Spin State and Spin Crossover Phenomena of Perovskite-type Cobalt Oxides

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1 Netsu Sokutei Spin State and Spin Crossover Phenomena of Perovskite-type Cobalt Oxides Shingo Tsubouchi, Tôru Kyômen, Mitsuru Itoh (Received April 7, 2003; Accepted June 20, 2003) It has been reported that the intermediate spin state becomes the ground state of Co ions in perovskite-type cobalt oxides and that peculiar spin-crossover phenomena occur in the oxides. The experimental facts and their interpretation on the spin state and the spin crossover phenomena of LaCoO 3 and Pr 0.5Ca 0.5CoO 3 are introduced in comparison with those in spincrossover complexes. The spin crossover phenomena of the perovskite-type cobalt oxides are correlated with the change in electrical resistivity. It has been considered that the correlation originates from the strong Co 3d e g -O 2p hybridization connecting with the intermediate spin state. In addition, many facts imply that the roles of vibrational and magnetic energies to the spin-crossover phenomena are different between perovskite-type cobalt oxides and spin-crossover complexes. d t 2g e g Fig t 2g Table 1 1) 2) d Table 1 3) d d 2003 The Japan Society of Calorimetry and Thermal Analysis. 180

2 E / B Fig.1 Transition-metal ion octahedrally coordinated by six anions. Energy level splitting of d orbitals of a transition-metal cation in the crystal field of. Dq / B Table 1 Possible ground spin states of d 4, d 5, d 6, and d 7 electronic configurations. E / B e g t 2g 10Dq 10 t 2g e g d 10 A B C A B C B d 5 d 6 Fig.2 Fig.2 1) 10Dq / B 10Dq / B Dq / B Fig.2 Tanabe-Sugano diagrams of d 5 and d 6 electronic configurations. Unit of B is cm 1. 1) Fe 2 Al 2O 3 Cr Potze SrCoO 3 LaCoO 3 4 d 5 3 d 6 t 2g4 e 1 g t 2g5 e 1 g 3) d 5 6 Fig.2 Fig.2 181

3 d 5 4 T 1 d 6 3 T 1 Potze 3d 3d SrCoO 3 LaCoO 3 SrCoO 3 LaCoO K 3d 3 4 3d 2p Co3d-O2p 3d Potze 3 4 ρ / Ω cm χ / emu mol 1 Oe 1 Co-O-Co 180 4) 2) 3 LnCoO 3 Ln Ln La 100 K 600 K Ln La 600 K Fig.3 Temperature dependences of resistivity and magnetic susceptibility of LaCoO 3. 5) LaCoO 3 LaCoO 3 Fig.3 5) Fig.3 5) Fig.4 6) Fig.5 7) 100 K 100 K 100 K 50 K R ln 2 0 K S K 100 K 100 K Goodenough 8) Goodenough 182

4 V / 3 (V VG E) / VG E / % Cp / JK 1 mol 1 Fig.4 Temperature dependences of unit cell volume and difference between the unit cell volume and Gruneisen-Einestein fit curve for LaCoO 3. 6) Goodenough Thornton 1300 K 9) Goodenough 10) Potze 3) LaCoO 3 S 1 S 2 Stolen 50 K R ln 2 S 1 triplet Fig.5 Temperature dependences of heat capacity of LaCoO 3. 7) singlet doublet singlet 7) ESR triplet singlet doublet 7 K 100 K 11) g 3.35 MgO Fe 2 3 5,12) 13) Naiman 14) Naiman Goodenough 15) LaCoO K Fig.3 Fig.4 9) 600 K 10 3 Ω cm 183

5 600 K Goodenough 10) 100 K 600 K 6) La 1 xsr xcoo x ) K 19) NMR Co 17) La 1 xsr xcoo 3 La Pr Nd Sm Sr Ca CaCoO 3 La 1 xsr xcoo 3 Pr 0.5Ca 0.5CoO 3 20) LnCoO 3 Pr 0.5Ca 0.5CoO 3 Fig K 5 K 5 K 350 K 100 K 2mΩcm 16) 80 K 2 90 K 50 K Cp / JK 1 mol 1 M/H / emu mol 1 ρ / Ω cm a, b/ 2, c / Fig.6 (c) (d) Unit cell volume / 3 Temperature dependences of resistivity, dc magnetization, (c) heat capacity, and (d) lattice constants and unit cell volume of Pr 0.5Ca 0.5CoO 3. The inset of shows the inverse of contribution to magnetic susceptibility from Co atoms obtained by subtracting Pr contributions (dashed line in ). The solid lines in the inset indicate the results of fitting according to Curie-Weiss law. Fig.6 5T 90 K Pr 3 21) 200 K Curie-Weiss Curie 1.69 emu mol 1 La 0.5Sr 0.5CoO emu mol 1 18) Weiss 80 K 184

