An Icosahedral Quasicrystal and Its 1/0 Crystalline Approximant in the Ca–Au–Al System

A new icosahedral quasicrystalline phase, CaAu_4.5–xAl_1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu_4.4Al_1.6, a_QC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu_3+xAl_1–x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)–9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca–Au–Al system. In the crystalline approximant, eight [Au_3–xAl_1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca_4/4[Au_3–xAl_1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au–Al interactions over Au–Au and Al–Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu_4.5–xAl_1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu_3+xAl_1–x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au_3–xAl_1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.