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Ordering of oxygen vacancies in LaBaCo2O6-δ epitaxial films
Scripta Materialia 181 (2020) 1–5
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Ordering of oxygen vacancies in LaBaCo2 O6-δ epitaxial films Jamal Shaibo a, Qin Yu Zhang b, Rui Yang a,∗, Xin Guo a,∗ a
State Key Laboratory of Material Processing and Die & Mould Technology, Laboratory of Solid State Ionics, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, School of Physics and Opto-electronic Technology, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
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Article history: Received 12 November 2019 Revised 29 January 2020 Accepted 2 February 2020
Keywords: Chemical ordering Oxygen vacancy Epitaxial film Perovskite cobaltite
a b s t r a c t Structure change resulting from oxygen vacancies can strongly influence the chemical and physical properties of transition metal oxides, leading to new functionalities for novel electronic devices. In this work, by annealing LaBaCo2 O6-δ epitaxial films in different atmospheres, e.g., O2 , N2 and H2 , we deduce that the concentration of oxygen vacancies and the stress in the film lattice imposed by the substrate play an important role in the ordering of oxygen vacancies in the LaBaCo2 O6-δ epitaxial films. Moreover, the structure changes resulting from the vacancy ordering have a direct influence on the electrical and optical properties of the epitaxial films.
Among strongly correlated electron oxides, perovskite cobaltites derived from LaCoO3 have been the object of numerous studies due to their fascinating magnetic and electrical properties. Perovskite cobaltites are also considered as the leading materials for replacing standard metal electrodes in solid oxide fuel cells (SOFCs) [1–8]. The stoichiometric perovskites with the generic formulation LnMCo2 O6 , where Ln = lanthanoid and M = Ba or Sr, exhibit order-disorder arrangements between Ln and M cations, thus significantly influencing the magnetic and electric properties. For example, Rautama et al. [9] showed that the ordered LaBaCo2 O6 (LBCO, space group P4/mmm) is semiconducting, however, the other two phases with the space group P m3m, corresponding to La and Ba disordering and ordering on nanometer scale, respectively, are semi-metallic conducting. Besides the cationic ordering, chemical ordering of oxygen vacancies in perovskite oxides is also an interesting topic, since the channels of oxygen vacancies were suggested to enhance the mobility of oxygen ions, which is helpful for reducing the working temperature of SOFCs, and thereby saves costs and prolongs the lifetime of SOFCs. In La0.5 Sr0.5 CoO3-δ (LSCO) materials, in which no cationic ordering was reported in bulk materials, electron microscopy studies on thin films demonstrated the occurrence of the ordering of oxygen vacancies [10,11]. In the epitaxial LSCO films on (001) SrTiO3 (STO), Donner et al. [12] observed an ordered phase at 650 K under reducing conditions using synchrotron X-ray diffraction (XRD). It is of interest that the new
∗
Corresponding authors. E-mail addresses: [email protected] (R. Yang), [email protected] (X. Guo).
https://doi.org/10.1016/j.scriptamat.2020.02.005 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
chemical ordering disappeared when venting the furnace to 1 atm air. By detailed comparison with the calculations of structure factors and the results calculated by first-principle methods, the chemical ordering was ascribed to the interplay between the epitaxial strain imposed by the substrate, change in the oxygen vacancy content and cationic mobility, and the oxygen vacancy ordering [12–16]. However, the origin of the chemical ordering is still elusive, and the correlation between the chemical ordering and the physical properties is still lack. Recently, thin films of LBCO in epitaxial form were fabricated [17–26], and suggested to have the ordered structure [17,18]. These LBCO epitaxial films exhibit potential applications in spintronics and magnetic memories, because the physical properties could be controlled by external means at room temperature [27,28]. Further, highly sensitive gas sensors operating in reducing (4% H2 /96% N2 ) or oxidizing (O2 ) environments were demonstrated [24–26], but the evidence supporting the fast channels of ordered oxygen vacancies is still lack in experiment. In this work, we report the chemical ordering of oxygen vacancies in epitaxial LBCO films. The structure of the films annealed in different atmospheres, namely O2 , N2 and H2 , is characterized and compared with XRD patterns calculated using structure factors, and evidences are presented to support the formation of the oxygen vacancy ordering at a relatively low temperature in H2 . The opportunity to manipulate the concentration of oxygen vacancies by post annealing allows us to better regulate the electrical and optical properties of perovskite oxide films. LBCO films were deposited on MgO (001) substrates by pulsed laser deposition (PLD) with a ceramic LBCO target under the op-
