M₃(Au,Ge)₁₉ and M_3.25(Au,Ge)₁₈ (M = Ca, Yb): Distinctive Phase Separations Driven by Configurational Disorder in Cubic YCd₆-Type Derivatives

Exploratory syntheses in the M−Au−Ge (M = Ca, Yb) systems have led to the discovery of two cleanly separated non-stoichiometric phases M3Au∼14.4Ge∼4.6 (I) and M3.25Au∼12.7Ge∼5.3 (II). Single crystal X-ray studies reveal that both (space group Im3̅) feature body-centered-cubic packing of five-shell multiply endohedral clusters that resemble those in the parent YCd6 (= Y3Cd18) and are akin to approximate phases in other quasicrystal systems. However, differences resulting from various disorders in these are distinctive. The innermost cluster in the M3Au∼14.4Ge∼4.6 phase (I) remains a disordered tetrahedron, as in the YCd6 parent. In contrast, its counterpart in the electron-richer M3.25Au∼12.7Ge∼5.3 phase (II) is a “rattling” M atom. The structural differentiations between I and II exhibit strong correlations between lattice parameters, cluster sizes, particular site occupancies, and valence electron counts

Electronic Tuning of Mg₂Cu₆Ga₅. A Route to Crystalline Approximant and Quasicrystalline Phases

Studies of Mg2Cu6Ga5 reveal that this compound contains incomplete Bergman clusters in its structure and shows a pseudogap and empty bonding states just above the Fermi energy according to band calculations. Under a rigid band assumption, such a compound may be tuned to approximant and quasicrystal phases in which the required number of electrons are attained. Here, we replace part of Mg in the isotypic Mg2Cu6Ga5 with Sc, and both 1/1 approximant and icosahedral quasicrystal phases are obtained after some fine-tuning. This method closely correlates the pseudogap and bonding with Hume−Rothery concepts, thus giving useful directions for future quasicrystal searches, especially when approximants are not known

Intermetallic Compounds with 1D Infinite Tunnels. Syntheses and Structures of AAu₄In₂ (A = K, Rb)

Four intermetallic compounds KxRb1-xAu4In2 (x = 0−1) synthesized by high-temperature solid-state reactions display 1D infinite tunnel constructions with Au−In frameworks. These compounds show small but different cation displacements in the tunnels and are also air and moisture inert at room temperature, even in concentrated HCl(aq)

Remarkable Metal-Rich Ternary Chalcogenides Sc₁₄M₃Te₈ (M = Ru, Os)

In this novel motif, scandium atoms define infinite parallel chains of alternate trans-face-sharing cubes and pairs of square antiprisms in which each polyhedron is also centered by an M atom (M = Ru, Os). These chains are further linked into a three-dimensional structure by Sc(Te2Te4/2) octahedra. Physical property measurements show Sc14Ru3Te8 to be metallic and Pauli-paramagnetic, consistent with the results of extended Hückel band structure calculations. Matrix effects are evident in the dimensions within the chains. The major interactions are Sc−M and Sc−Te

Synthesis and Structure of K₃Mg₂₀In₁₄, a Stuffed Variant of the BaHg₁₁ Structure Type with a Magnesium−Indium Network

The phase K3Mg20In14 was synthesized via high-temperature reactions of the elements in welded Ta tubes. The cubic crystal structure established by single-crystal X-ray diffraction means [space group Pm3̄m, Z = 1, a = 9.769(1) Å] features a 3D Mg−In network formed by K@Mg12In10 units plus cuboctahedral fillers, In@Mg12. This is the first example of a well-ordered stuffed BaHg11 structure (Pearson symbol cP37). On the basis of tight-binding linear muffin-tin orbital, atomic sphere approximation calculations, the electronic structure of the compound shows dominant Mg−In interactions and substantial participation of Mg in the overall network bonding. Both In−In and Mg−In bondings are effectively optimized at the Fermi level. The Fermi energy cuts through substantial densities of states, consistent with the measured metallic property

R₆TT‘₂, New Variants of the Fe₂P Structure Type. Sc₆TTe₂ (T = Ru, Os, Rh, Ir), Lu₆MoSb₂, and the Anti-typic Sc₆Te_0.80Bi_1.68

The Fe2P structure (P6̄2m) features two 3-fold Fe positions and both 2-fold and 1-fold P sites, and variations in occupancies of the latter pair yield the reported diversity of results. The known Sc6TTe2 examples for T = Fe−Ni are herein extended to four heavier transition metal T derivatives. An attempt to synthesize bismuth analogues led to the novel inverse derivative in which fractional Te (vice T) occupies the smaller tricapped trigonal prismatic (TTP) Sc polyhedron, and Bi rather than Te occurs in the larger TTP of Sc, with parallel reversal of polarity in the bonding. The reported Lu8Te, which is distributed as Lu6TeLu2, is the only example in which a transition metal occupies the normal 2-fold P or Te non-metal position, with corresponding large effects on the bonding. Lutetium otherwise does not form R6TTe2 analogues, but the novel Lu6MoSb2 isotype occurs instead. Extended Hückel calculations are presented for five examples, and the structural and bonding regularities and varieties are discussed further

