Dumbbells of Five-Connected Silicon Atoms and Superconductivity in the Binary Silicides MSi3 (M = Ca, Y, Lu)

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.

Stable Structures and Superconductivity in a Y-Si System under High Pressure

Recently, the discovery of superconductivity in compressed electrides offers a promising route toward searching for high superconductivity in a high-pressure community. However, only a few superconducting electrides have been successfully found thus far, thereby limiting the variety of superconducting electride examples. In this work, we performed extensive structure searches on a high-pressure Y-Si system by using CALYPSO structure prediction methodology. Our simulations identified several stable stoichiometries of YSi, YSi₂, YSi₃, Y₅Si₃, Y₂Si, and Y₃Si under high pressure. These structures contain a diversity of structure configurations, including silicon chains, Si₃ trilaterals, Si₄ quadrilaterals, Si₆ hexagons, Si₈ rings, a Si₄-Si₆-Si₈ frame, as well as a silicon layer. Remarkably, Y₃Si is predicted to be an electride with a superconducting critical temperature (T_c) of ∼11.2 and 14.5 K at 30 and 50 GPa, respectively. These results highlight the role of the electrons at the Fermi surface in determining the superconductivity of predicted structures.

High-pressure synthesis of SmGe3

The new samarium germanide SmGe3 is obtained by high-pressure high-temperature synthesis of pre-reacted mixtures of samarium and germanium at a pressure of 9.5 GPa and temperatures between 1073 and 1273 K. SmGe3 decomposes at 470(5) K into SmGe2, α-Sm3Ge5 and a hitherto unknown phase. SmGe3 exhibits a superstructure of the cubic Cu3Au-type. Transmission electron microscopy measurements of crystalline particles and prepared lamellae indicate a high density of defects on the nanoscale. Selected area electron diffraction and elaborate X-ray powder diffraction measurements consistently indicate a 2a 0 × 2a 0 × 2a 0 superstructure adopting space group F m 3 ¯ m $Fm\overline{3}m$ with a = 8.6719(2) Å.

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.

Ca-Ag compounds in ethylene epoxidation reaction

The ethylene epoxidation is a challenging catalytic process, and development of active and selective catalyst requires profound understanding of its chemical behaviour under reaction conditions. The systematic study on intermetallic compounds in the Ca-Ag system under ethylene epoxidation conditions clearly shows that the character of the oxidation processes on the surface originates from the atomic interactions in the pristine compound. The Ag-rich compounds Ca₂Ag₇ and CaAg₂ undergo oxidation towards fcc Ag and a complex Ca-based support, whereas equiatomic CaAg and the Ca-rich compounds Ca₅Ag₃ and Ca₃Ag in bulk remain stable under harsh ethylene epoxidation conditions. For the latter presence of water vapour in the gas stream leads to noticeable corrosion. Combining the experimental results with the chemical bonding analysis and first-principles calculations, the relationships among the chemical nature of the compounds, their reactivity and catalytic performance towards epoxidation of ethylene are investigated.

Characteristics of the s–Wave Symmetry Superconducting State in the BaGe3 Compound

Thermodynamic properties of the s–wave symmetry superconducting phase in three selected structures of the BaGe 3 compound ( P 6 3 / m m c , A m m 2 , and I 4 / m m m ) were discussed in the context of DFT results obtained for the Eliashberg function. This compound may enable the implementation of systems for quantum information processing. Calculations were carried out within the Eliashberg formalism due to the fact that the electron–phonon coupling constant falls within the range λ ∈ 0 . 73 , 0 . 86 . The value of the Coulomb pseudopotential was assumed to be 0 . 122 , in accordance with the experimental results. The value of the Coulomb pseudopotential was assumed to be 0 . 122 , in accordance with the experimental results. The existence of the superconducting state of three different critical temperature values, namely, 4 . 0 K, 4 . 5 K and 5 . 5 K, depending on the considered structure, was stated. We determined the differences in free energy ( Δ F ) and specific heat ( Δ C ) between the normal and the superconducting states, as well as the thermodynamic critical field ( H c ) as a function of temperature. A drop in the H c value to zero at the temperature of 4.0 K was observed for the P 6 3 / m m c structure, which is in good accordance with the experimental data. Further, the values of the dimensionless thermodynamic parameters of the superconducting state were estimated as: R Δ = 2 Δ ( 0 ) / k B T c ∈ { 3 . 68 , 3 . 8 , 3 . 8 } , R C = Δ C ( T c ) / C N ( T c ) ∈ { 1 . 55 , 1 . 71 , 1 . 75 } , and R H = T c C N ( T c ) / H c 2 ( 0 ) ∈ { 0 . 168 , 0 . 16 , 0 . 158 } , which are slightly different from the predictions of the Bardeen–Cooper–Schrieffer theory ( [ R Δ ] B C S = 3 . 53 , [ R C ] B C S = 1 . 43 , and [ R H ] B C S = 0 . 168 ). This is caused by the occurrence of small retardation and strong coupling effects.

