| Sumario: | The irreversible transformation from an icosahe-dral quasicrystal (i-QC) CaAu[sub 439]Al[sub 161] to its cubic 2/1 crystalline approximant (CA) Ca[sub 13]Au[sub 56 31]([sub 3])Al[sub 2169] (CaAu[sub 433]([sub 1])Al[sub 167], Pa3 (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[sub 59].[sub 86]([sub 2])Al[sub 17].[sub 14]0[sub 3].[sub 00]] and an icosahedral shell of only Al [Al[sub 105-15]]. 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, "Ca24Au[sub 88]Al64" (CaAu3.67Al2.67) is proposed.
|
|---|