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

Abstract

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu_4.39Al_1.61 to its cubic 2/1 crystalline approximant (CA) Ca₁₃Au_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_17.14⟂_3.00] and an icosahedral shell of only Al [Al_10.5⟂_1.5]. 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, “Ca₂₄Au₈₈Al₆₄” (CaAu_3.67Al_2.67) is proposed.

Authors:

Pham, Joyce  ^[1];

• Search DOE PAGES for author "Pham, Joyce"
• Search DOE PAGES for ORCID "0000-0002-5003-7114"

Meng, Fanqiang  ^[2];

• Search DOE PAGES for author "Meng, Fanqiang"
• Search DOE PAGES for ORCID "0000-0002-8677-8985"

Lynn, Matthew J. ^[2]; Ma, Tao ^[2]; Kreyssig, Andreas ^[3]; Kramer, Matthew J. ^[2]; Goldman, Alan I. ^[3]; Miller, Gordon J.  ^[4]

• Search DOE PAGES for author "Miller, Gordon J."
• Search DOE PAGES for ORCID "0000-0001-5717-8000"

---

+ Show Author Affiliations

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:

Fri Dec 29 00:00:00 EST 2017

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

OSTI Identifier:

1425483

Report Number(s):

IS-J-9592
Journal ID: ISSN 0002-7863; TRN: US1802115

Grant/Contract Number:  

AC02-07CH11358; AC02-06CH11357

Resource 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)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY