Ca10Pt7Tt3 (Tt = Si, Ge): New Platinide Phases Featuring Electron-Rich 4c–6e Bonded [Pt7Tt3]20– Intermetalloid Clusters

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4

The new phase Mg₇Pt₄Ge₄ (≡Mg₈□₁Pt₄Ge₄; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg₂PtSi (≡Mg₈Pt₄Si₄), reported in the Li₂CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg₇Pt₄Ge₄. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg₂PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg₂PtGe" reveal potential electronic instabilities at E_F in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg₃Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Na3Pt10Si5: A Non-Centrosymmetric Superconductor Having Rattling Na Atoms in the Tunnel Framework Structure

Single crystals of a novel Na-Pt-Si ternary compound, Na₃Pt₁₀Si₅, were synthesized by heating the constituent elements at 1423 K. It crystallizes in a non-centrosymmetric trigonal structure of space group R32 (Z = 3) with lattice constants of a = 10.1536(3) Å and c = 10.1539(3) Å at 300 K. The structure consists of a three-dimensional framework made of Pt and Si atoms, and the Na atoms are contained in the tunnels of the framework. The large magnitude and the temperature dependence of the atomic displacement parameter of the Na site reveal a large thermal vibration indicative of a "rattling" motion of Na atoms in the oversized tunnel. The electronic structure calculations explain the observed metallic properties on the basis of the covalent bonds between the Pt and Si atoms in the framework and the ionic bonding of the Na atoms to the framework. A type II superconductivity with a transition temperature of 2.9 K and an upper critical field of 2.5 kOe are observed for a polycrystalline sintered bulk sample of Na₃Pt₁₀Si₅ prepared by heating at 1353 K in Na vapor. Heat capacity measurements reveal a strong coupling superconductivity that is probably caused by an electron-phonon interaction enhanced by the rattling motion of the Na atoms.

Cation Clustering in Intermetallics: The Modular Bonding Schemes of CaCu and Ca2Cu

Electropositive metals such as the alkaline earths or lanthanides are generally assumed to act largely as spectator cations in solid state compounds. In polar intermetallic phases, atoms of such elements are indeed often placed at the peripheries of anions or polyanionic fragments. However, they also show a pronounced tendency to cluster with each other in these peripheral regions in a manner suggestive of multicenter bonding. In this Article, we theoretically investigate the bonding schemes that underlie these cationic cluster arrangements, focusing on CaCu (whose two polymorphs are based on the intergrowth of the FeB- and CrB-types) and Ca₂Cu (a Ca-intercalated derivative of CaCu). The structures of these phases are based on Cu zigzag chains embedded in matrices of Ca atoms arranged into increasingly well-developed fragments of closest-packed arrangements. Using reversed approximation Molecular Orbital (raMO) analysis, the Cu chains of both structures are revealed to be connected via nearly fully occupied Cu-Cu isolobal σ-bonds, such that the Cu atoms control 11.67 of the 13 and 15 electrons/formula unit of CaCu and Ca₂Cu, respectively. Most of the remaining electrons are drawn to multicenter bonding functions in the Ca sublattices despite the availability of additional Cu 4p orbitals, indicating that the electronegativity difference between Ca and Cu is insufficient to achieve formal Cu oxidation states far beyond -1. The metallic nature of the Ca-based bonding subsystem is reflected in the raMO analysis by a plurality of resonance structures that can be generated from the occupied crystal orbitals. Across these bonding schemes, a separation of the electronic structure into largely self-contained Ca-Ca and Ca-Cu states is a consistent theme. This modularity in the bonding can be correlated to the ease with which this and related systems rearrange FeB- and CrB-type features, which may provide clues to identifying other intermetallic families with similar degrees of structural versatility.

Exo-bonded six-membered heterocycle in the crystal structures of RE7Co2Ge4(RE = La–Nd)

The stability of the heterocyclic {Co₄Ge₆} clusters in RE₇Co₂Ge₄(RE = La–Nd) is determined by strong interactions with the surrounding RE atoms in the structures.

Synthesis, crystal structure, and bonding analysis of the hypoelectronic cubic phase Ca5Pd6Ge6

The title compound, Ca5Pd6Ge6, was obtained during a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The crystal structure was determined and refined from single-crystal X-ray diffraction data. It crystallizes in a new structure variant of the Y4PdGa12-type structure (Im3̅m, a = 8.7764(4) Å) that features an arrangement of vertex-sharing body-centered cubes of calcium, Ca@Ca8, with a hierarchical bcc network, interpenetrating a second (Pd6Ge6) network consisting of Ge2 dumbbells surrounded by Pd in a strongly flattened octahedron with Pd(μ(2)-η(2),η(4)-Ge2)-like motifs. These octahedra are condensed through the Pd to form a 3D open fcc network. Theoretical band structure calculations suggested that the compound is hypoelectronic with predominantly multicenter-type interatomic interactions involving all three elements and essentially a Hume-Rothery-like regime of electronic stabilization. The similar electronegativity between germanium and palladium atoms has a decisive impact on the bonding picture of the system.

Synthesis, structure, and superconductivity in the new-structure-type compound: SrPt6P2

A metal-rich ternary phosphide, SrPt(6)P(2), with a unique structure type was synthesized at high temperatures. Its crystal structure was determined by single-crystal X-ray diffraction [cubic space group Pa3̅; Z = 4; a = 8.474(2) Å, and V = 608.51(2) Å(3)]. The structure features a unique three-dimensional anionic (Pt(6)P(2))(2-) network of vertex-shared Pt(6)P trigonal prisms. The Sr atoms occupy a 12-coordinate (Pt) cage site and form a cubic close-packed (face-centered-cubic) arrangement, and the P atoms formally occupy tetrahedral interstices. The metallic compound becomes superconducting at 0.6 K, as evidenced by magnetic and resistivity measurements.