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.