Mixed Cations and Structural Complexity in (Eu_1−xCa_x)₄In₃Ge₄ and (Eu_1−xCa_x)₃In₂Ge₃—The First Two Members of the Homologous Series A_2[n+m]In_2n+mGe_2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

Abstract

Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu(1-x)Ca(x))(4)In(3)Ge(4) (0.35(1) <or= x <or= 0.70(1)) and (Eu(1-x)Ca(x))(3)In(2)Ge(3) (0.78(1) <or= x <or= 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu(1-x)Ca(x))(4)In(3)Ge(4) adopts the monoclinic Mg(5)Si(6)-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)-17.024(2) A, b = 4.496(3)-4.556(1) A, c = 7.473(4)-7.540(1) A, and beta = 107.306(10)-105.631(3) degrees . (Eu(1-x)Ca(x))(3)In(2)Ge(3) crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)-7.4010(9) A, b = 4.452(1)-4.4640(6) A, and c = 23.684(6)-23.734(3) A, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe(4) edge-shared tetrahedra, Ge(2) dimers, and bridging In atoms in a nearly square-planar environment. The (Eu(1-x)Ca(x))(4)In(3)Ge(4) structure can be viewed as a 1:1 intergrowth of Mo(2)FeB(2)-like and TiNiSi-like fragments, whereas (Eu(1-x)Ca(x))(3)In(2)Ge(3) can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.

Synthesis and structural characterization of A3In2Ge4 and A5In3Ge6 (A=Ca, Sr, Eu, Yb)—New intermetallic compounds with complex structures, exhibiting Ge–Ge and In–In bonding

“…All In-Ge and Ge-Ge bond distances in (Sr 1-x Ca x ) 5 In 3 Ge 6 increase slightly as the composition becomes Sr-richer. These observations are in a good agreement with the earlier reports on the (Eu 1-x Ca x ) 4 In 3 Ge 4 and (Eu 1-x Ca x ) 3 In 2 Ge 3 series [12], where similar structural motifs are present as well. Just like them, the A 5 In 3 Ge 6 phase is stabilized by mixtures of two types of cations -Sr and Ca or Eu and Yb -which are chemically similar, but spatially different species.…”

“…As can be seen, the Ge-Ge COHP curve shows an antibonding character at the Fermi level. This type of antibonding character was also observed from Ge 2 -dimers in the (Eu 1-x Ca x ) 4 In 3 Ge 4 and (Eu 1-x Ca x ) 3 In 2 Ge 3 phases [12]. As discussed therein, this antibonding character is caused by the 3-bonded Ge atom in a trigonal planar local environment, instead of being in a preferred pyramidal, as observed in the corrugated [Ge 2 ] 2-layers in EuGe 2 and CaGe 2 [14,16].…”

“…The title compounds were discovered in the attempts to substitute Eu or Ca with other rare-earth or alkaline-earth metals, following the optimized synthetic scheme, previously employed for growing crystals of (Eu 1-x Ca x ) 4 In 3 Ge 4 and (Eu 1-x Ca x ) 3 In 2 Ge 3 [12]. This procedure involved heating the mixture of elements to temperature 1223 K at a rate of 601/h, allowed to homogenize for 10-12 h, and followed by a slow cooling to 773 K at rates of 3-101/ h. Then, the silica ampoules were taken out from a furnace and instantly centrifuged to remove the molten flux, allowing for the isolation of small needle-shaped crystals with silver luster.…”

“…We have already reported on several new phases, which were synthesized for the first time and structurally characterized: (Eu 1-x Ca x ) 4 In 3 Ge 4 (0.3rx r0.7), (Eu 1-x Ca x ) 3 In 2 Ge 3 (0.7rx r0.9), and (Ca 1-x Yb x ) 8 In 5 Ge 8 (xE 0.4) [12]. Notably, these new phases were found to exist in mixed-cation systems only.…”

Bonding Schemes for Polar Intermetallics through Molecular Orbital Models: Ca-Supported Pt–Pt Bonds in Ca10Pt7Si3

“…At the other extreme are the Hume-Rothery phases in which the low Δχ and χ values result in the vanishing of clear charge assignments or localized bonds [3]. The divide between the Zintl and Hume-Rothery has inspired the synthetic probing of their border for new phases with unique bonding and structures [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A rich structural chemistry of polar intermetallic phases has resulted, which offers opportunities to chart how the close-shell electron configurations of the Zintl phases map to the delocalized bonding of the Hume-Rothery phases.…”