SrGe2B2O8 and Sr3Ge2B6O16: novel strontium borogermanates with three-dimensional and layered anionic architectures

Two new alkaline-earth strontium borogermanates, namely, SrGe2B2O8 and Sr3Ge2B6O16, have been successfully synthesized through high-temperature solid state reactions. They represent the first examples of strontium borogermanates. SrGe2B2O8 crystallized in space group Pnma, and its structure features a novel three-dimensional [Ge2B2O8](2-) framework composed of alternative linkages of the B2O7 and Ge2O7 dimeric units with one-dimensional (1D) tunnels of eight-membered rings (MRs) along the b axis that are filled by the Sr(2+) cations. Sr3Ge2B6O16 is isostructural with Ba3Ge2B6O16 and crystallizes in centrosymmetric space group P1. Its structure features a two-dimensional layer that is composed of circular B6O16 clusters and GeO4 tetrahedra that are interconnected via corner sharing, forming 1D four- and six-MR tunnels along a axis. Sr(1) cations are located in the six-MR tunnels, whereas Sr(2) cations are located in the interlayer space. Studies of their optical properties and thermal stability as well as band structure calculations based on density functional theory methods have been also performed.