Abstract
The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3̅ (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 [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□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. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed.
Highlights
From the conventional classification of solids using symmetry and atomic arrangements in real space, quasicrystals (QCs) occur between disordered amorphous materials and ordered, periodic crystalline solids because they are well-ordered but aperiodic, arising from their short-range, “crystallographically incompatible” five, seven, eight, or higher-order rotational symmetry.[1]
For the targeted compositions Ca1.00(4)Au4.50–xAl1.50(6)+x (0.11(6) ≤ x ≤ 0.44) (VEC = 1.60–1.70 e/a), a primitive icosahedral quasicrystal (i-QC) was produced from quenching, and its cubic 2/1 crystalline approximant (CA) was discovered from annealing
This i-QC and its cubic 2/1 CA belong to the general category of polar intermetallics in which the electronegative metals (Au + Al) share atomic shells and the formally electropositive Ca atoms form their own intervening shells to create significant polar-covalent Ca−(Au+Al) interactions for structural cohesion
Summary
From the conventional classification of solids using symmetry and atomic arrangements in real space, quasicrystals (QCs) occur between disordered amorphous materials and ordered, periodic crystalline solids because they are well-ordered but aperiodic, arising from their short-range, “crystallographically incompatible” five-, seven-, eight-, or higher-order rotational symmetry.[1]. Given the close chemical compositions and similar room-temperature PXRD patterns for the i-QC and 2/1 CA CaAu4.50−xAl1.50+x phases (Figure 1), in-situ high-energy, variable-temperature PXRD was carried out on the i-QC samples loaded as Ca13.2(5)Au55.0(1)Al23.8(7) and Ca12.8(5)Au52.6(1)Al25.2(7) (x = 0.32(6) and 0.40(6) in CaAu4.50– xAl1.50+x) to examine a possible structural evolution of the i-QC as a function of temperature.
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