AGe2O4Q (A = Ba, Sr; Q = S, Se): A series of heteroanionic oxychalcogenides with large birefringence

Heteroanionic materials are of current research interest owing to their varied structural chemistry and properties. Recently, a number of heteroanionic materials have been synthesized that can be used for water splitting, battery materials, thermoelectrics, and nonlinear optical applications. Birefringent heteroanionic materials are still rarely researched. Herein, we report a series of new oxychalcogenides, AGe2O4Q (A = Ba, Sr; Q = S, Se), that are synthesized by solid-state reactions in fused silica tubes with their structures determined by single-crystal X-ray diffraction. Optical property measurements show that AGe2O4Q (A = Ba, Sr; Q = S, Se) exhibit large birefringences, 0.13–0.19 @ 1064 nm. These are one-order of magnitude larger than their single-anionic homologue, BaGe2O5 - 0.03 @ 1064 nm. This suggests that constructing heteroanions through using chalcogenides to partially substitute O in tetrahedral foundation building units (FBUs) is a viable method for designing new birefringent materials. Furthermore, the crystal structures, UV–vis–NIR diffuse reflectance spectra, infrared and Raman spectra as well as the first-principles calculations for AGe2O4Q (A = Ba, Sr; Q = S, Se) are also reported.