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J. Shaibo, Q.Y. Zhang and R. Yang et al. / Scripta Materialia 181 (2020) 1–5
Fig. 1. (a) XRD patterns and (b) RSMs at (002) diffraction of the LBCO samples undergoing different annealing processes. The colors in the RSMs represent the XRD intensity varying in logarithmic scale, and the maximum scales for LBCO (002) and MgO (002) are 104 and 107 counts, respectively. The horizontal dotted lines indicate the deviation of the annealing in O2 , N2 and H2 with respect to as-grown, and the vertical dotted lines indicate the deviation of LBCO (002) with respect to MgO (002). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
timized conditions of 800 °C and 10−4 Pa. Before cooling down to room temperature, the films were in-situ annealed for 15 min at the same deposition pressure. To exclude any variation in the oxygen content, all as-grown thin films were firstly annealed at 900 °C for 3 h in oxygen prior to other annealing treatments. Subsequently, the samples were annealed at 350 °C for 30 min separately in reducing (H2 ) or inert (N2 ) atmosphere to create oxygen vacancies, and then slowly cooled down to room temperature in the same atmosphere for annealing. The structure of the films was characterized using transmission electron microscope (TEM, JEM-ARM200F), high-resolution XRD (Panalytical X’Per MRD), and scanning electron microscope (SEM, Nova NanoSEM 450). The electrical measurements were carried out using a four-probe system via Agilent B2900A, and the optical properties were characterized by the ultraviolet-to-near-infrared (UV-to-NIR) spectroscopy. TEM observation reveals that the films have a single-crystal feature, as shown in the supporting materials (Fig. S1). Fig. 1a shows the XRD patterns of the as-grown film in comparison with the films annealed in various atmospheres. As shown in Fig. 1a, all the XRD peaks can be indexed by (00l), indicating that the c-axes of the films orient normally to their surfaces. With respect to the as-grown sample, annealing process produces a shift in the positions of the XRD peaks with enhanced intensity, as shown in the inset of Fig. 1a. The peak shift is further demonstrated by reciprocal space maps (RSMs) shown in Fig. 1b, which is akin to the change in the lattice parameter of the LBCO film. The RSMs agree well with literature reports that annealing leads to change in lat-
tice parameter [5,14–15, 29–32]. Based on the XRD patterns and the RSMs recorded at the (002) reflections of the LBCO films and the MgO substrates, we safely conclude that the oxygen content in the film is the highest after annealing in O2 at 900 °C for 3 h, and then is reduced to lower levels when the films undergo annealing in atmospheres with low oxygen partial pressures. The LBCO sample, which was annealed in H2 , possesses the lowest oxygen content, thus the oxygen vacancy concentration is the highest. Because of very little difference between the atomic scattering factors of La3+ and Ba2+ ions, the cationic ordering in LBCO cannot be directly observed in the XRD and electron diffraction patterns. Though the chemical ordering of La and Ba ions can be deduced from the crystallographic symmetry using the powder XRD data, it is still hard to directly distinguish the ordered phase from the disordered one for LBCO films, because thin films are usually under strains, thus the tiny difference between the lattice parameters of a and c are blurred out. The XRD pattern of the as-grown LBCO film is usually indexed using the structure of the cubic perovskite. When undergoing annealing in O2 or N2 , the films maintain their crystal structure, but exhibit variation in the lattice parameters. After the film is annealed in H2 for 30 min, however, a new structural ordering appears in the XRD pattern collected at room temperature, which can be indexed by the structure of the 112-type perovskite with the symmetry of P4/mmm, and then H(00l) is used to index the XRD pattern. Apparently, the new chemical ordering cannot be due to the ordered arrangement of La and Ba ions. Considering that the LBCO sample contains a large amount of oxygen vacancies, we assign the new structural order to the chemical
J. Shaibo, Q.Y. Zhang and R. Yang et al. / Scripta Materialia 181 (2020) 1–5
Fig. 2. (a) Models of LaBaCo2 O6 , LaBaCo2 O5.5 and LaBaCo2 O5 for calculating XRD patterns. (b) Calculated XRD patterns of LaBaCo2 O6 (C-0), LaBaCo2 O5 without VO ordering (C-1), LaBaCo2 O5.5 and LaBaCo2 O5 with VO ordering (C-2 and C-3), respectively.