High-Pressure Synthesis and Chemical Bonding of Barium Trisilicide BaSi₃

BaSi₃ is obtained at pressures between 12(2) and 15(2) GPa and temperatures from 800(80) and 1050(105) K applied for one to five hours before quenching. The new trisilicide crystallizes in the space group I4¯2m (no. 121) and adopts a unique atomic arrangement which is a distorted variant of the CaGe₃ type. At ambient pressure and 570(5) K, the compound decomposes in an exothermal reaction into (hP3)BaSi₂ and two amorphous silicon-rich phases. Chemical bonding analysis reveals covalent bonding in the silicon partial structure and polar multicenter interactions between the silicon layers and the barium atoms. The temperature dependence of electrical resistivity and magnetic susceptibility measurements indicate metallic behavior.

Lutetium Trigermanide LuGe3: High-Pressure Synthesis, Superconductivity, and Chemical Bonding

LuGe₃ was obtained under high-pressure and high-temperature conditions at pressures between 8(1) and 14(2) GPa and at temperatures in the range from 1100(150) to 1500(150) K. The high-pressure phase is isotypic to DyGe₃ and decomposes at ambient pressure and T = 690 K mainly into ( cF8)Ge and LuGe_{2- x}. Chemical bonding analysis of LuGe₃ reveals two-center electron-deficient Ge-Ge bonds, multicenter polar Lu-Ge interactions, and lone pairs on germanium. Magnetic susceptibility, specific heat, and electrical conductivity measurements indicate transition into a superconducting state below T_c = 3.3(3) K.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture)

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L. Feringa describes the evolution of the field of molecular motors and explains how to program and control molecules by incorporating responsive and adaptive properties.

Germanium Dumbbells in a New Superconducting Modification of BaGe3

We report the high-pressure high-temperature synthesis (P = 15 GPa, T = 1300 K) of BaGe3(tI32) adopting a CaGe3-type crystal structure. Bonding analysis reveals layers of covalently bonded germanium dumbbells being involved in multicenter Ba-Ge interactions. Physical measurements evidence metal-type electrical conductivity and a transition to a superconducting state at 6.5 K. Chemical bonding and physical properties of the new modification are discussed in comparison to the earlier described hexagonal form BaGe3(hP8) with a columnar arrangement of Ge3 triangles.

TrigermanidesAEGe3(AE= Ca, Sr, Ba): chemical bonding and superconductivity

The crystal structures of the trigermanidesAEGe3(tI32) (AE= Ca, Sr, Ba; space groupI4/mmm, for SrGe3:a= 7.7873(1),c= 12.0622(3) Å) comprise Ge2dumbbells forming layered Ge substructures which enclose embeddedAEatoms. The chemical bonding analysis by application of the electron localizability approach reveals a substantial charge transfer from theAEatoms to the germanium substructure. The bonding within the dumbbells is of the covalent two-center type. A detailed analysis of SrGe3reveals that the interaction on the bond-opposite side of the Ge2groups is not lone pair-like – as it would be expected from the Zintl-like interpretation of the crystal structure with anionic Ge layers separated by alkaline-earth cations – but multi-center strongly polar between the Ge2dumbbells and the adjacent metal atoms. Similar atomic interactions are present in CaGe3and BaGe3. The variation of the alkaline-earth metal has a merely insignificant influence on the superconducting transition temperatures in thes,p-electron compoundsAEGe3.

High-Pressure Synthesis and Electronic Structure of a New Superconducting Strontium Germanide (SrGe3) Containing Ge2 Dumbbells

We obtained a new strontium germanide (SrGe3) by high-pressure and high-temperature synthesis. It was prepared at 13 GPa and 1100 °C. The space group and cell constants are I4/mmm (No. 139), a = 7.7800(8) Å, c = 12.0561(13) Å, and V = 729.74(17) Å(3). SrGe3 crystallizes in the CaSi3 structure composed of Ge-Ge dumbbells and Sr(2+) ions. SrGe3 is a type II superconductor with a transition temperature of 6.0 K.

High-pressure synthesis and superconductivity of the Laves phase compound Ca(Al,Si)2 composed of truncated tetrahedral cages Ca@(Al,Si))12

The Zintl compound CaAl2Si2 peritectically decomposes to a new ternary cubic Laves phase Ca(Al,Si)2 and an Al-Si eutectic at temperatures above 750 °C under a pressure of 13 GPa. The ternary Laves phase compound can also be prepared as solid solutions Ca(Al(1-x)Si(x))2 (0.35 ≤ x ≤ 0.75) directly from the ternary mixtures under high-pressure and high-temperature conditions. The cubic Laves phase structure can be regarded as a type of clathrate compound composed of face-sharing truncated tetrahedral cages with Ca atoms at the center, Ca@(Al,Si)12. The compound with a stoichiometric composition CaAlSi exhibits superconductivity with a transition temperature of 2.6 K. This is the first superconducting Laves phase compound composed solely of commonly found elements.