DOI: 10.1021/cg100178y
Orientation Change of an Infinite-Layer Structure LaNiO2 Epitaxial Thin Film by Annealing with CaH2
2010, Vol. 10 2044–2046
Masanori Kawai,*,† Kazuya Matsumoto,† Noriya Ichikawa,† Masaichiro Mizumaki,‡ Osami Sakata,‡ Naomi Kawamura,‡ Shigeru Kimura,‡ and Yuichi Shimakawa*,† †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, and Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
‡
Received February 4, 2010; Revised Manuscript Received March 14, 2010
ABSTRACT: Low-temperature annealing with CaH2 reduced a perovskite LaNiO3 thin film to a c-axis oriented LaNiO2 thin film with an infinite-layer structure. X-ray diffraction and X-ray absorption spectroscopy measurements revealed that a further annealing caused oxygen rearrangement that changed the c-axis oriented LaNiO2 to an a-axis oriented LaNiO2 thin film consisting of a twin structure without changing the monovalent Ni oxidation state. The NiO2 infinite-layer planes in the a-axis oriented and c-axis oriented LaNiO2 thin films are perpendicular to each other. Properties of transition-metal oxides are determined mainly by the electron configurations, i.e., ionic states, of the transitionmetals coordinated by oxygen ions. The d9 electron configuration with square-planar coordination is of particular interest because the Cu2þ (d9) in a CuO4 square plane is a mother electronic state of high-TC superconductivity.1 Isoelectronic Niþ (d9) is another ion whose electron configuration would be expected to give materials some interesting physical properties.2-4 The unusual ionic state of the monovalent Ni with the squareplanar coordination has been achieved in LaNiO2 with an infinite-layer structure, which is an oxygen-deficient perovskite made by using flowing hydrogen5,6,13 or metal hydrides such as CaH2 and NaH7,8 to reduce LaNiO3. In the reduction process, oxygen atoms are released from apical sites of NiO6 octahedra in the LaNiO3 perovskite without breaking the structural framework and the Ni oxidation state is adjusted by the oxygen content.7-13 In a previous study we confirmed that a low-temperature reduction with CaH2 caused a topotactic structural change from the perovskite LaNiO3 to LaNiO2 with the infinite-layer structure.9 A single-crystalline thin film of c-axis-oriented tetragonal LaNiO3 (c-LaNiO3) that had been epitaxially grown on a SrTiO3 (001) substrate changed first to a c-axis-oriented LaNiO2.5 (c-LaNiO2.5), in which oxygen vacancies order along the [1 1 0] direction, producing alternate units of NiO6 octahedra and NiO4 square planes, and then to a c-axis-oriented infinite-layer structure LaNiO2 (c-LaNiO2) (Figure 1). In the present study we found that a further annealing of the c-LaNiO2 thin film with CaH2 rearranged the oxygen atoms of the c-LaNiO2 and produced an a-axis-oriented LaNiO2 (a-LaNiO2). We report here details of the structural changes from c-LaNiO2 to a-LaNiO2 that were inferred from X-ray diffraction (XRD) and X-ray absorption spectrum (XAS) measurements. An 80-nm-thick precursor epitaxial single-crystalline thin film of c-LaNiO3 was prepared on a SrTiO3 (001) substrate by pulsed laser deposition in a manner similar to one described previously.9 A θ-2θ XRD pattern of the precursor film measured with Cu KR radiation is shown in Figure 2a, where one can see (00l) reflections from the LaNiO3 that confirm the epitaxial growth of the c-LaNiO3 (a = 3.91 A˚ and c = 3.84 A˚). Each small piece cut from the sample was then treated with CaH2 at 280 °C in an evacuated glass tube. The c-LaNiO3 thin film changed to *To whom correspondence should be addressed. E-mail: kawai@ msk.kuicr.kyoto-u.ac.jp,
[email protected]. pubs.acs.org/crystal
Published on Web 04/05/2010
Figure 1. Schematic drawings of (a) a LaNiO3 epitaxial thin film grown on a SrTiO3 (001) substrate, and (b) a c-axis oriented LaNiO2.5 thin film, and (c) a c-axis oriented LaNiO2 thin film. Low-temperature reduction with CaH2 changes the film in part a first to the film in part b and then to the film in part c.
Figure 2. θ-2θ XRD profiles in logarithmic scales of (a) a precursor LaNiO3 thin film grown on a SrTiO3 (001) substrate and of thin films obtained by annealing with CaH2 for (b) 0.5 h, (c) 2 h, (d) 2.5 h, and (e) 3 h. Diffraction peaks in the figures were indexed with the simple perovskite structures.
c-LaNiO2.5 (c = 3.74 A˚) after reduction for 0.5 h (Figure 2b) and to c-LaNiO2 (c = 3.40 A˚) after reduction for 2 h (Figure 2c).9 r 2010 American Chemical Society
Communication
Crystal Growth & Design, Vol. 10, No. 5, 2010
2045
Figure 3. (a) High-resolution synchrotron XRD-RSM measured around the (102) Bragg reflection of the SrTiO3 (001) substrate for the sample whose profile is shown in Figure 2d. (b) Schematic drawing of the thin film, which contains c-axis oriented LaNiO2 and twins of a-axis oriented LaNiO2.
