Development of the Ca−Au−In Icosahedral Quasicrystal and Two Crystalline Approximants: Practice via Pseudogap Electronic Tuning

Electronic tuning and syntheses to gain the icosahedral quasicrystal (i-QC) (Ca14.1(2)Au44.2(8)In41.7(7), e/a = 1.98) and two approximant crystals (ACs) are reported. The tuning was derived from Na2Au6In5, another cubic Mg2Zn11-type structure, for which the Fermi level (e/a = 1.77) should tune to a
calculated pseudogap (e/a = 2.02) under a rigid band assumption. The 1/1 AC, Ca3Au12.2(1)In6.3(2) (e/a =
1.73), crystallizes in space group Im3̄, with a = 15.152(2) Å, Z = 8, and the 2/1 AC, Ca12.6(1)Au37.0(2)In39.6(6)
(e/a = 2.01), in Pa3̄, with a = 24.632(3) Å , Z = 8. Both have substantially fixed compositions according
to lattice dimensions. Structure analyses reveal that both ACs contain triacontahedral clusters as the basic
building blocks at the body-centered and primitive cubic unit cell levels, respectively. Densities-of-states
(DOS) analyses for the 1/1 AC structure reveal a pseudogap at e/a = 2.00, close to the point at which the
i-QC was predicted and experimentally tuned. Phase relationships of the ACs and the i-QC are reported
according to DTA, XRD, and temperature-dependent XRD measurements. The QC is thermodynamically
metastable below ∼500 °C.