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, Ba(3)Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) and Ba(3)Ge(2)B(6)O(16), have been synthesized by hydrothermal or high-temperature solid-state reactions. They represent the first examples of alkaline-earth borogermanates. Ba(3)Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) crystallized in a polar space group Cc. Its structure features a novel three-dimensional anionic framework composed of B(7)O(16)(OH)(2)](13-) polyanions that are bridged by Ge atoms with one-dimensional (1D) 10-membered-ring (MR) tunnels along the b axis. The Ba(II) cations, hydroxide ions, and water molecules are located at the above tunnels. Ba(3)Ge(2)B(6)O(16) crystallizes in centrosymmetric space group P1. Its structure exhibits a thick layer composed of circular B(6)O(16) units connected by GeO(4) 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 Ba(3)Ge(2)B(7)O(16)(OH)(2)](OH)(H(2)O) displays a weak SHG response of about 0.3 times that of KH(2)PO(4). Optical, thermal stability, and ferroelectric properties as well as theoretical calculations have also been performed.