As clearly seen in Figure 2d, new diffraction peaks appear at 2θ ≈ 22.4° (d ≈ 3.97 A˚) and 45.8° (1.99 A˚), and their intensities increase with annealing time. After the annealing for 3 h, the diffraction peaks originating from the c-LaNiO2 phase completely disappear and the sample seems to change to a new phase (Figure 2e). Note that the peaks do not correspond to those of metallic Ni or La. The d values, 3.97 and 1.99 A˚, indicate that the new phase has an out-of-plane lattice of ∼3.97 A˚ and that the two diffraction peaks respectively correspond to (001) and (002). Interestingly, the lattice constant of ∼3.97 A˚ is quite close to that of the a-axis length of the infinite-layer structure LaNiO2. This suggests that the new phase is an a-axis-oriented LaNiO2 (a-LaNiO2) and that annealing with CaH2 changes the orientation of the infinite-layer LaNiO2 thin film from the c-axis orientation to the a-axis orientation. A high-resolution synchrotron XRD measurement of the sample whose profile is shown in Figure 2d was performed at beamline BL13XU in SPring-8 and confirmed the change from c-LaNiO2 to a-LaNiO2. Figure 3a shows an XRD reciprocal space map (RSM) measured on an hl plane around the (102) Bragg reflection of SrTiO3. There, one sees two weak diffraction peaks originating from the reduced thin film, one at (h, l) = (1.15, 1.98) and the other at (h, l) = (0.98, 1.98), in addition to the Bragg reflection from the substrate at (h, l) = (1.00, 2.00). They respectively give in-plane and out-of-plane lattice spaces of (1/din-plane, 1/dout-of-plane) that are (1/3.39, 1/1.97) and (1/3.98, 1/1.97) in a reciprocal space (/A˚). When we consider the tetragonal lattice of LaNiO2 with a = 3.959 A˚ and c = 3.375 A˚,7 the two peaks respectively can be represented as (1/c, 2/a) and (1/a, 2/a). Thus, they are (210) and (201) reflections respectively originating from a twin structure of a-LaNiO2. Because the sample whose profile is shown in Figure 2d also contains c-LaNiO2, the film structure should consist of three domains of c-LaNiO2 and twins of a-LaNiO2 as shown in Figure 3b. The corresponding two reflection peaks, which indicate the twin structure, were also observed in an RSM of the fully annealed sample shown in Figure 2e (data not shown). Since the NiO2 infinite layers of a-LaNiO2 and c-LaNiO2 align perpendicular to each other, angle-dependent XAS near the Ni K edge shows interesting contrast. The experiments were carried out with a fluorescence mode at beamline BL39XU with a horizontally linear polarized beam in SPring-8 for the LaNiO3, c-LaNiO2, and a-LaNiO2 samples, whose profiles are respectively shown in parts a, c, and e of Figure 2. As shown in Figure 4, the Ni K absorption edge energies for both the a-LaNiO2 and the c-LaNiO2 show ∼2.0 eV chemical shifts from the Ni K absorption edge energy for the LaNiO3 with Ni3þ, confirming the Niþ ionic state in LaNiO2. Note that when pre-edge structures in an energy range from 8330 to 8342 eV in the spectrum of the c-LaNiO2 are
Figure 4. X-ray absorption spectra near the Ni K edge of LaNiO3 and LaNiO2 thin films. The out-of-plane and in-plane data were respectively collected with incident beams whose polarization vectors were out of the plane of the film surface and in the plane of the film surface.