ordering of oxygen vacancies, similar to what was reported by Rautama et al. for LaBaCo2 O5.5 [33]. Then the XRD pattern is indexed using the structure of the 112-type perovskite, because the primitive tetragonal symmetry with the space group P4/mmm is consistent with our observations. Since the LBCO films are epitaxial, the domains with the c-axis parallel to the surface can be detected in the XRD patterns with symmetric Bragg geometry, leaving the reflections of other possible domains absent. The lattice parameter of the tetragonal phase with the oxygen vacancy ordering is determined to be c = 0.8055 nm by fitting the five orders of the XRD peaks, about 1.3% larger than that of the domains without the vacancy ordering (c = 0.7954 nm) in the same LBCO film. To explore the chemical ordering of oxygen vacancies, we further calculated the XRD (00l) patterns (using the Gaussian function with a base line, for details of building model please refer to [12,34]) of the ordered LBCO phases (LaBaCo2 O6 and LaBaCo2 O6-δ , in which δ varying from 0.5 to 1). LaBaCo2 O5 is used to simulate the LBCO phase containing oxygen vacancies, or one oxygen vacancy in per formation. The other two LBCO phases with the group space P m3m are not included because they cannot produce difference in the XRD pattern measurable using the symmetric Bragg geometry. The structure models for calculations are depicted in Fig. 2a, in which Co ions are octahedrally coordinated by O ions, and oxygen vacancies are created in the La-O and Ba-O layers with occupation probability varying from 0 to 0.5 for modeling the LBCO phases with different concentrations of oxygen vacancies. Though there is no definite conclusion for the occupation of oxygen vacancies in LBCO materials, oxygen vacancies in the La-O or Ba-O layer cannot produce a visible difference in the XRD pattern. Similarly, oxygen vacancies in the Co-O layers cannot influence the final conclusion either, because the XRD patterns are sensitive only to the vacancy ordering. In addition, the Gaussian function with a base line is used to simulate the lattice parameters in the epitaxial films.
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Fig. 3. Asymmetric RSMs at the (204) diffraction of the LBCO films and the MgO substrates. The colors in the RSMs represent the XRD intensity varying in logarithmic scale, and the maximum scales for LBCO (204) and MgO (204) are 102 and 107 counts, respectively. The dotted squares indicate the deviation of S-1, S-2, and S3 with respect to S-0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 2b depicts the calculated XRD patterns of the LBCO phases with and without the vacancy ordering. There is no chemical ordering observable in the XRD patterns of the LBCO phases without the vacancy ordering, such as C-0 (LBCO phase without oxygen vacancies) and C-1 (LBCO with disordered oxygen vacancies). However, the chemical ordering becomes visible in the XRD patterns when the vacancy ordering appears in the LBCO phase. By comparing the XRD data, we see that the domains with the vacancy ordering is negligible in the as-grown film and the film annealed in O2 or N2 , while the H2 -annealing produces a large amount of ordered vacancies, making the chemical ordering visible in the XRD pattern. However, it is still difficult to obtain a more confident conclusion that the chemical ordering is due to partially or fully ordered oxygen vacancies. As the annealing process of the LBCO sample was conducted in H2 at 350 °C, therefore, the ordered oxygen vacancies in the LBCO film are formed at a relatively low temperature, leaving the Co ion in a mixed-valence state (Co2.5+ ) [25]. This is of significance for understanding the fast resistance response between reducing and oxidizing environments at a temperature as low as ~200 °C [25,26]. The study by Donner et al. [12] revealed that the oxygen deficiency leads to the lift of Co ions in the LSCO film slightly from the pyramid base (z = 0.23). Furthermore, they suggested the size effect of La3+ and Sr2+ , together with the Coulomb repulsion between Co ions, leads to a lattice expansion as large as 5.7% in the LSCO thin film. To determine strains in our LBCO films, we recorded the asymmetric RSMs at the (204) reflections of the LBCO films and the MgO substrates, as shown in Fig. 3. The epitaxial relationships of all the films are determined to be [100]LBCO //[100]MgO and (001)LBCO //(001)MgO , being the same as previous studies [17–26]. The determined lattice constants and expansion are presented in Supplementary materials Table SI. It is clear that the in-plane parameters of all films are very close to each other (a/a0 = 1.0 ± 0.2%) and an obvious change is observed in the parameter normal to the film surfaces (c/c0 = 1.1 ~ 4.4%), leading to a volume expansion varying from ~ 2.1 to 4.2%. Therefore, the change in the lattice parameter due to the creation of
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Fig. 4. (a) Temperature-dependent resistance, (b) optical transmittance of the LBCO thin films subjected to various annealing atmospheres. (c) Conductivity as a function of temperature for LBCO films, and (d) absorption curves (absorption coefficient squared) for as-grown LBCO film annealed under various atmospheres.
