2019 Fiscal Year Annual Research Report
Metal-directed asymmetric spatial assembly of diverse building blocks - spheres, planes, and bowls
Publicly Offered Research
| Project Area | Coordination Asymmetry: Design of Asymmetric Coordination Sphere and Anisotropic Assembly for the Creation of Functional Molecules |
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| Project/Area Number |
19H04590
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| Research Institution | Osaka Prefecture University |
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Principal Investigator |
プラシデス コスマス 大阪府立大学, 工学(系)研究科(研究院), 教授 (90719006)
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| Project Period (FY) |
2019-04-01 – 2021-03-31
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| Keywords | mixed valence / valence transitions / spin frustration / rare earths / fullerides |
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| Outline of Annual Research Achievements |
Research focused on asymmetrically coordinated (cationogenic and anionogenic) mixed valence molecular solids. Progress was achieved in producing magnetically frustrated spin assemblies and driving insulator-to-metal transitions by valence transitions via the application of external stimuli.
[1] The electronic properties of d- and f-shell materials have drawn the bulk of research attention but although unconventional p-electron-based electronic, magnetic and conducting materials are rare, they are a fascinating topic. Among alkali oxides, the molecular sesquioxides A4O6 (A = alkali metal) are of special interest as they are rare examples of binary ionic compounds with molecular dioxygen in two different oxidation states. Our work has unveiled a complex cubic (charge-disordered) to tetragonal (charge-ordered) stability phase space as a function of temperature and pressure together with an intimate link between the lattice geometry and the absence of long-range magnetic ordering, which is suppressed because of geometric frustration.
[2] Rare-earth (RE) fullerides are an intriguing family of materials in which RE electronic instabilities couple to the electronic and lattice degrees-of-freedom of the strongly-correlated C60 sublattice. We revealed that the asymmetrically-coordinated mixed-valency (Sm1-xCax)2.75C60 materials show pressure-driven reversible phase transitions accompanied by drastic increases in Sm valence and C60 oxidation state. These are coincident with huge lattice contractions and concomitant insulator-to-metal transitions.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Progress has been very satisfactory with new directions and opportunities appearing. Work on f-electron fullerides has led to families of new materials and new properties are emerging. The wealth of asymmetrically coordinated non-stoichiometric mixed valency f-/p-electron families of molecular materials chemically synthesized in this work has led to numerous investigations of the fragility of the electronic states with changes in external stimuli such as temperature and pressure. For the purpose of the detailed investigations, we are using a variety of experimental techniques at both ambient and elevated pressures, including synchrotron X-ray diffraction, optical spectroscopy, magnetometry and synchrotron X-ray absorption spectroscopy in its high-resolution partial fluorescence-yield (PFY-XAS) variant.
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| Strategy for Future Research Activity |
The behavior of the asymmetrically coordinated molecular fullerides synthesized is reminiscent of the electronic and lattice response to pressure of highly correlated Kondo insulators like SmS and its ternary derivatives, Sm1-xRxS (R = Ca, Y, etc). However, a distinguishing feature of the fulleride systems is that the C60 anionic sublattice can act as an electron reservoir due to the availability of a close-lying band derived by the t1g orbitals and can accept excess charge as the 4f-electron occupation number decreases. Therefore, given that the C60 sublattice can support superconductivity, we plan to investigate the opportunities presented by the observed pressure-induced electronic response - this is opening new possibilities for accessing metallic fullerides at elevated pressures from which superconductivity can emerge at the higher C60 doping levels achieved by further increase in the average rare-earth valence. To this effect, we are planning to perform nuclear resonance scattering experiments at synchrotron facilities to search for the emergence of novel magnetic and/or superconducting behavior at high pressures.
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