TY - JOUR
T1 - Experimental Visualization of the Interstitialcy Diffusion of Anions in the LaOF-Based Oxyfluoride La0.9Sr0.1O0.45F2
AU - Hibino, Keisuke
AU - Tanaka, Mahiro
AU - Kozakai, Satoshi
AU - Fujii, Kotaro
AU - Ishihara, Tatsumi
AU - Hester, James R.
AU - Yashima, Masatomo
N1 - Funding Information:
We thank Dr. S. Torii, Dr. M. Hagihala, Dr. P. Miao, and Dr. S. Kawaguchi for the assistance in the neutron and synchrotron X-ray powder diffraction experiments. We express special thanks to Dr. E. Niwa, Dr. M. Shiraiwa, Dr. W. Zhang, Mr. K. Saito, Mr. T. Tsujiguchi, Mr. H. Yaguchi, Mr. M. Matsui, and Mr. Y. Yasui for useful discussion and assistance in the experiments/analyses. We would like to acknowledge Rigaku Co. for the XRF measurements. The neutron-diffraction measurements were carried out by the project approval (ANSTO Proposal No. PP51985, J-PARC MLF Proposal No. 2017L1301). Synchrotron X-ray experiments were done by the project approval 2017B1265 and 2018A1259. This study was partly supported by Grants-in-Aid for Scientific Research (KAKENHI, nos. JP15H02291, JP16H00884, JP16H06293, JP16H06440, JP16H06441, JP16H06438, JP16K21724, JP17K17717, JP17H06222, JP19H00821, JP20H05184, JP20K05086, and JP21K18182) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and JSPS Core-to-Core Programs, A. Advanced Research Networks (Solid Oxide Interfaces for Faster Ion Transport; Mixed Anion Research for Energy Conversion [JPJSCCA20200004]). Travel costs were partially supported by the Institute for Solid State Physics, The University of Tokyo (proposal nos. 17576 and 18584) and Japan Atomic Energy Agency, Tokai, Japan.
Publisher Copyright:
© 2021 American Chemical Society. All rights reserved.
PY - 2021/9/27
Y1 - 2021/9/27
N2 - Fluoride-ion and oxide-ion conductors are attractive materials due to their wide applications such as next-generation fluoride-ion batteries and solid oxide fuel cells. Crucial to the development of these anion conductors is the knowledge of crystal structures and the ion-diffusion mechanism at an atomic scale. Recently, mixed-anion compounds have attracted much attention, but experimental visualization of anion-diffusion pathways is very rare in mixed-anion compounds. Lanthanum oxyfluoride LaOF-based materials are mixed-anion compounds and exhibit high anion (fluoride-ion and oxide-ion) conductivities; however, their high-temperature crystal structures and anion-diffusion mechanism are not known satisfactorily. Herein, we report detailed information on the crystal structure and structural disorder of La0.9Sr0.1O0.45F2 and LaOF from -243 °C (30 K) to 600 °C. Trigonal R3¯ m β-LaOF undergoes a first-order phase transition into a cubic α-phase with the Fm3¯ m fluorite-type structure around 490 °C on heating, while La0.9Sr0.1O0.45F2 is a cubic Fm3¯ m α-phase between -243 and 600 °C. Neither significant amounts of interstitial anions at the 32f site nor significant anion vacancies at the lattice 8c site are observed in cubic LaOF at 600 °C, while both interstitial anions at the 32f site and anion vacancies at the lattice 8c site exist in cubic La0.9Sr0.1O0.45F2 from -243 to 600 °C. We have succeeded in experimental visualization of anion-diffusion pathways in La0.9Sr0.1O0.45F2. It was found that the anions migrate through both the interstitial 32f and lattice 8c sites (8c-32f-32f-8c anion-diffusion pathways), indicating an interstitialcy diffusion mechanism. The existence of interstitial anions and anion vacancies and the formation of the anion-diffusion pathways are the structural origins of the high anion conductivity of La0.9Sr0.1O0.45F2. The present elucidation of the crystal structure and anion-diffusion mechanism might provide useful knowledge for the design of superior anion conductors, which develop the next-generation fluoride-ion batteries and solid oxide fuel cells.
AB - Fluoride-ion and oxide-ion conductors are attractive materials due to their wide applications such as next-generation fluoride-ion batteries and solid oxide fuel cells. Crucial to the development of these anion conductors is the knowledge of crystal structures and the ion-diffusion mechanism at an atomic scale. Recently, mixed-anion compounds have attracted much attention, but experimental visualization of anion-diffusion pathways is very rare in mixed-anion compounds. Lanthanum oxyfluoride LaOF-based materials are mixed-anion compounds and exhibit high anion (fluoride-ion and oxide-ion) conductivities; however, their high-temperature crystal structures and anion-diffusion mechanism are not known satisfactorily. Herein, we report detailed information on the crystal structure and structural disorder of La0.9Sr0.1O0.45F2 and LaOF from -243 °C (30 K) to 600 °C. Trigonal R3¯ m β-LaOF undergoes a first-order phase transition into a cubic α-phase with the Fm3¯ m fluorite-type structure around 490 °C on heating, while La0.9Sr0.1O0.45F2 is a cubic Fm3¯ m α-phase between -243 and 600 °C. Neither significant amounts of interstitial anions at the 32f site nor significant anion vacancies at the lattice 8c site are observed in cubic LaOF at 600 °C, while both interstitial anions at the 32f site and anion vacancies at the lattice 8c site exist in cubic La0.9Sr0.1O0.45F2 from -243 to 600 °C. We have succeeded in experimental visualization of anion-diffusion pathways in La0.9Sr0.1O0.45F2. It was found that the anions migrate through both the interstitial 32f and lattice 8c sites (8c-32f-32f-8c anion-diffusion pathways), indicating an interstitialcy diffusion mechanism. The existence of interstitial anions and anion vacancies and the formation of the anion-diffusion pathways are the structural origins of the high anion conductivity of La0.9Sr0.1O0.45F2. The present elucidation of the crystal structure and anion-diffusion mechanism might provide useful knowledge for the design of superior anion conductors, which develop the next-generation fluoride-ion batteries and solid oxide fuel cells.
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U2 - 10.1021/acsaem.1c01097
DO - 10.1021/acsaem.1c01097
M3 - Article
AN - SCOPUS:85114293895
VL - 4
SP - 8891
EP - 8900
JO - ACS Applied Energy Materials
JF - ACS Applied Energy Materials
SN - 2574-0962
IS - 9
ER -