Remarkable Metal-Rich Ternary Chalcogenides Sc₁₄M₃Te₈ (M = Ru, Os)

In this novel motif, scandium atoms define infinite parallel chains of alternate trans-face-sharing cubes and pairs of square antiprisms in which each polyhedron is also centered by an M atom (M = Ru, Os). These chains are further linked into a three-dimensional structure by Sc(Te2Te4/2) octahedra. Physical property measurements show Sc14Ru3Te8 to be metallic and Pauli-paramagnetic, consistent with the results of extended Hückel band structure calculations. Matrix effects are evident in the dimensions within the chains. The major interactions are Sc−M and Sc−Te

Gold Derivatives of Eight Rare-Earth-Metal-Rich Tellurides: Monoclinic R₇Au₂Te₂ and Orthorhombic R₆AuTe₂ Types

Two series of rare-earth-metal (R) compounds, R7Au2Te2 (R = Tb, Dy, Ho) and R6AuTe2 (R = Sc, Y, Dy, Ho, Lu), have been synthesized by high-temperature techniques and characterized by X-ray diffraction analyses as monoclinic Er7Au2Te2-type and orthorhombic Sc6PdTe2-type structures, respectively. Single-crystal diffraction results are reported for Ho7Au2Te2, Lu6AuTe2, Sc6Au0.856(2)Te2, and Sc6Au0.892(3)Te2. The structure of Ho7Au2Te2 consists of columns of Au-centered tricapped trigonal prisms (TCTPs) of Ho condensed into 2D zigzag sheets that are interbridged by Te and additional Ho to form the 3D network. The structure of Lu6AuTe2 is built of pairs of Au-centered Lu TCTP chains condensed with double Lu octahedra in chains into 2D zigzag sheets that are separated by Te atoms. Tight binding–linear muffin-tin orbital–atomic sphere approximation electronic structure calculations on Lu6AuTe2 indicate a metallic property. The principal polar Lu–Au and Lu–Te interactions constitute 75% of the total Hamilton populations, in contrast to the small values for Lu–Lu bonding even though these comprise the majority of the atoms. A comparison of the theoretical results for Lu6AuTe2 with those for isotypic Lu6AgTe2 and Lu6CuTe2 provides clear evidence of the greater relativistic effects in the bonding of Au. The parallels and noteworthy contrasts between Ho7Au2Te2 (35 valence electrons) and the isotypic but much electron-richer Nb7P4 (55 valence electrons) are analyzed and discussed

Approximant Phases and an Icosahedral Quasicrystal in the Ca−Au−Ga System: The Influence of Size of Gallium versus Indium

Two crystalline approximants (ACs) and their corresponding icosahedral quasicrystal (i-QC) are obtained in the Ca−Au−Ga system through conventional solid-state exploratory syntheses. Single crystal structural analyses reveal that the 1/1 AC, Ca3AuxGa19-x (x = ∼ 9.3−12.1) [Im3̅, a = 14.6941(6)−14.7594(6) Å], has the empty cubes in the prototypic YCd6 (= Y3Cd18) now fully occupied by Ga, resulting in a 3:19 stoichiometry. In parallel, the distorted cubes in the 2/1 AC, Ca13Au57.1Ga23.4 [Pa3̅, a = 23.9377(8) Å] are fully or fractionally occupied by Ga. The valence electron count per atom (e/a) for the 2/1 AC (1.64) is smaller than that over the 1/1 AC composition range (1.76−2.02), and the e/a of the Ca15.2Au50.3Ga34.5 i-QC, 1.84, is somewhat distant from typical values for Tsai-type i-QCs (∼ 2.0). Comparisons of the gallium results with the corresponding In phases suggest that the structural differences result mainly from size rather than electronic factors. The 1/1 and 2/1 appear to be thermodynamically stable on slow cooling, as usual, whereas the i-QC isolated by quenching decomposes on heating at ∼660 °C, mainly into 2/1 AC and Ca3(Au,Ga)11. Calculations of the electronic structure of 1/1 AC suggest that the Fermi sphere−Brillouin zone interactions remain important for the Ca−Au−Ga i-QC

The 1/1 and 2/1 Approximants in the Sc−Mg−Zn Quasicrystal System: Triacontahedral Clusters as Fundamental Building Blocks

Single-crystal structures are reported for Sc3Mg0.18(1)Zn17.73(3), the 1/1 approximant crystal (AC), and Sc11.18(9)Mg2.5(1)Zn73.6(2), the 2/1 AC, in the corresponding icosahedral quasicrystal (i-QC) system. The 1/1 AC crystallizes in space group Im3̄, a = 13.863(2) Å, Z = 8, and the 2/1 AC, in Pa3̄, a = 22.412 (2) Å, Z = 8. The latter, which is valuable in pointing the way to the QC structure, is the best ordered and refined 2/1 example to date. The fundamental building blocks in both ACs are triacontahedral clusters centered by smaller multiply endohedral Tsai-type arrays; the former are condensed through body-centered-cubic packing in the 1/1 and primitive cubic packing in the 2/1 AC. Novel prolate rhombohedra centered by Sc−Sc dimers are also generated between triacontahedra in the 2/1 AC