Ba3[Ge2B7O16(OH)2](OH)(H2O) and Ba3Ge2B6O16: Novel Alkaline-Earth Borogermanates Based on Two Types of Polymeric Borate Units and GeO4 Tetrahedra

Two new barium borogermanates with two types of novel structures, namely, Ba3[Ge2B7O16(OH)2](OH)(H2O) and Ba3Ge2B6O16, have been synthesized by hydrothermal or high-temperature solid-state reactions. They represent the first examples of alkaline-earth borogermanates. Ba3[Ge2B7O16(OH)2](OH)(H2O) crystallized in a polar space group Cc. Its structure features a novel three-dimensional anionic framework composed of [B7O16(OH)2]13− polyanions that are bridged by Ge atoms with one-dimensional (1D) 10-membered-ring (MR) tunnels along the b axis. The BaII cations, hydroxide ions, and water molecules are located at the above tunnels. Ba3Ge2B6O16 crystallizes in centrosymmetric space group P1̅. Its structure exhibits a thick layer composed of circular B6O16 units connected by GeO4 tetrahedra via corner sharing, forming 1D 4- and 6-MR tunnels along the c axis. Ba1 ions reside in the tunnels of the 6-MRs, whereas Ba2 ions are located at the interlayer space. Both compounds feature new types of topological structures. Second-harmonic-generation (SHG) measurements indicate that Ba3[Ge2B7O16(OH)2](OH)(H2O) displays a weak SHG response of about 0.3 times that of KH2PO4. Optical, thermal stability, and ferroelectric properties as well as theoretical calculations have also been performed.