Materials Science: Electronic & Magnetic Properties Research Frontiers 2014 50 A complete high-to-low spin state transition of trivalent cobalt ion in octahedral symmetry in SrCo 0.5 Ru 0.5 O 3– δ The largest spin state variability of Co 3+ is reported mainly for metal oxide compound, often within crystal structures where cobalt is coordinated by four to six oxygen ions. Low-spin (LS) Co 3+ is found in LiCoO 2 , La 2 Li 0.5 Co 0.5 O 4 , and in LaCoO 3 at low temperature. LaCoO 3 is intensively studied and its controversial spin state cross-over upon heating is still debated [1,2]. High-spin (HS) Co 3+ in a tetrahedral local symmetry was found in a mixed valent cobalt oxide YBaCo 4 O 7 . The pure HS Co 3+ in a square pyramidal CoO 5 coordination occurs in Sr 2 CoO 3 Cl and BiCoO 3 . However, the pure HS Co 3+ oxide in octahedral local symmetry is rarely documented. Also, there is a steady debate on the issue whether the octahedrally coordinated Co 3+ has a mixed HS-LS state or an intermediate spin (IS) state in the new classes of cobaltates RBaCo 2 O 5.5 (R = rare-earth metal), Sr 1– x Y x CoO 3– δ , Pr 0.5 Ca 0.5 CoO 3 , as well as in LaCoO 3 at high temperature. All the extraordinary material properties of the cobaltates, including superconductivity, giant magnetoresistance, strong thermopower, metal-insulator transitions, as well as spin-blockade behavior are related to the spin state degree of freedom of the cobalt ion. Hence, if the debate around the IS state is settled the interpretations of the material properties of cobalt based metal oxides are simplified remarkably. The complex metal oxide SrCo 0.5 Ru 0.5 O 3– δ possesses a slightly distorted perovskite crystal structure. Figure 1 displays the unit cell of the body centered monoclinic crystal structure as well as the local oxygen coordination of the transition metal site. From the crystal structure refinement, the Co-O distances are unusually large, ranging from 1.96 to 2.00 Å. The Co- K EXAFS data reveal an average Co-O distance of 1.98 ± 0.02 Å [3]. This distance is even larger than that in LaCoO 3 at 1000 K. At 650 K the average distance in LaCoO 3 was found to be 1.949 Å, a state which was proposed to have half of the cobalt ions in a HS state, whereas Co-O distance for IS Co 4+ ion in SrCoO 3 is 1.918 Å. Based on soft-X-ray absorption spectroscopy in combination with multiplet calculations, we have demonstrated that the Co 3+ ion in SrCo 0.5 Ru 0.5 O 2.96 is high-spin, which is unique for Co 3+ in octahedral oxygen coordination [3]. To investigate whether the HS state in SrCo 0.5 Ru 0.5 O 2.96 can be converted to an IS or LS state on applying pressure, a K β emission experiment was carried out. The high pressure Co K β X-ray emission spectra were obtained at the Taiwan inelastic X-ray scattering beamline BL12XU . Figure 2(a) shows the evolution of the Co K β X-ray emission line of SrCo 0.5 Ru 0.5 O 2.96 as a function of pressure from ambient up to 39.6 GPa at room temperature. At ambient pressure, the Co K β emission spectrum reveals a main peak located at ~7650 eV referred to as the K β 1,3 line and a pronounced satellite peak located at ~7637 eV corresponding to the K β ’ line. At high pressures, e.g., 39.6 GPa, the satellite peak nearly disappears and the main line gets narrower. This is similar to what happens for the K β emission spectrum of LaCoO 3 at low temperatures, indicating that the LS state has been reached for SrCo 0.5 Ru 0.5 O 2.96 . In fact, the spectral changes are alike those found for the complete spin state transition in FeS from 0 to 11.5 GPa and for Mg 0.9 Fe 0.1 SiO 3 from 20 to 120 GPa, where Fe 2+ also has the 3 d 6 electronic configuration. This infers that the applied pressure induces a complete HS to LS transition in SrCo 0.5 Ru 0.5 O 2.96 . The inset of Fig. 2(a) contains a plot of the so-called integrated absolute difference (IAD) as a function of pressure and the total IAD changes by about 0.15 in going from ambient pressure to 39.6 GPa. This 0.15 value is consistent with what is expected for a complete HS to LS transition. Important is that the spectral changes with pressure are continuous without any signs for new features that otherwise could indicate the presence of spin states other than the HS and LS. This is apparent in Fig. 2(b), displaying a comparison between the difference spectrum of K β emissions taken at ambient pressure (AP) and at 39.6 GPa (black line) with the difference Fig. 1. Part of the SrCo 0.5 Ru 0.5 O 2.96 crystal structure (left) and the local transition metal surrounding with bond lengths in Å to the neighboring oxygen ions (right). 