The clathrate Is uperconductor Sr 8 Si 46 is obtained under high-pressureh igh-temperature conditions, at 5GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heatinga t T = 796(5) Ki nto Si and SrSi 2 .T he crystal structure of the clathrate is isotypic to that of Na 8 Si 46 .C hemical bonding analysisr eveals conventionalc ovalent bondingw ithin the silicon network as well as additional multi-atomic interactions between Sr and Si within the framework cages. Physical measurements indicateabulk BCS type II superconducting state below T c = 3.8(3) K.

Rb 8 B 8 Si 38 forms under high-pressure, high-temperature conditions at p = 8 GPa and T = 1273 K. The new compound (space group Pm 3̅ n , a = 9.9583(1) Å) is the second example for a clathrate-I borosilicide. The phase is inert against strong acids and bases and thermally stable up to 1300 K at ambient pressure. (Rb + ) 8 (B – ) 8 (Si 0 ) 38 is electronically balanced, diamagnetic, and shows semiconducting behavior with moderate Seebeck coefficient below 300 K. Chemical bonding analysis by the electron localizability approach confirms the description of Rb 8 B 8 Si 38 as Zintl phase.

The silicon-rich cage compound MgSi 5 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 MgSi 5 (space group Cmme, Pearson notation oS24, a = 4.4868(2) , b = 10.1066(5) , and c = 9.0753(4) ) constitutes a new type of framework of four-bonded silicon atoms forming Si 15 cages enclosing the Mg atoms. Two types of smaller Si 8 cages remain empty. The atomic interactions are characterized by two-center two-electron bonds within the silicon framework. In addition, there is evidence for multicenter Mg À Si bonding in the large cavities of the framework and for lone-pair-like interactions in the smaller empty voids.

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.

LuGe 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 DyGe and decomposes at ambient pressure and T = 690 K mainly into ( cF8)Ge and LuGe. Chemical bonding analysis of LuGe 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 = 3.3(3) K.