Mixed Valence and Unusual Germanium Coordination in SrGe8As10, BaGe8As10, and BaGe7P12

The new Zintl-compounds AEGe8As10 (AE = Sr, Ba) and BaGe7P12 were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGe8As10 and BaGe8As10 crystallize in the space group Cmce and show complex 3D networks composed of three different Ge–As motifs and As–As bonds with mixed valence of germanium in the oxidation states +2, + 3, and +4. Mixed valences of germanium +3 and +4 occur in BaGe7P12, which crystallizes in the space group R3̅ with a 3D network built up of Ge2P6 dumbbells and P–P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP6 trigonal antiprism. High temperature X-ray diffraction shows thermal stabilities up to 923–953 K. UV–Vis and resistivity measurements reveal a semiconducting nature with small indirect band gaps between 0.02 and 1.6 eV. Electronic band structure calculations confirm the semiconducting state and indicate covalent bonds within the Ge-Pn polyanions.