Ionic control of mixed-anion compounds and reduced oxides using electric field

2020 Fiscal Year Annual Research Report

• Project/Area Number: 19F19334
• Research Institution: Kyoto University
• Principal Investigator: 陰山 洋 京都大学, 工学研究科, 教授 (40302640)
• Co-Investigator(Kenkyū-buntansha): LI HAOBO 京都大学, 工学(系)研究科(研究院), 外国人特別研究員
• Project Period (FY): 2019-10-11 – 2022-03-31
• Keywords: Ionic manipulation / Electric-field control / Phase transition / Topochemical reaction / Proton insertion / Complex oxide / Mixed-anions / Thin film

Outline of Annual Research Achievements

The control of oxygen stoichiometry in complex oxides has been playing one of the key roles in designing novel function materials. By manipulating the oxygen vacancies, many unique behaviors can be observed such as superconductivity, polarization, metal-insulator transition and etc. Moreover, by taking 1/3 oxygen ions from a common cubic perovskite ABO3 lattice, the reduced perovskite ABO2 has shown many exotic properties such as magnetism in SrFeO2 and superconductivity and LaNiO2, which greatly enriches the functional materials.
Unfortunately, these ABO2 type materials are relatively difficult to obtain. As one of the most classic strategies, low-temperature topochemical reaction with metal-hydride agent (e.g., CaH2, NaH) has been widely adopted to synthesis reduced perovskite materials (BaFeO2, CaFeO2 and etc.). This strategy relies on the kinetic control of the reaction product which forms faster than the most thermodynamically stable state. This greatly hinders the further control of the products and the manipulation of the reaction energy landscape is most needed.
In year 2020, we had successfully injected proton into strongly-correlated oxide SrCoO2.5 and we were aiming to use the proton to trigger unexplored phase transition towards reduced oxide materials. By performing a series experiment, reduced oxide SrCoO2 was synthesized for the first time. Our findings reflected that electrochemically destablized materials could generate critical influence on the reaction energy landscape and finally helped to create new materials.

Current Status of Research Progress

1: Research has progressed more than it was originally planned.

Reason

Epitaxial SrCoO2.5 thin films were grown on SrTiO3 substrates using a pulsed laser deposition system. The growth conditions were optimized at 750 °C and an oxygen atmosphere of 10 Pa. The laser energy was kept at 1.9 J/cm2 with a repetition rate of 2 Hz. After deposition, the samples were cooled down to room temperature while maintaining the oxygen background pressure as during growth pressure to avoid over-oxidation. The electrochemical injected protons were stabilized in SrCoO2.5. By heating the protonated phase in high vacuum. The HSrCoO2.5 was unexpectedly transformed into a new oxide SrCoO2. We had performed soft X-ray absorption spectra, SIMS to verify the chemical composition. The crystal structure was carefully analyzed by STEM and DFT calculations with our collaborators. SrCoO2 showed the unique four-legged spin tube composed by CoO4 tetrahedra which had never been reported before. Besides, the dehydration was also verified by TDS measurement. This indicated that the injected protons were able to ‘drag’ oxygen out of the lattice. The recent SHG measurements also proved that SrCoO2 held a polar structure with space group of Amm2

Strategy for Future Research Activity

In year 2021, we will try to further explore the physical properties of SrCoO2. More importantly, the protonation and dehydration method will be expanded to other materials such as ReNiO3 series (Re = La, Nd, Sm, etc.). After summarizing all the data, we plan to publish our results in high-impact journal and conferences.