oxygen vacancies is essentially along the c-axis for the LBCO films on the (001) MgO substrates. As all the samples have the same cationic ordering, we deduce that the enhanced Coulomb repulsion between Co ions or between La and Ba ions due to the removal of oxygen is the major reason responsible for the c-axis expansion in the LBCO films [12], instead of the cationic size effect, though the difference in ionic radius is much larger between La3+ (0.116 nm) and Ba2+ (0.142 nm) than that between La3+ and Sr2+ (0.124 nm) [35]. In the work of Donner et al. [12], they reported an enormous lattice expansion at 650 K, together with the disappearance of the chemical ordering when venting the furnace to 1 atm air, but no description whether the chemical ordering was visible if the sample was cooled down to room temperature under reducing conditions. Comparing the XRD patterns of the sample annealed in N2 with the one annealed in H2 , we see that the vacancy ordering is thermodynamically stable at high temperatures in the atmospheres with low oxygen partial pressures, and the transition from the ordered to the disordered oxygen vacancies can take place due to the release of strains in the cooling process. Such a deduction is evidenced by the asymmetric RSMs at the (204) diffractions of the substrates and the cross-sectional SEM photographs. One can see that the (204) diffraction of the substrate splits into 2 or 3 spots when the sample is annealed in N2 or H2 , as shown in Fig. 3c and d, indicating that the MgO substrate has a layered structure with different lattice parameters. The layered MgO substrate is further confirmed by the cross-sectional SEM observation shown in the Fig. S2 of the Supplementary materials. The layered structure of the MgO substrate can be attributed to the release of stress caused by the creation of oxygen vacancies. Therefore, the stress imposed by the substrate may play a crucial role in maintaining the vacancy ordering visible in the sample at room temperature. As a conse-
quence, the parameters of the cooling process, including the cooling rate, the atmosphere, and the pressure are of significance in controlling the physical properties of the samples undergoing annealing in reducing conditions. The ordering of oxygen vacancies has also effect on the electrical and optical properties of the thin films. Fig. 4a shows the temperature-dependent electrical property of the films. The increased oxygen vacancy concentration (reduction of Co valence from 3+ to 2+ by annealing, details are given in Fig. S3) increases the resistance of the LBCO film by about one order of magnitude, as compared with that of the as-grown or the LBCO film annealed in O2 . This lower conductivity realized in the sample annealed in H2 (Fig. 4c) results from the wide-band gap (Ea ) characteristics of the film [36,37], which is verified by transmittance spectrum measurements shown in Fig. 4b. It is worth to note that, after annealing in H2 , the absorption shifts gradually to higher energies (Fig. 4d), which is also related to the oxygen vacancy concentration in the films [38,39]. As reported previously [40], the higher is the oxygen defect concentration, the larger is the shift towards high energies in the absorption spectrum. These data suggest that the largest increase in the concentration of oxygen defects is realized after annealing in H2 . In conclusion, the effect of the annealing atmosphere on the structure of the LBCO epitaxial films and its effect on the electrical and optical properties are investigated. By changing the annealing atmosphere, the concentration and distribution of oxygen vacancies, and stress in the film lattice imposed by the substrate are directly related to the chemical ordering of oxygen vacancies. Moreover, these highly defective structures exhibit higher resistivity and optical transmittance in the films. Therefore, by controlling the oxygen pressure in the annealing process, it is possible to tune the physical properties of thin films.
J. Shaibo, Q.Y. Zhang and R. Yang et al. / Scripta Materialia 181 (2020) 1–5
Declaration of Competing Interest The authors declare no conflict of interest. There are no interests to declare. Acknowledgments This work is supported by the National Key Research and Development Program of China (Grant No. 2018YFE0203802), the National Natural Science Foundation of China (Grant No. U1832116 and 51772112), the Fundamental Research Funds for the Central Universities (HUST: 2016YXZD058) and the HUAWEI Project (YBN2019055139). The Analytical and Testing Center of Huazhong University of Science and Technology is acknowledged for the TEM investigations. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.scriptamat.2020.02. 005. References [1] T. Kawada, K. Masuda, J. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, H. Yokokawa, Solid State Ion. 121 (1999) 271–279. [2] S. Ohara, R. Maric, X. Zhang, K. Mukai, T. Fukui, H. Yoshida, T. Inagaki, K. Miura, J. Power Sources 86 (20 0 0) 455–458. [3] T. Ishihara, S. Fukui, H. Nishiguchi, Y. Takita, Solid State Ion. 152 (2002) 609–613. [4] K. Zhang, L. Ge, R. Ran, Z.P. Shao, S.M. Liu, Acta Mater. 56 (2008) 4876–4889. [5] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Q.Y. Zhang, Mater. Chem. Phys. 131 (2012) 642–646. [6] C. Setevich, L. Mogni, A. Caneiro, F. Prado, J. Electrochem. Soc. 159 (2012) B73–B80. [7] S.L. Pang, X.N. Jiang, X.N. Li, H.X. Xu, L. Jiang, Q.L. Xu, Y.C. Shi, Q.Y. Zhang, J. Power Sources 240 (2013) 54–59. [8] D. Garces, C.F. Setevich, A. Caneiro, G.J. Cuello, L. Mogni, J. Appl. Crystallogr. 47 (2014) 325–334. [9] E.-L. Rautama, P. Boullay, A.K. Kundu, V. Caignaert, V. Pralong, M. Karppinen, B. Raveau, Chem. Mater. 20 (2008) 2742–2750. [10] J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagwa, J. Solid State Chem. 80 (1989) 102–111. [11] R.P. Haggerty, R. Seshadri, J. Phys. Condens Mat. 16 (2004) 6477–6484. [12] W. Donner, C.L. Chen, M. Liu, A.J. Jacobson, Y.L. Lee, M. Gadre, D. Morgan, Chem. Mater. 23 (2011) 984–988. [13] X.W. Wu, J. Walter, T.L. Feng, J. Zhu, H. Zheng, J.F. Mitchell, N. Biskup, M. Varela, X.L. Ruan, C. Leighton, X.J. Wang, Adv. Func. Mater. 27 (2017) 1704233.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Ordering of oxygen vacancies in LaBaCo2 O6-δ epitaxial films Jamal Shaibo a, Qin Yu Zhang b, Rui Yang a,∗, Xin Guo a,∗ a
State Key Laboratory of Material Processing and Die & Mould Technology, Laboratory of Solid State Ionics, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, School of Physics and Opto-electronic Technology, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 12 November 2019 Revised 29 January 2020 Accepted 2 February 2020
Keywords: Chemical ordering Oxygen vacancy Epitaxial film Perovskite cobaltite
a b s t r a c t Structure change resulting from oxygen vacancies can strongly influence the chemical and physical properties of transition metal oxides, leading to new functionalities for novel electronic devices. In this work, by annealing LaBaCo2 O6-δ epitaxial films in different atmospheres, e.g., O2 , N2 and H2 , we deduce that the concentration of oxygen vacancies and the stress in the film lattice imposed by the substrate play an important role in the ordering of oxygen vacancies in the LaBaCo2 O6-δ epitaxial films. Moreover, the structure changes resulting from the vacancy ordering have a direct influence on the electrical and optical properties of the epitaxial films.
