Crystal structure of η″-Fe3Al7+x determined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy

Norihiko L. Okamoto, Masaya Higashi, Haruyuki Inui

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)543-556
Number of pages14
JournalScience and Technology of Advanced Materials
Issue number1
Publication statusPublished - 2019 Dec 31


  • 10 Engineering and Structural materials
  • 106 Metallic materials
  • 212 Surface and interfaces
  • 302 Crystallization / Heat treatment / Crystal growth
  • 503 TEM, STEM, SEM
  • 504 X-ray / Neutron diffraction and scattering
  • Intermetallic compound
  • icosahedron
  • nanoscale twins
  • orientation variants
  • superlattice structure

ASJC Scopus subject areas

  • Materials Science(all)


Dive into the research topics of 'Crystal structure of η″-Fe<sub>3</sub>Al<sub>7+x</sub> determined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy'. Together they form a unique fingerprint.

Cite this