Mixed Valence and Unusual Germanium Coordination in SrGeAs, BaGeAs, and BaGeP.

The new Zintl-compounds GeAs ( = Sr, Ba) and BaGeP were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGeAs and BaGeAs crystallize in the space group 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 BaGeP, which crystallizes in the space group 3̅ with a 3D network built up of GeP dumbbells and P-P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP 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- polyanions.