2020同步年報

028 ACTIVITY REPORT 2020 Fig. 3 : XAS of PbCoO 3 measured at varied pressures and room temperature. (a) Co K-edge XAS at some representative pressures. (b) Relative energy shift of the Co K-edge as a function of pressure at normalized intensity μ = 0.8 after subtracting the background, and the va- lence-state change of Co under pressure. (c) Pre-edge peak S d in the Pb L 3 -edge PFY-XAS. For comparison, the XAS collected at 0.5 GPa is shown for data recorded at varied pressure. (d) Integral area of S d and valence-state change of Pb as a function of pressure. The dotted and dashed curve is for visual guidance. [Reproduced from Ref. 1] Fig. 4 : Pressure- and temperature-dependent phase diagram of PbCoO 3 . The circle ( ○ ), cross (×) and square ( □ ) respectively stand for the cubic, tetra-I, and tetra-II phases as determined from XRD measurements. The dashed curves show the approximate phase boundaries. [Repro- duced from Ref. 1] of Pb. Given the initial average valence state Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 of Pb 3.5+ at lower pressure, the relative variation of the integral area of S d indicates the change from Pb 3.5+ to Pb 3.0+ . The Co K-edge (Co 2.5+ → Co 3.0+ ) and Pb L 3 -edge (Pb 3.5+ → Pb 3.0+ ) XAS thus confirm the Pb 4+ -Co 2+ intermetallic charge transfer, which occurs about 15 GPa with a sharp change near 20 GPa and is complete about 30 GPa. According to the high-pressure resistance, XES, XAS and X-ray diffraction (XRD) results, the authors obtained an interesting phase diagram of the dependence on pressure and temperature of PbCoO 3 as shown in Fig. 4 . 1 i) Below ~20 GPa, the compound maintains the A- and B-site-ordered quadruple perovskite struc- ture in cubic Pn -3 symmetry with charge combination Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 (Pb 3.5+ Co 2.5+ O 3 on average). The HS-LS transition is expected to be complete about 15 GPa. When that spin-state transition is complete, pressure-in- duced intermetallic charge transfer begins between Co 2+ and Pb 4+ ions. ii) Between approximately 20 and 30 GPa, metallization is observed due to the accumulated effect of the Pb-Co charge transfer near 300 K, strongly indicating the melting of the ordered low-spin Co 2+ and Co 3+ states into mixed Co 2.5+ on average, consistent with a first-order structural phase transition to the tetra-I phase. The Pb 4+ -Co 2+ intermetallic charge transfer still occurs in the tetra-I phase, making the LS-Co 2+ state become oxidized to the LS-Co 3+ state. iii) With pressure up to ~30 GPa at 300 K, the Pb-Co inter- metallic charge transfer is complete, changing the charge combination to be Pb 3.0+ Co 3.0+ O 3 on average. Because the charge transfer greatly changes the charge states and the electronic configurations for both Pb and Co ions, the compound experiences another first-order structure phase transition toward the tetra-II phase with a con- siderable shrinkage of volume. In the tetra-II phase, all transition-metal sites are occupied with LS-Co 3+ with spin moment = 0. In summary, the authors present the first example in which spin- state, charge-transfer, crystal-structure and metal-insulator transitions occur collectively in the same material sys- tem, PbCoO 3 , producing a series of intriguing variations and potential