Among strongly correlated electron oxides, perovskite cobaltites derived from LaCoO3 have been the object of numerous studies due to their fascinating magnetic and electrical properties. Perovskite cobaltites are also considered as the leading materials for replacing standard metal electrodes in solid oxide fuel cells (SOFCs) [1–8]. The stoichiometric perovskites with the generic formulation LnMCo2 O6 , where Ln = lanthanoid and M = Ba or Sr, exhibit order-disorder arrangements between Ln and M cations, thus significantly influencing the magnetic and electric properties. For example, Rautama et al. [9] showed that the ordered LaBaCo2 O6 (LBCO, space group P4/mmm) is semiconducting, however, the other two phases with the space group P m3m, corresponding to La and Ba disordering and ordering on nanometer scale, respectively, are semi-metallic conducting. Besides the cationic ordering, chemical ordering of oxygen vacancies in perovskite oxides is also an interesting topic, since the channels of oxygen vacancies were suggested to enhance the mobility of oxygen ions, which is helpful for reducing the working temperature of SOFCs, and thereby saves costs and prolongs the lifetime of SOFCs. In La0.5 Sr0.5 CoO3-δ (LSCO) materials, in which no cationic ordering was reported in bulk materials, electron microscopy studies on thin films demonstrated the occurrence of the ordering of oxygen vacancies [10,11]. In the epitaxial LSCO films on (001) SrTiO3 (STO), Donner et al. [12] observed an ordered phase at 650 K under reducing conditions using synchrotron X-ray diffraction (XRD). It is of interest that the new
∗
Corresponding authors. E-mail addresses: [email protected] (R. Yang), [email protected] (X. Guo).
https://doi.org/10.1016/j.scriptamat.2020.02.005 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
chemical ordering disappeared when venting the furnace to 1 atm air. By detailed comparison with the calculations of structure factors and the results calculated by first-principle methods, the chemical ordering was ascribed to the interplay between the epitaxial strain imposed by the substrate, change in the oxygen vacancy content and cationic mobility, and the oxygen vacancy ordering [12–16]. However, the origin of the chemical ordering is still elusive, and the correlation between the chemical ordering and the physical properties is still lack. Recently, thin films of LBCO in epitaxial form were fabricated [17–26], and suggested to have the ordered structure [17,18]. These LBCO epitaxial films exhibit potential applications in spintronics and magnetic memories, because the physical properties could be controlled by external means at room temperature [27,28]. Further, highly sensitive gas sensors operating in reducing (4% H2 /96% N2 ) or oxidizing (O2 ) environments were demonstrated [24–26], but the evidence supporting the fast channels of ordered oxygen vacancies is still lack in experiment. In this work, we report the chemical ordering of oxygen vacancies in epitaxial LBCO films. The structure of the films annealed in different atmospheres, namely O2 , N2 and H2 , is characterized and compared with XRD patterns calculated using structure factors, and evidences are presented to support the formation of the oxygen vacancy ordering at a relatively low temperature in H2 . The opportunity to manipulate the concentration of oxygen vacancies by post annealing allows us to better regulate the electrical and optical properties of perovskite oxide films. LBCO films were deposited on MgO (001) substrates by pulsed laser deposition (PLD) with a ceramic LBCO target under the op-
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Fig. 1. (a) XRD patterns and (b) RSMs at (002) diffraction of the LBCO samples undergoing different annealing processes. The colors in the RSMs represent the XRD intensity varying in logarithmic scale, and the maximum scales for LBCO (002) and MgO (002) are 104 and 107 counts, respectively. The horizontal dotted lines indicate the deviation of the annealing in O2 , N2 and H2 with respect to as-grown, and the vertical dotted lines indicate the deviation of LBCO (002) with respect to MgO (002). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
timized conditions of 800 °C and 10−4 Pa. Before cooling down to room temperature, the films were in-situ annealed for 15 min at the same deposition pressure. To exclude any variation in the oxygen content, all as-grown thin films were firstly annealed at 900 °C for 3 h in oxygen prior to other annealing treatments. Subsequently, the samples were annealed at 350 °C for 30 min separately in reducing (H2 ) or inert (N2 ) atmosphere to create oxygen vacancies, and then slowly cooled down to room temperature in the same atmosphere for annealing. The structure of the films was characterized using transmission electron microscope (TEM, JEM-ARM200F), high-resolution XRD (Panalytical X’Per MRD), and scanning electron microscope (SEM, Nova NanoSEM 450). The electrical measurements were carried out using a four-probe system via Agilent B2900A, and the optical properties were