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

[(thf)5Ln(Ge9{Si(SiMe3)3}2)] (Ln = Eu, Sm, Yb): Capping Metalloid Germanium Cluster with Lanthanides

We report the synthesis of three neutral complexes with different coordination modes of a di-silylated metalloid germanium cluster to divalent lanthanides [(thf)₅Ln(ηⁿ-Ge₉(Hyp)₂)] (Ln = Yb (1, n = 1); Eu (2, n = 2, 3), Sm (3, n = 2, 3); Hyp = Si(SiMe₃)₃) by the salt metathesis of LnI₂ with K₂[Ge₉(Hyp)₂] in THF. The complexes were characterized by elemental analysis, nuclear magnetic resonance and UV-vis-NIR spectroscopy, and single-crystal X-ray diffraction. In thf solution, the formation of contact or solvate-separated ion pairs depending on the concentration is assumed. Compound 2 exhibits a blue luminescence typical for Eu²⁺. The solid-state magnetic measurements of compounds 2 and 3 confirm the presence of divalent europium and samarium, respectively.

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.

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.

Synthesis and Superconductivity of a Strontium Digermanide SrGe2-δ with ThSi2 Structure

We have succeeded in crystallizing a new strontium digermanide (SrGe_2-δ) with the ThSi₂-type structure (tetragonal SrGe₂), which is theoretically predicted to compete with the EuGe₂-type one (trigonal SrGe₂) under pressure. The tetragonal SrGe₂ appeared as a metastable phase in samples at approximately 900 °C under a pressure of 2 GPa. X-ray diffraction studies show that the tetragonal SrGe₂ is formed by the reaction between trigonal SrGe₂ and excess Sr. The composition of the tetragonal SrGe₂ was analyzed to be SrGe_1.66(4). Lattice parameters for the tetragonal SrGe₂ are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe₂ shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (T_c) of 7.3 K, which is the highest among compounds with the ThSi₂-type structure. Superconducting properties of the tetragonal SrGe₂, such as the upper critical field, and the effect of pressure on T_c, are presented and superconductivity is discussed on the basis of electronic band structure calculations.

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.