DOI: 10.1021/cg101133w
Topotactic Changes in Thin Films of Brownmillerite SrFeO2.5 Grown on SrTiO3 Substrates to Infinite-Layer Structure SrFeO2
2010, Vol. 10 4713–4715
Yuichi Shimakawa,* Satoru Inoue, Mitsutaka Haruta, Masanori Kawai, Kazuya Matsumoto, Aya Sakaiguchi, Noriya Ichikawa, Seiji Isoda, and Hiroki Kurata Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received August 27, 2010; Revised Manuscript Received September 26, 2010
ABSTRACT: A brownmillerite SrFeO2.5 thin film grown on a SrTiO3(001) substrate contained an a-axis-oriented single-crystalline domain near the substrate and multiple domains with three orthogonal orientations. When the film was reduced with CaH2, it changed topotactically to a homogeneous c-axis-oriented infinite-layer structure, SrFeO2. A brownmillerite SrFeO2.5 thin film grown on a SrTiO3(111) substrate and treated with CaH2 at 250 °C also changed to (111)-oriented SrFeO2 with an infinite-layer structure. During reduction at low temperatures, the framework of the perovskite-structure stays intact, some oxygen atoms are removed from it, and others are rearranged. SrFeO2 is an oxygen-deficient perovskite with an infinitelayer structure consisting of FeO2 planes with corner-sharing square-planar oxygen coordination of divalent Fe ions.1 The compound is obtained by reducing perovskite-like SrFeO2.875 or SrFeO2.5 at 250-280 °C with a metal hydride such as CaH2. This low-temperature reduction removed some oxygen ions from the perovskite-structure framework and rearranged others in it.2 We recently obtained a SrFeO2 single-crystalline thin film by topotactic reduction of a single-crystalline brownmillerite SrFeO2.5 film.3 The b-axis-oriented precursor SrFeO2.5 thin film grown on a KTaO3 substrate changed to the c-axis-oriented infinite-layerstructure single-crystalline SrFeO2 film. We define here that a pseudotetragonal structure of the brownmillerite SrFeO2.5 (orthorhombic; a = 5.67, b = 15.59, and c = 5.53 A˚) is ap (= cp) ∼ d202 and bp ∼ d040. Because ap (∼3.96 A˚) of SrFeO2.5 is close to the KTaO3 cubic lattice (3.99 A˚), the b-axis-oriented precursor film epitaxially grew on the substrate. During the low-temperature reduction with CaH2, the perovskite-structure framework stayed intact and the topotactic relation of the structures held. Like SrFeO2.5, the brownmillerite CaFeO2.5 (orthorhombic; a = 5.43, b = 14.76, and c = 5.60 A˚) is also reduced to infinite-layerstructure CaFeO2 by low-temperature reduction with CaH2.4,5 Because of the large anisotropy of ap (∼d202 = 3.90 A˚)/bp (d040 = 3.69 A˚), the orientation of an epitaxially grown CaFeO2.5 thin film can be controlled by choosing the substrates.6 Deposition on SrTiO3(001) (cubic lattice constant, a = 3.905 A˚) and (La0.3Sr0.7)(Al0.65Ta0.35)O3(001) (LSAT, ap = 3.868 A˚) substrates yielded b-axis-oriented CaFeO2.5 brownmillerite thin films, while deposition on LaAlO3(001) (LAO, ap = 3.793 A˚) and LaSrAlO4(001) (LSAO, ap = 3.756 A˚) yielded ap-axis-oriented films. Thus, the lattice mismatch to the substrate strongly affects the orientation of the brownmillerite CaFeO2.5. Importantly, c-axis-oriented single-crystalline CaFeO2 thin films with the infinite-layer structure were obtained irrespective of the orientation of the precursor CaFeO2.5 thin films.7 In this study we prepared precursor SrFeO2.5 thin films on SrTiO3 substrates. Because of the large compressive strain of the film due to the lattice mismatch to the substrate, the SrFeO2.5 brownmillerite precursor thin film grown on a SrTiO3(001) substrate had multiple domains with different orientations. We also made a SrFeO2.5 thin film on a SrTiO3(111) substrate. The results of low-temperature reduction of those films are presented. Brownmillerite SrFeO2.5 precursor thin films were prepared by pulsed laser deposition in a manner similar to one described
Figure 1. X-ray diffraction patterns of precursor SrFeO2.5 (top, blue) and reduced SrFeO2 (bottom, red) thin films on SrTiO3(001) substrates.
