Unconventional Metal-Framework Interaction in MgSi5

Electron Diffraction Tomography on Two-Phase Nanolamellae of Topochemically Synthesized Cu(Sb2S3)Cl

The dark red semiconductor Cu(Sb2S3)Cl was obtained by leaching the layered precursor Cu(Sb2S3)[AlCl4] in a 0.1 M aqueous HCl solution. The selective extraction of AlCl3 yielded a mica-like lamellar product of poor crystallinity. Misalignment of lamellae down to the nanoscale prevented structure determination by conventional single-crystal X-ray diffraction, but a combination of transmission electron microscopy, selected area electron diffraction, and selected area electron precession diffraction tomography on a nanoscale spot with largely ordered crystalline lamellae revealed the crystal structures of two intergrown modifications. Orthorhombic o-Cu(Sb2S3)Cl and monoclinic m-Cu(Sb2S3)Cl have similar layers to the precursor and differ only in the stacking of the layers. These consist of uncharged Sb2S3 strands, whose sulfide ions, together with chloride ions, coordinate the copper(I) cations. Only one chloride ion remained from the [AlCl4]- group. DFT calculations confirm the structure solution for the orthorhombic form and suggest that the monoclinic structure is metastable against transformation to o-Cu(Sb2S3)Cl.

Ge32 Co9-x : Creating "Empty" Space by High Pressure

The compound Ge₃₂ Co_9-x (x=0.54(6), a=10.9861(3) Å, space group Im 3 ‾ $\bar 3$ m) prepared under high pressure and at high temperature is metastable under ambient conditions. It crystallizes in a new structure type, Pearson symbol cI82-1.08. The crystal structure represents a slightly distorted cubic primitive arrangement of germanium atoms with part of the Ge cubes filled by cobalt. Analysis of the chemical bonding by real-space methods revealed three-core cluster units Ge₁₆ Co₃ and seemingly empty regions comprising either covalent inter-polyhedral Ge-Ge bonds or lone-pairs located at the germanium atoms. The electrical conductivity is metal-like.

Gadolinium trisilicide − a paramagnetic representative of the YbSi3 type series

Binary gadolinium trisilicide GdSi3 was prepared by high-pressure high-temperature synthesis at typically 9.5(5) GPa and 870 K before quenching to ambient conditions. At ambient pressure, GdSi3 exhibits an exothermic decomposition at 647(10) K into the thermodynamically stable phases GdSi2–x and Si, indicating its metastable character. Powder X-ray diffraction data is consistent with the YbSi3-type crystal structure comprising slabs of condensed Si2 dumbbells, which enclose layered arrangements of gadolinium cations. Quantum chemical analysis of the chemical bonding shows, that the framework is formed by silicon dumbbells with homopolar bonds. The magnetic moment of 8.13(8) µ B is consistent with Gd4f 7 (Gd3+ state) and antiferromagnetic ordering is observed below 64(1) K.

Type-II Clathrate Na24-δ Ge136 from a Redox-Preparation Route

The metastable type-II clathrate Na24-δ Ge136 was obtained from Na12 Ge17 by applying a two-step procedure. At first, Na12 Ge17 was reacted at 70 °C with a solution of benzophenone in the ionic liquid (IL) 1,3-dibutyl-2-methylimidazolium-bis(trifluoromethylsulfonyl) azanide. The IL was inert towards Na12 Ge17 , but capable of dissolving the sodium salts formed in the redox reaction. By annealing at 340 °C under an argon atmosphere, the X-ray amorphous intermediate product was transformed to crystalline Na24-δ Ge136 (δ≈2) and α-Ge in an about 1 : 1 mass ratio. The product was characterized by X-ray powder diffraction, chemical analysis, and 23 Na solid-state NMR spectroscopy. Metallic properties of Na24-δ Ge136 were revealed by a significant Knight shift of the 23 Na NMR signals and by a Pauli-paramagnetic contribution to the magnetic susceptibility. At room temperature, Na24-δ Ge136 slowly ages, with a tendency to volume decrease and sodium loss.

