for all structural phases. Schematic illustrations of the electron-beam-induced phases of LSMO are shown in Figure S3 in the Supporting Information. The time-evolution of the structural phase transitions in LSMO was monitored by the acquisition of high-resolution transmission electron microscopy (HRTEM) images during electron-beam irradiation. From such a series of images, the outof-plane lattice spacing was extracted as a function of irradiation time at different sample locations. A typical result for a current density of ca. 110 pA/cm2 is shown in Figure 3. In this experiment, the brownmillerite structure nucleates in the vicinity of position P1. At P1, first a rapid increase of the out-of-plane lattice spacing in every other layer is observed (c1), which is followed by a more gradual decrease of this parameter towards the perovskite-like structure. Although the brownmillerite structure is already obtained after an irradiation time of 2 min, the data evidences a steady increase of the lattice modulation (i.e. the parameters c1 and c2 change incessantly during the formation and ordering of oxygen vacancies). The second phase transition involving the disordering of oxygen vacancies also evolves via a continuous variation of the out-of-plane lattice. The difference between c1 and c2 gradually decreases with irradiation time until the perovskite-like structure is fully established after about 10 min. The evolution of the out-of-plane lattice spacing at P2 is similar but slightly delayed with respect to P1, demonstrating fast lateral growth of the brownmillerite phase. Oxygen vacancy formation during electron-beam irradiation is further confirmed by electron energy loss spectroscopy (EELS). The O–K edge fine structure in EELS spectra provides
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Figure 3. a,b,c) HRTEM image along the [100] zone axis of the LSMO film (a) and time-evolution of the out-of-plane lattice spacing at locations P1 (b) and P2 (c) during electron-beam irradiation with a current density of ca. 110 pA/cm2. The parameters c1 and c2 indicate the average lattice spacing of alternating layers in the LSMO structure. A full series of HRTEM images is shown in Figure S1 in the Supporting Information.
information on excitations from O 1s electrons to 2p bands. Three main features can be identified for LSMO:[25–27] a pre-peak at about 530 eV, the first main peak around 535 eV which reflects hybridization with La 5d and Sr 3d bands, and a second main peak at about 540 eV containing contributions from Mn 4sp bands. The pre-peak is strongly associated with the filling of the Mn 3d band and thus the Mn oxidation state (valence). The intensity of the pre-peak and the energy separation with the adjacent main peak have been found to decrease with a lowering of Mn valence.[26,27] Figure 4a compares EELS
Figure 4. a,b) STEM-EELS spectra of the O–K edge (a) and the Mn-L2 and Mn-L3 edges (b) for the three structural phases of LSMO. The dashed lines in (a) indicate the position of the O–K edge pre-peak.
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Figure 5. a,b,c) HRTEM images of (a) the original perovskite (a), the brownmillerite (b) and the second perovskite-like LSMO structural phases (c) obtained using NCSI (Cs = −30 µm) conditions. The images were recorded along the [110] zone axis. The insets show corresponding structural models for this projection (La/Sr (yellow), Mn (green), and O (red in (a) and (b), grey in (c)). d,e,f) Line profiles of two neighbouring MnOx layers (indicated by A and B in the HRTEM images).