*To whom correspondence should be addressed. Telephone: þ81-774-383110. Fax: þ81-774-38-3118. E-mail:
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previously.3 A SrFeO2.5 ceramic target was first prepared in a solidstate reaction of SrCO3 and Fe2O3 raw materials. SrFeO2.5 precursor thin films about 90 nm thick were deposited on singlecrystal SrTiO3(001) and (111) substrates by pulsed laser deposition with a KrF excimer laser (λ = 248 nm). During the depositions, the oxygen partial pressure was 10-5 Torr, and the substrate temperature was kept at 700-800 °C. The X-ray diffraction (XRD) pattern of the brownmillerite SrFeO2.5 precursor thin film grown on a SrTiO3(001) substrate is shown in Figure 1. A broad diffraction peak with a shoulder structure is seen at 2θ ∼ 46°, which appears to originate from (2 0 0)p and (0 2 0)p fundamental diffractions of the simple perovskite structure. Weak superstructure reflections such as (0 3/2 0)p and (0 5/2 0)p are also seen, suggesting the presence of b-axis-oriented brownmillerite SrFeO2.5 on the substrate. Details of the thin-film structure were further studied by scanning transmission electron microscopy (STEM) with JEM-2200FS and JEM-9980TKP1 equipped with a Cs aberration corrector. Figure 2 shows a cross-sectional high-angle annular darkfield (HAADF) STEM image of the SrFeO2.5 film. We can clearly see that adjacent to the substrate there is a homogeneous region about 30 nm thick, and over that is a multidomain region. A magnified view of the structure near the interface of the film and the substrate is shown in Figure 3. In a Sr column intensity profile parallel to the film plane, one sees two distinct spacings (∼4.3 and ∼3.4 A˚) that correspond to the stacking of FeO6 octahedra and FeO4 tetrahedra. Thus, the bottom region near the substrate consists of a single-crystalline domain in which the b-axis of the brownmillerite lies in the film plane, and the orientation relation between the film and the substrate
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Figure 2. Cross-sectional HAADF-STEM image of a SrFeO2.5 thin film grown on SrTiO3(001).
Figure 3. (a) Cross-sectional HAADF-STEM image of a region near the interface between a SrFeO2.5 thin film and a SrTiO3 substrate. (b) Sr column intensity profile parallel to the film plane (along the light-blue line in part a). The arrangement of Sr and Fe atoms is shown over the intensity profile.
Figure 4. Cross-sectional HAADF-STEM image of multidomain structure in the upper region of a SrFeO2.5 thin film. Structural models of the three orthogonal domains are also shown.
is SrFeO2.5(100)p//SrTiO3(001) and SrFeO2.5(010)p//SrTiO3(100) (a-axis-oriented domain). This growth orientation relation is quite similar to that of CaFeO2.5 brownmillerite thin films grown on LAO and LSAO substrates.7 Although 4-fold orthorhombic domain structures (rhombohedral perovskite) with ∼1.4° tilting angles have been seen in a SrFeO2.5 film on SrTiO3(001),8 we did not apparently see such structures in the present experiment resolution. In the upper multidomain region, on the other hand, we saw the brownmillerite SrFeO2.5 domains with different orientations. A HAADF-STEM image of the region (Figure 4), for example, clearly shows three domains with orthogonally oriented b axes. In addition to the a-b lattice plane images (orientation relationships
Shimakawa et al.
Figure 5. (a) Cross-sectional HAADF-STEM image of a SrFeO2 thin film obtained by low-temperature reduction. Magnified lattice images of (b) a tetragonal infinite-layer structure SrFeO2 thin film and (c) a cubic SrTiO3 substrate. (d) Selected area electron diffraction pattern obtained from a SrFeO2 thin film on a SrTiO3 substrate. The (002) diffraction spots of SrFeO2 and SrTiO3 appear at different positions.
