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.