title: Centric and non-centric Ca3Au(approximately 7.5)Ge(approximately 3.5): electron-poor derivatives of La3Al11. Syntheses, structures, and bonding analyses.
abstract: Two La(3)Al(11) type derivatives have been discovered in the Ca-Au-Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca(3)Au(7.16(6))Ge(3.84(6)) (1) and Ca(3)Au(7.43(9))Ge(3.57(9)) (2) lie within a non-centric Imm2 phase region with a approximately 4.40 A, b approximately 13.06 A, c approximately 9.60 A. The Au-richer and electron-poorer Ca(3)Au(7.50(1))Ge(3.50(1)) (3) and Ca(3)Au(8.01(1))Ge(2.99(1)) (4) occur within a centric Pnnm phase region with a approximately 9.50 A, b approximately 13.20 A, c approximately 4.43 A. Both phases contain complex [Au,Ge](11)(6-) polyanionic networks made up of hexagonal and pentagonal prisms that are filled with the electropositive Ca atoms. Both 3:11 phases represent opposed 1 x 3 x 1 superstructure distortions of CaAu(2)Ge(2) (ThCr(2)Si(2) type, I4/mmm), the structure of which has also been re-determined in this work. Linear muffin-tin-orbital (LMTO) calculations reveal that the symmetry variations induced by changes of the Au contents in the present 3:11 phases are consequences of bonding and structural optimizations. The hypothetical "CaAu(2.33-2.67)Ge(1.33-1.00)cube(0.33)" compositions, which are close to those of 1-4, follow through creation and elimination of vacancies within the electronegative networks of CaAu(2)Ge(2).
authors.affiliation: Ames Laboratory, USDOE, and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Qisheng
authors.initials: Q
authors.lastname: Lin
authors.forename: John D
authors.initials: JD
authors.lastname: Corbett