1.965 1.965 1.957 1.957 1.995 1.995 Sr 2+ Co/Ru O 2– (a) (b) Research Frontiers 2014 51 spectrum of emissions taken at 15.8 GPa and at 39.6 GPa (green line). The black curve represents the spectral changes associated with a change in the average spin expectation value S from 2 to 0, and the green curve for a change of S from ≈1 to 0. Note that the difference spectra are identical apart from the expected scaling factor of two. This infers that the system at 15.8 GPa is in a ~1:1 mixed HS:LS state, and not in an IS state, as otherwise the difference spectrum would show new features [4]. To understand the above experimental observations, the total energy level diagram of the cobalt ions as a function of Co-O distance was calculated (Fig. 3). Clearly, the ground state of Co 3+ ions is either HS or LS (not IS) with a crossover at Co-O distance about 1.93 Å, where the HS and LS states are strongly mixed. Thus, Fig. 3 describes the evolution of the spin state during the compression induced by high pressure for SrCo 0.5 Ru 0.5 O 2.96 and reversely, during the expansion upon increasing the temperature for LaCoO 3 . Note that the IS states are remarkably unfavorable in energy in comparison to the HS and LS states. In fact, the lowest of the IS states is already ~0.4 eV (~4000 K) higher than the degenerated LS and HS at the Co-O distance 1.932 Å. These results allow us to draw an energy diagram depicting relative stabilities of the high, intermediate, and low spin states as functions of the metal-oxygen bond length for a Co 3+ ion in an octahedral coordination. A gradual high-to-low spin state transition in SrCo 0.5 Ru 0.5 O 2.96 is completed by applying high hydrostatic pressure of up to ~40 GPa. Across this spin state transition, the Co K β emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. We have not found any signs of the much debated intermediate spin state at any stage during the pressure induced spin state transition in this material. Fig. 2. (a) Evolution of the Co K β X-ray emission spectra of SrCo 0.5 Ru 0.5 O 2.96 under pressure up to 39.6 GPa at room-temperature. The inset shows IAD values of Co K β emission spectra as a function of applied pressure with respect to 39.6 GPa. (b) Difference spectrum of the Co K β emissions taken at ambient pressure (AP) and 39.6 GPa (full black line) and taken at 15.8 GPa and 39.6 GPa (green dashed line). Fig. 3. Energy level diagram of a CoO 6 cluster as a function of the Co-O distance. References [1] J.B. Goodenough and P.M. Raccah: J. Appl. Phys. 36 (1965) 1031. [2] M.W. Haverkort et al .: Phys. Rev. Lett. 97 (2006) 176405 [3] J.-M. Chen, Y.-Y. Chin, M. Valldor, Z. Hu, J.-M. Lee, S.-C. Haw, N. Hiraoka, H. Ishii, C.-W. Pao, K.-D. Tsuei, J.-F. Lee, H.-J. Lin, L.-Y. Jang, A. Tanaka, C.-T. Chen and L. H. Tjeng: J. Am. Chem. Soc. 136 (2014) 1514 . [4] J.P. Rueff et al .: Phys. Rev. B 73 (2006) 024424. Jin-Ming Chen a, *, Zhiwei Hu b and Liu Hao Tjeng b a National Synchrotron Radiation Research Center, Taiwan b Max Planck Institute for Chemical Physics of Solids, Germany *E-mail: jmchen@nsrrc.org.tw Energy (eV) Co-O (Å) – 6.0 – 6.5 – 7.0 – 7.5 – 8.0 – 8.5 2.00 1.98 1.96 1.94 1.92 1.90 IS IS LS HS d z 2 d _ xy d x 2 –y 2 d _ xy ~ J=2 ~ J=1 ~ J=1 ~ J=1 ~ J=0 ~ J=0 ~ J=2 ~ J=2 ~ J=3 0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 40 0.00 0.04 0.08 0.12 0.16 7625 7630 7635 7640 7645 7650 7655 7660 7625 7630 7635 7640 7645 7650 7655 7660 – 0.4 – 0.2 0.0 0.2 0.4 0.6 (b) Pressure (GPa) IAD Normalized Intensity (arb. units) Emission Energy (eV) AP 5.0 10.0 15.8 21.2 30.2 39.6 P (GPa) K β ' 1,3 K β (a) Normalized Intensity (arb. units) Emission Energy (eV) (AP – 39.6 GPa ) × 6 (15.8 GPa – 39.6 GPa) × 12
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