Pressure-induced spin-state and insulator-metal transition in Sr 2 CoO 3 F by first principles Xue-dong Ou and Hua Wu Figure 1a. The structure of Sr 2 CoO 3 F Figure 1b. The parameter D, defined as the distance between the Co ion and the O4 basal square, is related to bandwidth of the x2-y2 pdσ state. It may tune an insulator-metal and/or AFM-FM transition in layered cobalt oxides Figure 2.Configuration states of Co 3+ ion.(a). A competition between crystal field and Hund exchange leads to different spin states. (b) A multiplet effect in the IS state is due to inter-orbital interactions. Ⅰ. Introductio n Ⅱ. Spin state transition and multiplet effect Ⅲ. Pressure-induced spin-state and insulator-metal transitions Figure 3a. Pressure dependence of the electric resistivity of Sr 2 CoO 3 F [3]. Figure 3b.The E-V and P-V curve by GGA+U calculations. Figure 3a shows a decreasing electric resistivity of Sr 2 CoO 3 F under pressure, implying the onset of a metallic state [3]. Through configuration-state-constrained GGA+U calculations (Fig. 3b), we find that an HS-IS transition and an associated AF insulator-FM metal transition occur at about 6 GPa (with about 3% volume compression). This agrees with the experiment data [3]. The P-V curve is fitted by Brich-Murnaghan equation of state as follows: 3 2 2 3 2 3 3 2 0 0 0 0 4 6 1 0 ' 1 0 16 9 V E V V V V B V V B V E ) ( Figure 4. Total density of states (DOS) and orbitally resolved Co-3d DOS. (a) High spin (HS) insulator with G-type AF order is the ground state at ambient pressure, which agrees well with the experimental results [3]. (b) Under the pressure of about 6 GPa, spin state and I-M transition occurr, and the IS half-metalli state becomes most stable. HS IS (a ) (b ) Strongly correlated cobalt oxides are interesting materials which display spectacular properties such as giant magneto resistance, superconductivity and large thermoelectric power [1- 2]. They have an important spin-state issue. Spin state and its transition are often associated with fascinating magnetic and electronic properties. An intermediate-spin state could give rise to a FM half-metallic state that is of great interest. We have studied the electronic structure and magnetic property of the pressurized Sr 2 CoO 3 F, using configuration-state-constrained GGA+U calculations. The obtained results show that a moderate pressure(about 6 GP) could readily induce a transition from antiferromagentic (AF) insulator with high spin (HS, S=2) to ferromagnetic (FM) half-metal with intermediate-spin (IS, S=1). Our results account well for the recent experimental data, and it is suggested that a novel FM half- metallic ground state may be explored by Ca-doping. In 3d transition-metal oxides, there could be a spin crossover, if the Hund exchange energy E ex and the crystal field energy E CF are comparable. The former favors a high-spin state, while the latter prefers to stabilize a low- spin state. The energy difference between E ex and E CF could be tuned effectively by an external pressure or a chemical substitution. Then interesting magnetic and electronic properties may emerge. Sr 2 CoO 3 F is found to have an interesting transition from HS AF insulator to IS FM half-metal, under a moderate pressure of about 6 GPa. As the D parameter (or the Co-O-Co bond angle) controls the x2-y2 bandwidth effectively, one would need to further reduce it to increase the bandwidth. This plays a vital role in forming the IS FM half- metallic state. We suggest that Ca-doping could shrink the lattice and reduce the D parameter, and as a result, the IS FM half-metallic phase could become the ground state at ambient pressure. Currently we are exploring such a possible half-metallic oxide. References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian, and T. Sasaki, Nature (London) 422, 53 (2003). [2] T. Jia, H. Wu, G. Zhang, X. Zhang, Y. Guo, Z. Zeng, H.Q. Lin. Phys. Rev. B 83, 174433 (2011). [3] Y.Tsujimoto, C.I. Sathish, K. P. Hong, K. Oka, M. Azuma, Y. F. Guo, Y. Matsushita, K. Yamaura, and E. Takayama-Muromachi, Inorg. Chem. 51 ,4802 (2012). Ⅳ. Discussion and conclusion