6 Cp / JK 1 mol 1 dt/dt / µks 1 Cp T 1 / JK 2 mol 1 Fig.7 Heat capacities of Pr 0.5Ca 0.5CoO 3. Temperatures in the figure indicate the lowest limit of precooling for respective runs. Two thick solid lines represent the base lines for estimating the transition enthalpy and entropy. Spontaneous temperature-drift rates observed in the heat capacity measurements. 90 K Curie-Weiss Curie 0.27 emu mol 1 Weiss 0K Curie-Weiss 90 K Curie Fig.6(c) 90 K Fig K 4, 85, 90, 95 K 200 K 90 K 12.5 Fig.7 90 K Fig.6(c) Fig.7 H S K K 90 K H S 417 Jmol 1, 4.66 JK 1 mol 1 5 K Fig.8 C pt 1 vs. T plot of Pr 0.5Ca 0.5CoO 3 in the temperature range of K. The solid and dashed lines indicate the data of PrCoO 3 and LaCoO 3, respectively. Table 2 Bond lengths and bond angles of Pr 0.5Ca 0.5CoO 3 determined by Rietveld refinements of neutron diffraction data. 10 K 297 K Co-O(1) / (3) (3) Co-O(1) / (7) 1.921(1) (7) 1.927(1) <Co-O> / (6) 1.922(1) O(2)-Co-O(2) / 90.70(1) 90.99(1) O(1)-Co-O(2) / 90.07(5) 90.12(7) 90.10(4) 90.22(6) < O-Co-O> / 90.29(3) 90.44(5) Co-O(1)-Co / (7) 158.3(1) Co-O(2)-Co / (4) (6) < Co-O-Co> / (6) 158.0(1) 9T Fig.8 2 K M-H ρ -H X Pnma Fig.6(d) K 90 K a b, c 2 % 185

7 0.5 % 22) Rietveld 297, 10 K Pnma 297, 10 K Co-O O-Co-O Co-O-Co Table 2 Co-O 297 K K O- Co-O 297 K 10 K Co-O-Co 297 K K CoO 6 Fig.6 90 K Curie Fig.8 5 K Fig.8 PrCoO 3 LaCoO 3 LaCoO 3 PrCoO 3 Pr 3 4f 2 6 K PrCoO 3 LaCoO 3 Pr 0.5Ca 0.5CoO 3 Pr 0.5Ca 0.5CoO 3 Pr 3 Pr 0.5Ca 0.5CoO K 2.26 JK 1 mol 1 Co 3 t 6 2 g : S 0 Co 4 t 5 2g :S 1/2 S 2.88 JK 1 mol 1 0.5R ln 1 0.5R ln JK 1 mol K 13 K 3 4 2K 0.5 µ B Co 3 t 2g5 e g1 :S 1 4 Co 4 t 2g5 :S 1/2 S 4.56 Fig. 9 Proposed electronic structures of Pr 0.5Ca 0.5CoO 3 for the low- and high-temperature phases. JK 1 mol 1 0.5R (ln 3 ln 2) 0.5R (ln 1 ln 2) S 4.66 JK 1 mol t 5 2g e g K Rietveld Co- O Fe(phen) 2(NCS) 2 Fe 2 Fe-N ) Pr 0.5Ca 0.5CoO 3 Pr 0.5Ca 0.5 CoO 3 Co- O Co-O Fig.9 σ * Co e g O2p 186

