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 Sr 6 Ge 3 OSe 11 . Sr 6 Ge 3 OSe 11 crystallizes in the NCS trigonal space group P 3 m 1 with lattice parameters of a = 10.268(5) Å, c = 6.363(3) Å, Z = 1. Spectroscopic studies revealed that Sr 6 Ge 3 OSe 11 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 KH 2 PO 4 standard. The second-order nonlinear susceptibilities ( d ij ) of Sr 6 Ge 3 OSe 11 were calculated to be d 15 = −12.9 pm V –1, d 22 = −15.4 pm V –1, d 33 = 15.0 pm V –1, and d eff = 17.0 pm V –1 . Size-dependent second harmonic generation (SHG) intensity experiments revealed that Sr 6 Ge 3 OSe 11 is phase-matchable at 1064 nm with an intensity equal to 0.62 × KH 2 PO 4 . Additionally, this material exhibits ultralow thermal conductivity despite its relatively low molar mass, which is of interest in the thermoelectric energy conversion.