2020同步年報

Chemical Science 027 Fig. 2 : Co Kβ XES of PbCoO 3 at varied pressures and room temperature; the inset shows the pressure dependence of IAD. [Reproduced from Ref. 1] Fig. 1 : Schematic crystal structure of both A- and B-site-ordered PbCoO 3 (Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 ) quadruple perovskite with symmetry Pn - 3. The corner-sharing Co 2+ /Co 3+ O 6 octahedra and isolated Pb 4+ O 4 squares are shown. [Reproduced from Ref. 1] Sequential Spin-State Transition and Intermetallic Charge Transfer in PbCoO 3 under High Pressure The authors report for the first time that spin-state, charge-state, crystal structure and metal-insu- lator transitions take place collectively in the same material system PbCoO 3 . A lthough a spin-state transition and intermetallic charge transfer have received much attention in the past, these two phenomena were never found to occur together in a particular material. PbCoO 3 synthesized at high pressure (12 GPa and 1323 K) provides a unique opportunity for this study by Jin-Ming Chen (NSRRC) and his international collaborators. 1 Figure 1 shows the schematic quadruple perovskite structure of both A- and B-site-ordered PbCoO 3 (Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 ) quadruple perovskite with symmetry Pn -3, containing corner-sharing Co 2+ /Co 3+ O 6 octahedra and isolated Pb 4+ O 4 squares. Figure 2 shows the pressure-dependent Co-Kβ X-ray emission spectra (XES) of PbCoO 3 measured at SP 12U1 , a Taiwan beam- line at SPring-8 in Japan. The ratio of intensity of the low-energy Kβ' line to the main emission Kβ 1,3 line is proportional to the number of unpaired electrons in the incomplete 3 d shell and can serve to determine the spin state of Co ion as a function of temperature or pressure. With increasing pressure, the intensity of the low-energy Kβ' line decreases and almost disappears above ~15 GPa ( Fig. 2 ), indicating a decreasing spin moment of Co 2+ ion and a transition of Co 2+ from high spin (HS) to low spin (LS). The inset of Fig. 2 contains a plot of the integrated absolute difference (IAD) as a function of pressure; this IAD value presents a linear relation with the average spin number. In PbCoO 3 , the total IAD changes by about 0.043 (10) with pressure increasing from 0.5 to 22.1 GPa. Figure 3(a) shows the Co K-edge X-ray absorption spectra (XAS) of PbCoO 3 measured at varied pressures and normalized intensity μ = 0.8. At pressures below 15 GPa or above 30 GPa one can see a weak linear shift of the absorption edge with increasing pressure. There is an abrupt shift to higher energy from 20.2 to 29.3 GPa accompanying a variation of spectral profile, indicating the changes of valence state and crystal structure. The observed Co K-edge energy shift in PbCoO 3 indicates an average valence change about 0.5 ( i.e ., from Co 2.5+ to Co 3.0+ on average), as illustrated in Fig. 3(b) (right scale). To fulfil the requirement of charge balance, one expects some Pb 4+ to change to Pb 2+ correspondingly. Figure 3(c) presents the pressure-dependent Pb L 3 -edge XAS of PbCoO 3 . The high-resolution Pb L 3 -edge partial-fluorescence- yield (PFY) XAS provides an opportunity to identify the valence state of Pb. As shown in Fig. 3(c) , there is a sharp shoulder S d at lower energy, 13,030 eV, in the PFY-XAS; this feature is assigned to the dipole-allowed transition from the 2 p 3/2 core level to the unoccupied 6 s states. The spectral intensity of pre-edge peak S d represents the number of 6 s holes and can serve to deter- mine the valence change of Pb as a function of pressure. The spectral integral area of S d as a function of pressure is presented in Fig. 3(d) . The intensity sharply decreasing with pressure increasing from 15 GPa to 30 GPa indicates the valence decrease