8 Co-O-Co t 2g σ * Co e g O 2p σ * t 2g t 2g σ * σ * Pr 1 xca xcoo x 0.45) x ,24) 30 K 2 30 K LaCoO 3 23,24) Pr 1 xca xcoo x 0.45 x 0.5 Pr 0.5Ca 0.5CoO 3 Co 3 Co 4 1:1 LaCoO 3 Pr 0.5Ca 0.5CoO Co-O-Co Co e g -O 2p Pr 0.5Ca 0.5CoO 3 Co-O 80 K LaCoO 3 1),,, (1993). 2) P. Gutlich, A. Hauser, and H. Spiering, Angew. Chem. Int. Ed. Engl. 33, 2024 (1994);, 29, 102 (2002). 3) R. H. Potze, G. A. Sawatzky, and M. Abbate, Phys. Rev. B51, (1995). 4), 26, 64 (1999). 5) S. Yamaguchi, Y. Okamoto, H. Taniguchi, and Y. Tokura, Phys. Rev. B53, R2926 (1996). 6) K. Asai, A. Yoneda, O. Yokokura, J. M. Tranquada, G. Shirane, and K. Kohn, J. Phys. Soc. Japan 67, 290 (1998). 7) S. Stolen, F. Gronvold, H. Brinks, T. Atake, and H. Mori, Phys. Rev. B55, (1997). 8) J. B. Goodenough, J. Phys. Chem. Solids 6, 287 (1958); P. M. Raccah and J. B. Goodenough, Phys. Rev. B155, 932 (1967). 9) G. Thornton, B. C. Tofield, and A. W. Hewat, J. Solid State Chem. 61, 301 (1986). 10) M. A. Senaris-Rodriguez and J. B. Goodenough, J. Solid State Chem. 116, 224 (1995). 11) S. Noguchi, S. Kawamata, K. Okuda, H. Nojiri, and M. Motokawa, Phys. Rev. B66, (2002). 12) M. Itoh, M. Sugahara, I. Natori, and K. Motoya, J. Phys. Soc. Japan 64, 3967 (1995) 13) K. Asai, O. Yokokura, N. Nishimori, H. Chou, J. M. Tranquada, G. Shirane, S. Higuchi, Y. Okajima, and K. Kohn, Phys. Rev. B50, 3025 (1994). 14) C. S. Naiman, R. Gilmore, B. DiBartolo, A. Linz, and R. Santoro, J. Appl. Phys. 36, 1044 (1965). 15) T. Kyomen, Y. Asaka, and M. Itoh, Phys. Rev. B67, (2003). 16) P. Ganguly, P. S. Anil Kumar, P. N. Santhosh, and I. S. Mulla, J. Phys.: Condens. Matter 6, 533 (1994). 17) M. Itoh, I. Natori, S. Kubota, and K. Motoya, J. Phys. Soc. Jpn. 63, 1486 (1994); M. Itoh and I. Natori, J. Phys. Soc. Japan 64, 970 (1995). 18) M. A. Senaris-Rodriguez and J. B. Goodenough, J. Solid State Chem. 118, 323 (1995). 19) S. Tsubouchi, T. Kyomen, and M. Itoh, Phys. Rev. B67, (2003). 20) S. Tsubouchi, T. Ky_men, M. Itoh, P. Ganguly, M. Oguni, Y. Shimojo, Y. Morii, and Y. Ishii, Phys. Rev. B66, (2002). 21) K. Sekizawa, M. Kitagawa, and Y. Takano, J. Magn. 187

9 Magn. Mater , 541 (1998). 22) H. Kawano, H. Yoshizawa, and Y. Ueda, J. Phys. Soc. Japan. 63, 2857 (1994); P. G. Radaelli, D. E. Cox, M. Marezio, S-W. Cheong, P. E. Schiffer, and A. P. Ramirez, Phys. Rev. Lett. 75, 4488 (1995); J. L. Garcia-Munoz, J. Rodriguez-Carvajal, P. Lacorre, and J. B. Torrance, Phys. Rev. B46, 4414 (1992). 23),, (2003). 24),,, 58, 28pWG-2, p.521 (2003). LaCoO 3 Pr 0.5Ca 0.5CoO 3 Co e g -O 2p Shingo Tsubouchi, Materials and Structures Laboratory, Tokyo Institute of Technology, TEL , FAX , e- mail: stsubo@msl.titech.ac.jp Tôru Kyômen, Materials and Structures Laboratory, Tokyo Institute of Technology, TEL , FAX , e- mail: tkyomen@msl.titech.ac.jp Mitsuru Itoh, Materials and Structures Laboratory, Tokyo Institute of Technology, TEL , FAX , e- mail: Mitsuru_Itoh@msl.titech.ac.jp 188

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