characterized by the ultraviolet-to-near-infrared (UV-to-NIR) spectroscopy. TEM observation reveals that the films have a single-crystal feature, as shown in the supporting materials (Fig. S1). Fig. 1a shows the XRD patterns of the as-grown film in comparison with the films annealed in various atmospheres. As shown in Fig. 1a, all the XRD peaks can be indexed by (00l), indicating that the c-axes of the films orient normally to their surfaces. With respect to the as-grown sample, annealing process produces a shift in the positions of the XRD peaks with enhanced intensity, as shown in the inset of Fig. 1a. The peak shift is further demonstrated by reciprocal space maps (RSMs) shown in Fig. 1b, which is akin to the change in the lattice parameter of the LBCO film. The RSMs agree well with literature reports that annealing leads to change in lat-
tice parameter [5,14–15, 29–32]. Based on the XRD patterns and the RSMs recorded at the (002) reflections of the LBCO films and the MgO substrates, we safely conclude that the oxygen content in the film is the highest after annealing in O2 at 900 °C for 3 h, and then is reduced to lower levels when the films undergo annealing in atmospheres with low oxygen partial pressures. The LBCO sample, which was annealed in H2 , possesses the lowest oxygen content, thus the oxygen vacancy concentration is the highest. Because of very little difference between the atomic scattering factors of La3+ and Ba2+ ions, the cationic ordering in LBCO cannot be directly observed in the XRD and electron diffraction patterns. Though the chemical ordering of La and Ba ions can be deduced from the crystallographic symmetry using the powder XRD data, it is still hard to directly distinguish the ordered phase from the disordered one for LBCO films, because thin films are usually under strains, thus the tiny difference between the lattice parameters of a and c are blurred out. The XRD pattern of the as-grown LBCO film is usually indexed using the structure of the cubic perovskite. When undergoing annealing in O2 or N2 , the films maintain their crystal structure, but exhibit variation in the lattice parameters. After the film is annealed in H2 for 30 min, however, a new structural ordering appears in the XRD pattern collected at room temperature, which can be indexed by the structure of the 112-type perovskite with the symmetry of P4/mmm, and then H(00l) is used to index the XRD pattern. Apparently, the new chemical ordering cannot be due to the ordered arrangement of La and Ba ions. Considering that the LBCO sample contains a large amount of oxygen vacancies, we assign the new structural order to the chemical
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Fig. 2. (a) Models of LaBaCo2 O6 , LaBaCo2 O5.5 and LaBaCo2 O5 for calculating XRD patterns. (b) Calculated XRD patterns of LaBaCo2 O6 (C-0), LaBaCo2 O5 without VO ordering (C-1), LaBaCo2 O5.5 and LaBaCo2 O5 with VO ordering (C-2 and C-3), respectively.
ordering of oxygen vacancies, similar to what was reported by Rautama et al. for LaBaCo2 O5.5 [33]. Then the XRD pattern is indexed using the structure of the 112-type perovskite, because the primitive tetragonal symmetry with the space group P4/mmm is consistent with our observations. Since the LBCO films are epitaxial, the domains with the c-axis parallel to the surface can be detected in the XRD patterns with symmetric Bragg geometry, leaving the reflections of other possible domains absent. The lattice parameter of the tetragonal phase with the oxygen vacancy ordering is determined to be c = 0.8055 nm by fitting the five orders of the XRD peaks, about 1.3% larger than that of the domains without the vacancy ordering (c = 0.7954 nm) in the same LBCO film. To explore the chemical ordering of oxygen vacancies, we further calculated the XRD (00l) patterns (using the Gaussian function with a base line, for details of building model please refer to [12,34]) of the ordered LBCO phases (LaBaCo2 O6 and LaBaCo2 O6-δ , in which δ varying from 0.5 to 1). LaBaCo2 O5 is used to simulate the LBCO phase containing oxygen vacancies, or one oxygen vacancy in per formation. The other two LBCO phases with the group space P m3m are not included because they cannot produce difference in the XRD pattern measurable using the symmetric Bragg geometry. The structure models for calculations are depicted in Fig. 2a, in which Co ions are octahedrally coordinated by O ions, and oxygen vacancies are created in the La-O and Ba-O layers with occupation probability varying from 0 to 0.5 for modeling the LBCO phases with different concentrations of oxygen vacancies. Though there is no definite conclusion for the occupation of oxygen vacancies in LBCO materials, oxygen vacancies in the La-O or Ba-O layer cannot produce a visible difference in the XRD pattern. Similarly, oxygen vacancies in the Co-O layers cannot influence the final conclusion either, because the XRD patterns are sensitive only to the vacancy ordering. In addition, the Gaussian function with a base line is used to simulate the lattice parameters in the epitaxial films.
