TY - JOUR
T1 - Crystal structure of η″-Fe3Al7+x determined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy
AU - Okamoto, Norihiko L.
AU - Higashi, Masaya
AU - Inui, Haruyuki
N1 - Funding Information:
This work was supported by JSPS KAKENHI [grant numbers 15H02300, 16K14415, 16H04516, and 16K14373], and the Elements Strategy Initiative for Structural Materials (ESISM) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. It was supported in part by Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST). It was also supported by JKA and its promotion funds from KEIRIN RACE and Program for Advanced Technology, Kansai Research Foundation for Technology Promotion, as well as the 24th Research Promotion Grant from the Iron and Steel Institute of Japan (ISIJ). The synchrotron radiation experiments were performed at the BL02B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal Nos. 2016B1096, 2017A1243, and 2019A1568).
Publisher Copyright:
© 2019, © 2019 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group.
PY - 2019/12/31
Y1 - 2019/12/31
N2 - The crystal structure of η″-Fe3Al7+x, the low-temperature phase of η-Fe2Al5 with a composition on the Fe-rich side of the solid solubility range, has been determined by synchrotron X-ray single-crystal diffraction combined with scanning transmission electron microscopy. The η″ phase possesses commensurate long-period-ordered superlattice structures (space group Pmcn) based on the parent orthorhombic unit cell of η-Fe2Al5, consisting of twin domains (orientation variants) alternately stacked along the long-periodicity axis. Each of the twin domains possesses a motif structure belonging to the base-centered monoclinic space group C2/m, with a cell volume twice that of the parent orthorhombic unit cell (space group Cmcm). One-fourth of the c-axis chain sites corresponding to Al2- and Al3-sites in the η phase are respectively occupied by both Fe and Al atoms and exclusively by Al atoms in a regular manner. This regularity is disturbed in the twin-boundary region, giving rise to structural/compositional modulation. Because of the different chemical compositions between the motif structure and twin-boundary region, the η″ phase with various compositions can be constructed only by changing the number of the parent orthorhombic unit cells to be stacked along the orthorhombic c-axis, without changing the atomic arrangements for the motif structure or the twin boundary to account for the observed solid solubility range. The chemical formula of the η″ phase can thus be expressed as Fe3Al7+x under a simple assumption on the occupancies for Al/Fe atoms in the c-axis chain sites.
AB - The crystal structure of η″-Fe3Al7+x, the low-temperature phase of η-Fe2Al5 with a composition on the Fe-rich side of the solid solubility range, has been determined by synchrotron X-ray single-crystal diffraction combined with scanning transmission electron microscopy. The η″ phase possesses commensurate long-period-ordered superlattice structures (space group Pmcn) based on the parent orthorhombic unit cell of η-Fe2Al5, consisting of twin domains (orientation variants) alternately stacked along the long-periodicity axis. Each of the twin domains possesses a motif structure belonging to the base-centered monoclinic space group C2/m, with a cell volume twice that of the parent orthorhombic unit cell (space group Cmcm). One-fourth of the c-axis chain sites corresponding to Al2- and Al3-sites in the η phase are respectively occupied by both Fe and Al atoms and exclusively by Al atoms in a regular manner. This regularity is disturbed in the twin-boundary region, giving rise to structural/compositional modulation. Because of the different chemical compositions between the motif structure and twin-boundary region, the η″ phase with various compositions can be constructed only by changing the number of the parent orthorhombic unit cells to be stacked along the orthorhombic c-axis, without changing the atomic arrangements for the motif structure or the twin boundary to account for the observed solid solubility range. The chemical formula of the η″ phase can thus be expressed as Fe3Al7+x under a simple assumption on the occupancies for Al/Fe atoms in the c-axis chain sites.
KW - 10 Engineering and Structural materials
KW - 106 Metallic materials
KW - 212 Surface and interfaces
KW - 302 Crystallization / Heat treatment / Crystal growth
KW - 503 TEM, STEM, SEM
KW - 504 X-ray / Neutron diffraction and scattering
KW - Intermetallic compound
KW - icosahedron
KW - nanoscale twins
KW - orientation variants
KW - superlattice structure
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U2 - 10.1080/14686996.2019.1613174
DO - 10.1080/14686996.2019.1613174
M3 - Article
AN - SCOPUS:85076056654
VL - 20
SP - 543
EP - 556
JO - Science and Technology of Advanced Materials
JF - Science and Technology of Advanced Materials
SN - 1468-6996
IS - 1
ER -