measured with an incident beam whose polarization vector is along the out-of-plane direction of the film sample (c-LaNiO2 out-of-plane), they are enhanced compared to those measured with a beam whose polarization vector is along the in-plane direction (c-LaNiO2 in-plane). This is due to a large anisotropy of the square-planar coordination of Ni in the infinite-layer crystal structure of LaNiO2. As expected from the orientation of the sample configuration relative to the direction of the polarization vector of the incident beam, the a-LaNiO2 in-plane XAS is enhanced compared to the a-LaNiO2 out-of-plane XAS because the twin-structure sample contains the domains whose c-axis is parallel to the in-plane polarization vector of the beam. These results indicate that the NiO2 infinite-layer planes in the two films are perpendicular to each other, confirming the a-axis orientation of the LaNiO2 in one film and the c-axis orientation of the LaNiO2 in the other film. The results also confirm that the Ni oxidation states in the two films are identical and that the oxidation state of Ni ion is kept monovalent even after the 3-h annealing. During the first half hour of the annealing with CaH2, Ni is reduced and LaNiO3 changes to c-LaNiO2. Over the next 1.5 h of the annealing, however, rearrangement of oxygen changes c-LaNiO2 to a-LaNiO2 without changing the Ni oxidation state. A similar orientation change was reported from the changes of the XRD peak positions in reduction of a LaNiO3 thin film with H2 gas.13 However, such a change was never observed for the infinite-layer structure thin film containing Fe prepared by
2046
Crystal Growth & Design, Vol. 10, No. 5, 2010
annealing with CaH2.14,15 There is no convincing explanation so far for this structural change during the annealing with CaH2. Since the lattice mismatch for an a-axis length of LaNiO2 on cubic SrTiO3 (001) (-13.6%) is much larger than that for a c-axis length of LaNiO2 on SrTiO3 (001) (1.38%), the lattice mismatch to the substrate lattice cannot be the main parameter controlling the orientation of the film. Because the a-LaNiO2 film still keeps the epitaxial relationship to the substrate lattice, epitaxial strain from the substrate lattice might play an important role in stabilizing the orientation of the film. This possibility needs further studies. In conclusion, we have found that low-temperature annealing with CaH2 first reduces a single-crystalline epitaxially grown LaNiO3 thin film to a c-axis oriented LaNiO2 film and then changes it to an a-axis oriented LaNiO2 film consisting of the twin structure. Rearrangement of oxygen changes the orientation of the infinite-layer structure LaNiO2 thin film without changing the Ni oxidation state. Acknowledgment. The synchrotron radiation experiments (Proposal Nos. 2008B1222 and 2008B1314) were performed with the approval of the Japan Synchrotron Radiation Research Institute. This work was partly supported by the Grants-in-Aid for Scientific Research (Grant Nos. 19GS0207, 18350097, and 18655076), by the JSPS Research Fellow program, by the Global COE Program “International Center for Integrated Research and Advanced Education in Materials Science”, and by the Joint Project of Chemical Synthesis Core Research Institutions from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
Kawai et al.
References (1) Siegrist, T.; Zahurak, S. M.; Murphy, D. W.; Roth, R. S. Nature (London) 1998, 334, 231. (2) Lee, K. W.; Pickett, W. E. Phys. Rev. B 2004, 70, 165109. (3) Choisnet, J.; Evarestov, R. A.; Tupitsyn, I. I.; Veryazov, V. A. J. Phys. Chem. Solids 1996, 57, 1839. (4) Anisimov, V. I.; Bukhvalov, D.; Rice, T. M. Phys. Rev. B 1999, 59, 7901. (5) Crespin, M.; Levitz, P.; Gatineau, L. J. Chem. Soc., Faraday Trans. 1983, 79, 1181. (6) Crespin, M.; Levitz, P.; Gatineau, L. J. Chem. Soc., Faraday Trans. 1983, 79, 1195. (7) Hayward, M. A.; Green, M. A.; Rosseinsky, M. J.; Sloan, J. J. Am. Chem. Soc. 1999, 121, 8843. (8) Crespin, M.; Isnard, O.; Dubois, F.; Choisnet, J.; Odier, P. J. Solid State Chem. 2005, 178, 1326. (9) Kawai, M.; Inoue, S.; Mizumaki, M.; Kawamura, N.; Ichikawa, N.; Shimakawa, Y. Appl. Phys. Lett. 2009, 94, 082102. (10) Sanchez, R. D.; Causa, M. T.; Caneiro, A.; Butera, A.; Vallet-Regı´ , M.; Sayagues, M. J.; Gonzalez-Calbet, J.; Garcı´ a-Sanz, F.; Rivas, J. Phys. Rev. B 1996, 54, 16574. (11) Alonso, J. A.; Martı´ nez-Lope, M. J.; Garcı´ a-Mu~ noz, J. L.; Fernandez, M. T. Physica B 1997, 234, 18. (12) Moriga, T.; Usaka, O.; Imamura, T.; Nakabayashi, I.; Matsubara, I.; Kinouchi, T.; Kikkawa, S.; Kanamaru, F. Bull. Chem. Soc. Jpn. 1994, 67, 687. (13) Kaneko, D.; Yamagishi, K.; Tsukada, A.; Manabe, T.; Naito, M. Physica C 2009, 469, 936. (14) Inoue, S.; Kawai, M.; Shimakawa, Y.; Mizumaki, M.; Kawamura, N.; Watanabe, T.; Tsujimoto, Y.; Kageyama, H.; Yoshimura, K. Appl. Phys. Lett. 2008, 92, 161911. (15) Inoue, S.; Kawai, M.; Ichikawa, N.; Kageyama, H.; Paulus, W.; Shimakawa, Y. Nature Chem. 2010, 2, 213.