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Fig. 3. Asymmetric RSMs at the (204) diffraction of the LBCO films and the MgO substrates. The colors in the RSMs represent the XRD intensity varying in logarithmic scale, and the maximum scales for LBCO (204) and MgO (204) are 102 and 107 counts, respectively. The dotted squares indicate the deviation of S-1, S-2, and S3 with respect to S-0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 2b depicts the calculated XRD patterns of the LBCO phases with and without the vacancy ordering. There is no chemical ordering observable in the XRD patterns of the LBCO phases without the vacancy ordering, such as C-0 (LBCO phase without oxygen vacancies) and C-1 (LBCO with disordered oxygen vacancies). However, the chemical ordering becomes visible in the XRD patterns when the vacancy ordering appears in the LBCO phase. By comparing the XRD data, we see that the domains with the vacancy ordering is negligible in the as-grown film and the film annealed in O2 or N2 , while the H2 -annealing produces a large amount of ordered vacancies, making the chemical ordering visible in the XRD pattern. However, it is still difficult to obtain a more confident conclusion that the chemical ordering is due to partially or fully ordered oxygen vacancies. As the annealing process of the LBCO sample was conducted in H2 at 350 °C, therefore, the ordered oxygen vacancies in the LBCO film are formed at a relatively low temperature, leaving the Co ion in a mixed-valence state (Co2.5+ ) [25]. This is of significance for understanding the fast resistance response between reducing and oxidizing environments at a temperature as low as ~200 °C [25,26]. The study by Donner et al. [12] revealed that the oxygen deficiency leads to the lift of Co ions in the LSCO film slightly from the pyramid base (z = 0.23). Furthermore, they suggested the size effect of La3+ and Sr2+ , together with the Coulomb repulsion between Co ions, leads to a lattice expansion as large as 5.7% in the LSCO thin film. To determine strains in our LBCO films, we recorded the asymmetric RSMs at the (204) reflections of the LBCO films and the MgO substrates, as shown in Fig. 3. The epitaxial relationships of all the films are determined to be [100]LBCO //[100]MgO and (001)LBCO //(001)MgO , being the same as previous studies [17–26]. The determined lattice constants and expansion are presented in Supplementary materials Table SI. It is clear that the in-plane parameters of all films are very close to each other (a/a0 = 1.0 ± 0.2%) and an obvious change is observed in the parameter normal to the film surfaces (c/c0 = 1.1 ~ 4.4%), leading to a volume expansion varying from ~ 2.1 to 4.2%. Therefore, the change in the lattice parameter due to the creation of
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Fig. 4. (a) Temperature-dependent resistance, (b) optical transmittance of the LBCO thin films subjected to various annealing atmospheres. (c) Conductivity as a function of temperature for LBCO films, and (d) absorption curves (absorption coefficient squared) for as-grown LBCO film annealed under various atmospheres.
