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Title: From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca–Au–Al: In Situ Variable-Temperature Transformation

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu 4.39Al 1.61 to its cubic 2/1 crystalline approximant (CA) Ca 13Au 56.31(3)Al 21.69 (CaAu 4.33(1)Al1.67, Pa$$\bar{3}$$ (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ~570 °C and complete by ~650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au 59.86(2)Al 17.143.00] and an icosahedral shell of only Al [Al 10.51.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au–Al nearest neighbor contacts over homoatomic Al–Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. Finally, according to electronic structure calculations, a cubic 1/1 CA, “Ca 24Au 88Al 64” (CaAu 3.67Al 2.67) is proposed.
Authors:
ORCiD logo [1] ; ORCiD logo [2] ;  [2] ;  [2] ;  [3] ;  [2] ;  [3] ; ORCiD logo [4]
  1. Iowa State Univ., Ames, IA (United States). Dept. of Chemistry
  2. Ames Lab., Ames, IA (United States)
  3. Iowa State Univ., Ames, IA (United States). Dept. of Physics and Astronomy; Ames Lab., Ames, IA (United States)
  4. Iowa State Univ., Ames, IA (United States). Dept. of Chemistry; Ames Lab., Ames, IA (United States)
Publication Date:
Report Number(s):
IS-J-9592
Journal ID: ISSN 0002-7863
Grant/Contract Number:
AC02-07CH11358; AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 140; Journal Issue: 4; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Research Org:
Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
OSTI Identifier:
1425483