Crystal Structure and Physical Properties of the Cage Compound Hf2B2-2δIr5+δ

Hf₂B_2–2δIr_5+δ crystallizes with a new type of structure: space group Pbam, a = 5.6300(3) Å, b = 11.2599(5) Å, and c = 3.8328(2) Å. Nearly 5% of the boron pairs are randomly replaced by single iridium atoms (Ir_5+δB_2–2δ). From an analysis of the chemical bonding, the crystal structure can be understood as a three-dimensional framework stabilized by covalent two-atom B–B and Ir–Ir as well as three-atom Ir–Ir–B and Ir–Ir–Ir interactions. The hafnium atoms center 14-atom cavities and transfer a significant amount of charge to the polyanionic boron–iridium framework. This refractory boride displays moderate hardness and is a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient, and thermal conductivity. The metallic character of this system is also confirmed by electronic structure calculations revealing 5.8 states eV^–1 fu^–1 at the Fermi level. Zr₂B_2–2δIr_5+δ is found to be isotypic with Hf₂B_2–2δIr_5+δ, and both form a continuous solid solution.

Hf₂Ir_5+δB_2−2δ is a cage compound with a three-dimensional anionic boron−iridium framework composed of [B₂Ir₈] units with cavities bearing the hafnium cations. Zr₂Ir_5+δB_2−2δ is found to be isotypic with Hf₂Ir_5+δB_2−2δ, and both form a continuous solid solution.

Solid state interactions in the La-Au-Mg system: phase equilibria, novel compounds and chemical bonding

Gold intermetallic chemistry is very rich, covering different classes of compounds ranging from the Hume-Rothery to Zintl phases to polar intermetallics to quasicrystals. Au's relativistic effects are frequently mentioned as responsible for the peculiar structural and physical properties of its compounds, nonetheless the aspects of chemical bonding are far to be clearly understood. In this work, the La-Au-Mg system was targeted for the discovery of new gold intermetallics and their structural and chemical bonding characterization. Studies on solid state interactions resulted in the construction of a partial La-Au-Mg isothermal section at 400 °C. The high reactivity between the constituents is reflected by the formation of five intermetallic compounds in the concentration range of less than 50 at% of Au. A complete crystallographic study was conducted for four of them, namely La1.82Au3+xMg14.36-x (0 ≤ x ≤ 0.90, hP42-3.64-CeMg10.3), La3Au4-xMg12+x (0 ≤ x ≤ 0.75, hP38-Gd3Ru4Al12), LaAuMg2 (oS16-MgCuAl2) and LaAu1+xMg1-x (0 ≤ x ≤ 0.15, hP9-ZrNiAl). A unifying description based on the different stacking sequences of equal slabs along the c-axis is proposed for these intermetallics. Chemical bonding in LaAuMg2 was studied by following the position space approach and including relativistic effects. Among the peculiarities of this LaMg2Au auride, there are two-atomic La-Au bonds showing a classical polar covalent character and that form distorted hexagonal planar layers and multi-atomic bonds involving Mg species. One of these is interpreted as a Mg-Mg bond supported by the neighbouring La and Au atoms, explaining the Mg reduced oxidation state (close to +1) in this compound.

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.

Unconventional Metal-Framework Interaction in MgSi5

The silicon‐rich cage compound MgSi₅ was obtained by high‐pressure high‐temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X‐ray diffraction experiments. The crystal structure of MgSi₅ (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four‐bonded silicon atoms forming Si₁₅ cages enclosing the Mg atoms. Two types of smaller Si₈ cages remain empty. The atomic interactions are characterized by two‐center two‐electron bonds within the silicon framework. In addition, there is evidence for multi‐center Mg−Si bonding in the large cavities of the framework and for lone‐pair‐like interactions in the smaller empty voids.