oxygen vacancies is essentially along the c-axis for the LBCO films on the (001) MgO substrates. As all the samples have the same cationic ordering, we deduce that the enhanced Coulomb repulsion between Co ions or between La and Ba ions due to the removal of oxygen is the major reason responsible for the c-axis expansion in the LBCO films [12], instead of the cationic size effect, though the difference in ionic radius is much larger between La3+ (0.116 nm) and Ba2+ (0.142 nm) than that between La3+ and Sr2+ (0.124 nm) [35]. In the work of Donner et al. [12], they reported an enormous lattice expansion at 650 K, together with the disappearance of the chemical ordering when venting the furnace to 1 atm air, but no description whether the chemical ordering was visible if the sample was cooled down to room temperature under reducing conditions. Comparing the XRD patterns of the sample annealed in N2 with the one annealed in H2 , we see that the vacancy ordering is thermodynamically stable at high temperatures in the atmospheres with low oxygen partial pressures, and the transition from the ordered to the disordered oxygen vacancies can take place due to the release of strains in the cooling process. Such a deduction is evidenced by the asymmetric RSMs at the (204) diffractions of the substrates and the cross-sectional SEM photographs. One can see that the (204) diffraction of the substrate splits into 2 or 3 spots when the sample is annealed in N2 or H2 , as shown in Fig. 3c and d, indicating that the MgO substrate has a layered structure with different lattice parameters. The layered MgO substrate is further confirmed by the cross-sectional SEM observation shown in the Fig. S2 of the Supplementary materials. The layered structure of the MgO substrate can be attributed to the release of stress caused by the creation of oxygen vacancies. Therefore, the stress imposed by the substrate may play a crucial role in maintaining the vacancy ordering visible in the sample at room temperature. As a conse-
quence, the parameters of the cooling process, including the cooling rate, the atmosphere, and the pressure are of significance in controlling the physical properties of the samples undergoing annealing in reducing conditions. The ordering of oxygen vacancies has also effect on the electrical and optical properties of the thin films. Fig. 4a shows the temperature-dependent electrical property of the films. The increased oxygen vacancy concentration (reduction of Co valence from 3+ to 2+ by annealing, details are given in Fig. S3) increases the resistance of the LBCO film by about one order of magnitude, as compared with that of the as-grown or the LBCO film annealed in O2 . This lower conductivity realized in the sample annealed in H2 (Fig. 4c) results from the wide-band gap (Ea ) characteristics of the film [36,37], which is verified by transmittance spectrum measurements shown in Fig. 4b. It is worth to note that, after annealing in H2 , the absorption shifts gradually to higher energies (Fig. 4d), which is also related to the oxygen vacancy concentration in the films [38,39]. As reported previously [40], the higher is the oxygen defect concentration, the larger is the shift towards high energies in the absorption spectrum. These data suggest that the largest increase in the concentration of oxygen defects is realized after annealing in H2 . In conclusion, the effect of the annealing atmosphere on the structure of the LBCO epitaxial films and its effect on the electrical and optical properties are investigated. By changing the annealing atmosphere, the concentration and distribution of oxygen vacancies, and stress in the film lattice imposed by the substrate are directly related to the chemical ordering of oxygen vacancies. Moreover, these highly defective structures exhibit higher resistivity and optical transmittance in the films. Therefore, by controlling the oxygen pressure in the annealing process, it is possible to tune the physical properties of thin films.
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Declaration of Competing Interest The authors declare no conflict of interest. There are no interests to declare. Acknowledgments This work is supported by the National Key Research and Development Program of China (Grant No. 2018YFE0203802), the National Natural Science Foundation of China (Grant No. U1832116 and 51772112), the Fundamental Research Funds for the Central Universities (HUST: 2016YXZD058) and the HUAWEI Project (YBN2019055139). The Analytical and Testing Center of Huazhong University of Science and Technology is acknowledged for the TEM investigations. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.scriptamat.2020.02. 005. References [1] T. Kawada, K. Masuda, J. Suzuki, A. Kaimai, K. Kawamura, Y. Nigara, J. Mizusaki, H. Yugami, H. Arashi, N. Sakai, H. Yokokawa, Solid State Ion. 121 (1999) 271–279. [2] S. Ohara, R. Maric, X. Zhang, K. Mukai, T. Fukui, H. Yoshida, T. Inagaki, K. Miura, J. Power Sources 86 (20 0 0) 455–458. [3] T. Ishihara, S. Fukui, H. Nishiguchi, Y. Takita, Solid State Ion. 152 (2002) 609–613. [4] K. Zhang, L. Ge, R. Ran, Z.P. Shao, S.M. Liu, Acta Mater. 56 (2008) 4876–4889. [5] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Q.Y. Zhang, Mater. Chem. Phys. 131 (2012) 642–646. [6] C. Setevich, L. Mogni, A. Caneiro, F. Prado, J. Electrochem. Soc. 159 (2012) B73–B80. [7] S.L. Pang, X.N. Jiang, X.N. Li, H.X. Xu, L. Jiang, Q.L. Xu, Y.C. Shi, Q.Y. Zhang, J. Power Sources 240 (2013) 54–59. [8] D. Garces, C.F. Setevich, A. Caneiro, G.J. Cuello, L. Mogni, J. Appl. Crystallogr. 47 (2014) 325–334. [9] E.-L. Rautama, P. Boullay, A.K. Kundu, V. Caignaert, V. Pralong, M. Karppinen, B. Raveau, Chem. Mater. 20 (2008) 2742–2750. [10] J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagwa, J. Solid State Chem. 80 (1989) 102–111. [11] R.P. Haggerty, R. Seshadri, J. Phys. Condens Mat. 16 (2004) 6477–6484. [12] W. Donner, C.L. Chen, M. Liu, A.J. Jacobson, Y.L. Lee, M. Gadre, D. Morgan, Chem. Mater. 23 (2011) 984–988. [13] X.W. Wu, J. Walter, T.L. Feng, J. Zhu, H. Zheng, J.F. Mitchell, N. Biskup, M. Varela, X.L. Ruan, C. Leighton, X.J. Wang, Adv. Func. Mater. 27 (2017) 1704233.
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