Structural and Thermal Properties of La 1-x Sr x CoO 3-δ Johann Mastin, Mari-Ann Einarsrud, and Tor Grande* Department of Materials Science and Engineering, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway ReceiVed July 4, 2006. ReVised Manuscript ReceiVed September 22, 2006 The crystal structure and the thermal properties of La 1-x Sr x CoO 3-δ (0 e x e 0.5) perovskites have been investigated by high-temperature X-ray diffraction. At ambient temperature, the crystal structure changes from a rhombohedral structure with the space group R3 hc to a cubic structure with space group Pm3 hm at x g 0.55. The thermal evolution of the lattice parameters was determined by Rietveld refinement of the diffraction data. The rhombohedral distortion from cubic symmetry decreased nearly linearly with increasing temperature up to the phase transition to the cubic perovskite structure. The linear thermal expansion coefficients of the lattice parameters were found and the rhombohedral to cubic phase transition temperature was determined. The phase transition temperature decreases rapidly with increasing Sr content in La 1-x Sr x CoO 3-δ . The phase transition is discussed with relation to the angle of rotation and the strain parameter of the CoO 6 octahedra. Finally, the structural and thermal properties of Sr- and Ca-substituted LaCoO 3 are compared, demonstrating the significant difference in the effect of the two alkaline earth cations. Introduction La 1-x A x CoO 3 with A ) Sr 2+ or Ca 2+ crystallizes in the perovskite ABO 3 structure with the ideal cubic Pm3 hm space group at elevated temperatures. LaCoO 3 undergoes a dis- placive phase transition at a critical temperature, T c , and transforms to a rhombohedrally distorted cubic structure with the R3 hc space group at low temperature. 1 With substitution of La 3+ by Sr 2+ or Ca 2+ , the phase transition temperature, T c , is lowered from about 1340 °C for LaCoO 3 1 to ambient temperature for ∼50 mol % Ca 2+ 2,3 and ∼55 mol % Sr 2+ . 4 Substitution of La 3+ with Sr 2+ or Ca 2+ is compensated by a mixed valence of Co (Co 3+ /Co 4+ ) and/or by creation of oxygen vacancies at high Sr or Ca substitution level. At high temperature, thermal reduction of Co introduces chemical expansion in La 1-x A x CoO 3-δ . 5,6 At high substitution level the oxidation of the materials during cooling becomes sluggish, resulting in an apparent constant oxygen defect concentration at low temperature. The deviation from cubic symmetry is caused by rotation and compression of the CoO 6 octahedra along one of the four diagonals in the cubic unit cell. 7 In the rhombohedral symmetry, the compressed pseudo cubic [111] direction becomes equal to the hexagonal c-axis, giving four possible equivalent domain states. These domains meet on the pseudo- cubic (100) and (110) planes to form (100) and (110) twins. 8,9 Twins in LaCoO 3 -based materials have been confirmed by electron microscopy. 10 Under external stress, the domains having the compressed body diagonal close to the stress direction will have lower free energy and grow at the expense of the less energetically favorable domain orientations. Domain reorientation during uniaxial compression has been confirmed by synchrotron X-ray diffraction. 11 The reorientation of ferroelastic domains as a response to mechanical stress is the microscopic origin of ferroelastic behavior of LaCoO 3 -based materials. Ferroelastic materials are characterized by a ferroelastic hysteresis defined by the cohersive stress and spontaneous strain. 12 Kleveland et al. 13 and Faaland et al. 14 have shown that LaCoO 3 -based materials display nonlinear behavior under mechanical compression and a remnant strain after unloading. Domain reorientation under mechanical stress in ferroelastic materials opens up for mechanical toughening of the materials. During propaga- tion of a crack, domain switching will absorb energy and, * Corresponding author. E-mail: Tor.Grande@material.ntnu.no. (1) Kobayashi, Y.; Mitsunaga, T.; Fujinawa, G.; Arii, T.; Suetake, M.; Asai, K.; Harada, J. J. Phys. Soc. Jpn. 2000, 69 (10), 3468-3469. (2) Mastin, J.; Einarsrud, M.-A.; Grande, T. Chem. Mater. 2006, 18 (6), 1680-1687. (3) Faaland, S.; Einarsrud, M.-A.; Grande, T. Chem. Mater. 2001, 13 (3), 723-732. (4) Mineshige, A.; Inaba, M.; Yao, T. S.; Ogumi, Z.; Kikuchi, K.; Kawase, M. J. Solid State Chem. 1996, 121 (2), 423-429. (5) Chen, X. Y.; Yu, J. S.; Adler, S. B. Chem. Mater. 2005, 17 (17), 4537-4546. (6) Lein, H. L.; Wiik, K.; Grande, T. Solid State Ionics 2006, 177, 1795- 1798. (7) Thornton, G.; Tofield, B. C.; Hewat, A. W. J. Solid State Chem. 1986, 61 (3), 301-307. (8) Kim, C. H.; Jang, J. W.; Cho, S. Y.; Kim, I. T.; Hong, K. S. Physica B 1999, 262 (3-4), 438-443. (9) Kim, C. H.; Cho, S. Y.; Kim, I. T.; Cho, W. J.; Hong, K. S. Mater. Res. Bull. 2001, 36 (9), 1561-1571. (10) Walmsley, J. C.; Bardal, A.; Kleveland, K.; Einarsrud, M.-A.; Grande, T. J Mater. Sci. 2000, 35, 4251-4260. (11) Vullum, P. E.; Mastin, J.; Wright, J.; Einarsrud, M.-A.; Holmestad, R.; Grande, T. Acta Mater. 2006, 54, 2615-2624. (12) Salje, E. K. H. Phase transitions in ferroelastic and co-elastic crystals; Cambridge University Press: Cambridge, 1990. (13) Kleveland, K.; Orlovskaya, N.; Grande, T.; Moe, A. M. M.; Einarsrud, M.-A.; Breder, K.; Gogotsi, G. J. Am. Ceram. Soc. 2001, 84 (9), 2029- 2033. (14) Faaland, S.; Grande, T.; Einarsrud, M.-A.; Vullum, P. E.; Holmestad, R. J. Am. Ceram. Soc. 2005, 88 (3), 726-730. 6047 Chem. Mater. 2006, 18, 6047-6053 10.1021/cm061539k CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006