are (i) SrFeO2.5(100)p//SrTiO3(001) and SrFeO2.5(010)p//SrTiO3(100), and (iii) SrFeO2.5(010)p//SrTiO3(001) and SrFeO2.5(100)p// SrTiO3(100)), a nearly square lattice image (∼3.9∼3.9 A˚), which originates from the a-c lattice plane of the brownmillerite ((ii) b-axis of the brownmillerite perpendicular to the image, and the orientation relationship is SrFeO2.5(001)p//SrTiO3(001) and SrFeO2.5(100)p//SrTiO3(100)), is observed. One of the domains whose b-axis is perpendicular to the film surface should give (0k/20)p peaks in the θ-2θ XRD pattern. From the above results of the XRD measurement and the STEM observation, we can conclude that SrFeO2.5 multiple domains with (100)p, (010)p, and (001)p orientations grow after the single-crystalline a-axis-oriented domain grows to the thickness of about 30 nm on the SrTiO3(001) substrate. This multidomain growth seems to be due to the large lattice mismatch between SrFeO2.5 and SrTiO3. Where the SrFeO2.5 is more than 30 nm thick, the strain from the substrate lattice induces misfit dislocations that result in the formation of multiple domains. The multidomain-structure brownmillerite SrFeO2.5 thin film obtained was then treated at 250 °C for 35 h with CaH2 in a manner similar to one described previously.3,7 As shown in Figure 1, after the reduction the diffraction peaks originating from the precursor brownmillerite SrFeO2.5 were gone and there was a (002) XRD peak of the infinite-layer structure SrFeO2. Interestingly, the cross-sectional HAADF-STEM image of the reduced film shown in Figure 5a looks homogeneous and no multidomain structure is evident. The magnified atomic column lattice image (Figure 5b) clearly shows the tetragonal structure due to the absence of apical oxygen atoms. (The corresponding image of the cubic SrTiO3 structure is shown in Figure 5c for comparison.) It should be noted that in the selected area electron diffraction pattern obtained from the reduced thin-film sample (Figure 5d) the in-plane (200) diffractions of the SrFeO2 and the SrTiO3 coincide while the out-of-plane (002) diffractions appear at different positions. This implies that after the reduction the epitaxial relationship still holds and the in-plane lattice of the infinite-layer structure well matches the substrate lattice. All the results presented above clearly indicate that the reduction with CaH2 changes the multiple brownmillerite SrFeO2.5 domains with different orientations into a single domain of c-axis-oriented SrFeO2 with infinite-layer structure. The result is consistent with the previous observations in which both a-axis-oriented and b-axis-oriented single-crystalline CaFeO2.5 changed to c-axis-oriented CaFeO2 with infinite-layer structure.7 In the present SrFeO2.5 thin film grown on the SrTiO3 (001) substrate, the perovskite structure framework, i.e. the cation arrangement in the structure, is basically the same even in the domains with different orientations. Thus, the oxygen rearrangement during the low-temperature reduction makes the multiple domains merge into
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multiple domains with three orthogonal orientations on the a-axisoriented domain. Low-temperature reduction with CaH2 changed that film to the homogeneous c-axis-oriented infinite-layerstructure SrFeO2. A (111)-oriented SrFeO2.5 brownmillerite thin film was also grown on a SrTiO3(111) substrate, and the low-temperature reduction with CaH2 changed it to a (111)-oriented SrFeO2 film with infinite-layer structure. Therefore, the lowtemperature reduction topotactically changes brownmillerite SrFeO2.5 to infinite-layer SrFeO2 and leaves the perovskitestructure framework intact. Only some of the oxygen atoms are removed from the structure, and others are rearranged in it. It should also be noted that the rearrangement of oxygen atoms occurs at a temperature of 250 °C. Figure 6. X-ray diffraction patterns of precursor SrFeO2.5 (top, blue) and reduced SrFeO2 (bottom, red) thin films on SrTiO3(111) substrates.
a single domain. Because of the large anisotropy in the tetragonal structure of SrFeO2, the c-axis-oriented infinite-layer structure SrFeO2 is stabilized on the SrTiO3 substrate. To see what happens in the low-temperature reduction of a (111)-oriented SrFeO2.5 thin film, we prepared a brownmillerite SrFeO2.5 precursor thin film on a SrTiO3(111) substrate under the same condition described above. As shown in Figure 6, XRD peaks due to (111)p and (222)p reflections of the pseudocubic brownmillerite structure were observed. 6-fold orthorhombic (monoclinic) SrFeO2.5 domains8 seem to be present in the film even though such domains are not evident in the θ-2θ XRD data. After the reduction with CaH2 at 250 °C for 35 h, we clearly see the (111) and (222) XRD peaks of the infinite-layer-structure SrFeO2. The low-temperature reduction of the (111)-oriented brownmillerite SrFeO2.5 thus changed it to the (111)-oriented infinite-layer-structure SrFeO2. This result also implies that the perovskite-structure framework stays intact during the lowtemperature reduction and that only oxygen atoms are removed from the structure. The structural change from the brownmillerite SrFeO2.5 to the infinite-layer SrFeO2 is topotactic. In conclusion, we prepared a brownmillerite precursor SrFeO2.5 thin film on a SrTiO3(001) substrate. It consisted of a 30-nm-thick a-axis-oriented single-crystalline domain near the substrate and
Acknowledgment. This work was partly supported by Grantin-Aid for Scientific Research (19GS0207), by the Global COE Program “International Center for Integrated Research and Advanced Education in Materials Science”, and by the Project of Integrated Research on Chemical Synthesis from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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