Reduction of Germanium Oxides-The Mixed-Valence Germanates A 2 Ge 4 O 7 ( A = Na, K).

We report on the synthesis of two-layered alkali germanates, Na 2 Ge 4 O 7 and K 2 Ge 4 O 7 . Both compounds were synthesized by using the ammonothermal method at 823 K and 100 MPa. Under these conditions, germanium is partially reduced from the +IV state to +II, forming mixed-valence compounds with the rarely observed [Ge (II) O 3 ] 4- unit. The valence state was verified by X-ray photoelectron spectroscopy (XPS) and was accompanied by theoretical calculations alongside vibrational spectroscopy and single-crystal X-ray structure determination. The compounds crystallize in the trigonal space groups (Na 2 Ge 4 O 7 : P 3̅ c 1 and K 2 Ge 4 O 7 : P 3̅ m 1) and feature layers of corner sharing [Ge (II) O 3 ] 4- and [Ge (IV) 2 O 7 ] 6- units forming [Ge (II) 2 Ge (IV) 2 O 7 ] 2- polyanions. These layers are separated by alkali metal ions. The compounds are colorless insulators with band gaps of 4.0-4.2 eV. According to the Robin-Day classification, both compounds can be described as class I materials, where the valences are trapped on specific sites.