spectra of the O–K edge for the initial LSMO film, the brownmillerite structure, and the over-irradiated perovskite-like phase of LSMO. The shift of the pre-peak to higher energy losses indicates that the Mn oxidation state is reduced via the creation of oxygen vacancies during the first perovskite–brownmillerite transition. The nearly identical EELS spectra of the two electron-beam-induced structures confirm that the second phase transition is driven by a disordering of oxygen vacancies without a significant change in oxygen stoichiometry. The same conclusion can be drawn from a comparison of the Mn L3/L2 peak ratio (Figure 4b), which is another measure of the Mn valence.[26,27] The increase of the L3/L2 ratio during the first structural phase transition evidences a lowering of the Mn oxidation state, yet the smaller enhancement of the L3/L2 ratio after the second phase transition indicates a slowdown in the formation of oxygen vacancies upon prolonged electron-beam irradiation. Additional information on the distribution of oxygen and oxygen vacancies in the three structural phases of LSMO was obtained by HRTEM under negative Cs imaging (NCSI) conditions.[28] Figure 5 shows HRTEM images for Cs = −30 µm along the [110] zone axis. The images were recorded at the same sample area after sequential electron-beam irradiation periods. Oxygen atoms are visible in all MnOx layers of the original perovskite LSMO structure (Figure 5a), as clearly demonstrated by the line profiles at A and B (Figure 5d), indicating octahedral oxygen coordination of Mn cations. The NCSI contrast from the oxygen columns is reduced in every second MnOx layer of the brownmillerite structure (Figure 5b and 5e). In the [110] projection, oxygen in the MnOx planes almost aligns with Mn if the layer consists of MnO4 tetrahedra (see structural model in 2792
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the inset of Figure 5b). The absence of oxygen contrast in every other MnOx layer thus manifests a depletion of oxygen and predominant tetrahedral coordination of Mn. The modulation of imaging contrast from oxygen columns disappears when the LSMO crystal transforms into the oxygen-deficient perovskitelike structure with enhanced out-of-plane lattice parameter (Figure 5c and 5f). In this case, the oxygen content is randomly distributed, which is facilitated by oxygen diffusion from MnO6 octrahedra to MnO4 tetrahedra during the second structural phase transition. In summary, we have demonstrated full control over the formation of ordered and disordered oxygen-deficient structural phases in La2/3Sr1/3MnO3 films using electron-beam irradiation in a transmission electron microscope. The ability to continuously change and monitor the lattice structure, oxygen content and degree of vacancy ordering opens up promising new routes for the exploration of transition metal oxides and other complex materials. In particular, we anticipate that our results will stimulate research on ionic conduction using in situ TEM techniques for simultaneous electrical transport and structural characterization during electron-beam exposure.
Experimental Section Film Growth: Epitaxial LSMO films were grown on single-crystalline STO (001) substrates using pulsed laser deposition (PLD). Prior to film growth, the substrates were first etched in buffered HF for 30 s and subsequently annealed in oxygen atmosphere at 950 °C for 1 h. This process resulted in fully TiO2-terminated surfaces as confirmed by atomic force microscopy (AFM) images showing regular terraces with straight and one-unit-cell-high step edges.[30] The LSMO films were
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the Academy of Finland (Grant Nos. 260361 and 252301) and by the European Research Council (ERC2012-StG 307502). TEM analysis was conducted at the Aalto University Nanomicroscopy Center (Aalto-NMC). S.M. acknowledges financial support from the Jenny and Antti Wihuri Foundation and L.Ä. was supported by the Väisälä Foundation.
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grown at a substrate temperature of 700 °C, an oxygen partial pressure of 0.5 mbar, a pulse repetition rate of 4 Hz and a laser fluence of 2.5 J/cm2. After deposition, the films were cooled down in about 30 min under 0.5 mbar oxygen. High-Resolution TEM Characterization: High-resolution microstructural analysis and electron energy loss spectroscopy (EELS) were performed on a JEOL 2200FS TEM with double Cs correctors, operated at 200 keV. The cross-sectional TEM specimens were prepared by standard mechanical methods using a MultiPrep polishing machine (Allied High-Tech) and subsequent argon ion milling. For Z-contrast imaging, a high-angle annular dark field (HAADF) detector (inner diameter 0.3 mm and outer diameter 8 mm) was used. In this configuration, the camera length was set to 50 cm, so that the inner and outer angles were about 100 mrad and 170 mrad. For HRTEM imaging under NCSI conditions, the Cs value was set to −30 µm. The noise in all TEM images was reduced using a Wiener filter in the DigitalMicrograph software. The thickness of the TEM specimens was roughly estimated to be less than 15 nm by measuring the intensity ratio of the plasmon loss and the zero-loss peaks in EELS. For the analysis of EELS core-loss peaks, background subtraction was performed using a power-law fit. The Pearson method[29] was used for quantitative calculations of the white line intensity of the Mn valence state. Maps of the lattice spacing were calculated with a Matlab script applying 2D Gaussian fits to determine the peak coordinates. EDX measurements were carried out to analyze the composition of all LSMO phases.
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