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) CaAuAl to its cubic 2/1 crystalline approximant (CA) CaAuAl (CaAuAl, 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 [AuAl□] and an icosahedral shell of only Al [Al□]. 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, "CaAuAl" (CaAuAl) is proposed.

Metallic alloys at the edge of complexity: structural aspects, chemical bonding and physical properties*

“…Initial studies showed that these three types are quite well separated electronically based on valence electron count, showing just a minor overlap for the B and T types (1.9-2.1 e/a). However, recent discoveries proved the almost complete overlap of their areas of existence [103,108]. On the other hand, the valence electron count for the transition metals has always been in part controversial leading to somewhat arbitrary numbers.…”

Chemical Bonding and Physical Properties in Quasicrystals and Their Related Approximant Phases: Known Facts and Current Perspectives

“…There are several hints signaling the importance of short-range effects in the emergence of some unusual physical properties of QCs. For instance, electrical conductivity measurements show that (i) amorphous precursors of i-AlCuFe phase already exhibit an increase of conductivity with temperature, and (ii) the structural evolution from the amorphous to the quasicrystalline state is accompanied by a progressive enhancement of the electronic transport anomalies (Figure 6a) [40,41]. It is worthy noticing that the σ(T) curves reported for both amorphous and QC phases are almost parallel to each other, hence displaying the inverse Matthiessen rule (see Figure 2a).…”

Superconducting YAu3Si and Antiferromagnetic GdAu3Si with an Interpenetrating Framework Structure Built from 16-Atom Polyhedra

“…Polar intermetallic compounds of gold with electropositive and post-transition metals/semimetals from groups 12–14 display peculiarities in their structural chemistry and physical properties, which has been attributed to the extraordinarily high electronegativity of gold (which is the highest among metallic elements) and associated relativistic effects in chemical bonding . The family of gold polar intermetallics is rather diverse and also includes icosahedral quasicrystals (iQCs), such as i-Na–Au–Ga, i-Ca–Au–Al­(Ga)­(In), i-RE–Au–Al (RE = Yb and Tm), i-RE–Au–Sn (RE = Ca and Yb), and a larger range of 1/1 cubic approximant crystal (AC) phases. − …”