Sr₆Ge₃OSe₁₁: A Rationally Designed Noncentrosymmetric Oxyselenide with Polar [GeOSe₃] Building Blocks

The strategy of partial isovalent anion substitution was used to discover the new noncentrosymmetric (NCS) quaternary oxyselenide Sr6Ge3OSe11. Sr6Ge3OSe11 crystallizes in the NCS trigonal space group P3m1 with lattice parameters of a = 10.268(5) Å, c = 6.363(3) Å, Z = 1. Spectroscopic studies revealed that Sr6Ge3OSe11 has a wide transparent range in the IR region and an optical band gap of 2.39 eV. The nonlinear optical (NLO) properties of this compound were calculated via DFT as well as measured experimentally using a KH2PO4 standard. The second-order nonlinear susceptibilities (dij) of Sr6Ge3OSe11 were calculated to be d15 = −12.9 pm V–1, d22 = −15.4 pm V–1, d33 = 15.0 pm V–1, and deff = 17.0 pm V–1. Size-dependent second harmonic generation (SHG) intensity experiments revealed that Sr6Ge3OSe11 is phase-matchable at 1064 nm with an intensity equal to 0.62 × KH2PO4. Additionally, this material exhibits ultralow thermal conductivity despite its relatively low molar mass, which is of interest in the thermoelectric energy conversion.