Exploratory syntheses and structures of SrAu(4.3)In(1.7) and CaAg(3.5)In(1.9): electron-poor intermetallics with diversified polyanionic frameworks that are derived from the CaAu4In2 approximant

The phase regions around quasicrystals and approximants (QC/ACs) are rich pools for electron-poor intermetallics with novel, complex structures, and bonding patterns. The present SrAu(4.30(1))In(1.70(1)) (1) and CaAg(3.54(1))In(1.88(1)) (2) were synthesized through chemical tunings of the model CaAu(4)In(2) (YCd(6)-Type) AC. Single crystal X-ray diffraction analyses reveals that crystal 1 has Pnma (CeCu(6)-type) symmetry, with a = 9.102(1) Å, b = 5.6379(9) Å, and c = 11.515(2) Å. The building block in 1 is a 19-vertex cluster Sr@Au(9)In(4)M(6) (M = Au/In), which vividly mimics Ca@(Au,In)(18) in Ca(3)Au(12.4)In(6.1) (YCd(6)-type) in geometry. These clusters aggregate into one-dimensional columns extending along the b axis. Crystal 2 (P6/mmm, a = 20.660(3) Å, c = 9.410(2) Å) is closely related to Na(26)Cd(141) (hP167) and Y(13)Pd(40)Sn(31) (hP168), which are differentiated by the selective occupation of Wyckoff 1a (0 0 0) or 2d (1/3 2/3 1/2) sites by Cd or Pd. Crystal 2 adopts the Na(26)Cd(141) structure, but the 1a site is split into two partially occupied sites. The synergistic disorder in the hexagonal tunnels along c is a major property. The valence electron count per atom (e/a) values for 1 and 2 are 1.63 and 1.74, respectively, the lowest among any other ternary phases in each system. These values are close to those of ACs in the Ca-Au-M (M = Ga, In) systems. Electronic structures for both are discussed in terms of the results of TB-LMTO-ASA calculations.