Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations

We investigated the Sr(2-)xPb(x)GeSe(4) series from 0 <= x <= 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds gamma-Sr2GeSe4 and alpha-Pb2GeSe4 have already been reported previously, the substitution variants Sr1.31Pb0.69GeSe4 (space group Ama2, a = 10.31220(1) angstrom, b = 10.39320(1) angstrom, c = 7.42140(1) angstrom) and Sr0.19Pb1.81GeSe4 (I (4) over bar 3d, a = 14.6177(3) angstrom) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from gamma-Sr2GeSe4 to Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and alpha-Pb2GeSe4 from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds gamma-Sr2GeSe4 and Sr1.3Pb0.7GeSe4 (approximated with the supercell Sr3PbGe2Se8) were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, d(eft), of gamma-Sr(2)GeSe(4 )and Sr3PbGe2Se8 at the static limit to be 10.8 and 8.8 pm V-1, respectively. The experimental d(eff) values of gamma-Sr2GeSe4, Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and alpha-Pb2GeSe4 were 2.6, 2.3, 0.68, and 0.79 pm V-1, respectively.