Zintl-Phase Sr3LiAs2H: Crystal Structure and Chemical Bonding Analysis by the Electron Localizability Approach

The untypical high-pressure Zintl phase SrGe6

The binary strontium germanide SrGe6 was synthesized at high-pressure high-temperature conditions of approximately 10 GPa and typically 1400 K before quenching to ambient conditions. At ambient pressure, SrGe6 decomposes in a monotropic fashion at T = 680(10) K into SrGe2 and Ge, indicating its metastable character. Single-crystal X-ray diffraction data indicate that the compound SrGe6 adopts a new monoclinic structure type comprising a unique three-dimensional framework of germanium atoms with unusual cages hosting the strontium cations. Quantum chemical analysis of the chemical bonding shows that the framework consists of three- and four- bonded germanium atoms yielding the precise electron count Sr[(4bGe0]4[(3b)Ge−]2 in accordance with the 8 − N rule and the Zintl concept. Conflicting with that, a pseudo-gap in the electronic density of states appears clearly below the Fermi level, and elaborate bonding analysis reveals additional Sr–Ge interactions in the concave coordination polyhedron of the strontium atoms.

In-Cage Interactions in the Clathrate Superconductor Sr8 Si46

The clathrate I superconductor Sr₈Si₄₆ is obtained under high‐pressure high‐temperature conditions, at 5 GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heating at T=796(5) K into Si and SrSi₂. The crystal structure of the clathrate is isotypic to that of Na₈Si₄₆. Chemical bonding analysis reveals conventional covalent bonding within the silicon network as well as additional multi‐atomic interactions between Sr and Si within the framework cages. Physical measurements indicate a bulk BCS type II superconducting state below T_c=3.8(3) K.

Modulated vacancy ordering in SrGe6− x (x≈0.45)

The structural properties of modulated SrGe6− x (x≈0.45) were investigated by means of single-crystal and powder X-ray diffraction combined with quantum chemical calculations. The framework compound SrGe6− x adopts a defect variant of the EuGa2Ge4-type crystal structure. Samples of the binary compound with nominal compositions 0≤x≤0.5 were synthesized at pressures from 5 to 6 GPa and a temperature of typically 1400 K. The product reveals diffraction peaks of the EuGa2Ge4-type subcell plus additional reflections indicating an ordered superstructure. Detailed crystal structure analysis evidences the incommensurate nature of the superstructure and a modulation of the vacancy ordering in the germanium network. The computations have shown that the non-stoichiometric composition of the framework with its local defect organization affects the calculated charge of the strontium anions. Although the chemical composition is close to a charge-balanced situation, temperature-dependent resistivity measurements showed metal-type conductivity. At ambient pressure SrGe6− x decomposes exothermally and irreversibly at T=680(10) K into SrGe2 and germanium, indicating its metastable nature at ambient pressure.

Cluster Formation in the Superconducting Complex Intermetallic Compound Be21Pt5

Materials with the crystal structure of γ-brass type (Cu₅Zn₈ type) are typical representatives of intermetallic compounds. From the electronic point of view, they are often interpreted using the valence electron concentration approach of Hume-Rothery, developed previously for transition metals. The γ-brass-type phases of the main-group elements are rather rare. The intermetallic compound Be₂₁Pt₅, a new member of this family, was synthesized, and its crystal structure, chemical bonding, and physical properties were characterized. Be₂₁Pt₅ crystallizes in the cubic space group F4̅3m with lattice parameter a = 15.90417(3) Å and 416 atoms per unit cell. From the crystallographic point of view, the binary substance represents a special family of intermetallic compounds called complex metallic alloys (CMA). The crystal structure was solved by a combination of synchrotron and neutron powder diffraction data. Besides the large difference in the scattering power of the components, the structure solution was hampered by the systematic presence of very weak reflections mimicking wrong symmetry. The structural motif of Be₂₁Pt₅ is described as a 2 × 2 × 2 superstructure of the γ-brass structure (Cu₅Zn₈ type) or 6 × 6 × 6 superstructure of the simple bcc structural pattern with distinct distribution of defects. The main building elements of the crystal structure are four types of nested polyhedral units (clusters) with the compositions Be₂₂Pt₄ and Be₂₀Pt₆. Each cluster contains four shells (4 + 4 + 6 + 12 atoms). Clusters with different compositions reveal various occupation of the shells by platinum and beryllium. Polyhedral nested units with the same composition differ by the distance of the shell atoms to the cluster center. Analysis of chemical bonding was made applying the electron localizability approach, a quantum chemical technique operating in real space that is proven to be especially efficient for intermetallic compounds. Evaluations of the calculated electron density and electron localizability indicator (ELI-D) revealed multicenter bonding, being in accordance with the low valence electron count per atom in Be₂₁Pt₅. A new type of atomic interactions in intermetallic compounds, cluster bonds involving 8 or even 14 atoms, is found in the clusters with shorter distances between the shell atoms and the cluster centers. In the remaining clusters, four- and five-center bonds characterize the atomic interactions. Multicluster interactions within the polyhedral nested units and three-center polar intercluster bonds result in a three-dimensional framework resembling the structural pattern of NaCl. Be₂₁Pt₅ is a diamagnetic metal and one of rather rare CMA compounds revealing superconductivity (T_c = 2.06 K).