Classical and Nonclassical Germanium Environments in High-Pressure BaGe₅

A new crystalline form of BaGe₅ was obtained at a pressure of 15(2) GPa in the temperature range from 1000(100) to 1200(120) K. Single-crystal electron and powder X-ray diffraction patterns indicate a body-centered orthorhombic structure (space group <i>Imma</i>, Pearson notation <i>oI</i>24) with unit cell parameters <i>a</i> = 8.3421(8) Å, <i>b</i> = 4.8728(5) Å, and <i>c</i> = 13.7202(9) Å. The crystal structure of <i>hp</i>-BaGe₅ consists of four-bonded Ge atoms forming complex layers with Ge–Ge contacts between 2.560(6) and 2.684(3) Å; the Ba atoms are coordinated by 15 Ge neighbors in the range from 3.341(6) to 3.739(4) Å. Analysis of the chemical bonding using quantum chemical techniques in real space reveal charge transfer from the Ba cations to the anionic Ge species. Ge atoms having nearly tetrahedral environments show an electron-localizability-based oxidation number close to 0; the four-bonded Ge atoms with a Ψ-pyramidal environment adopt a value close to 1-. In agreement with the calculated electronic density of states, the compound is a metallic conductor (electrical resistivity of ca. 240 μΩ cm at 300 K), and magnetic susceptibility measurements evidence diamagnetic behavior with χ₀ = −95 × 10^–6 emu mol<sup>–1