Nonlinear Optical Properties of Mixed Oxide Crystals CsNbMoO₆ and CsTaMoO₆: A Periodic CPHF/KS Study

Inorganic crystals are advantageous for use as second harmonic generation (SHG) materials because of their large laser damage threshold and mechanical strength. Here, we focus on the defect pyrochlores based on mixed oxides of MoO3, Ta2O5, Cs2O, and Nb2O5, which were synthesized recently. We predict the linear optical properties (refractive indices and linear polarizabilities) and nonlinear ones [first hyperpolarizabilities and nonlinear optical (NLO) tensors] for the crystals of the defect pyrochlores CsNbMoO6 and CsTaMoO6 using the coupled-perturbed Hartree–Fock/Kohn–Sham method (CPHF/KS) in both full-electron and pseudopotential basis sets. To identify the more accurate computational protocol, the crystals KH2PO4, LiNbO3, (NH2)2CO, and α-SiO2 were used as benchmarks. The best agreement of the predicted NLO parameters with experimental ones is achieved in calculations at the density functional theory/PBE0 level in the POB-TZVP basis. This agreement becomes quantitative when the dimensionless relative value of the NLO activity ρ is used. The calculated relative SHG characteristics of the pyrochlore crystals are ρ­(CsNbMoO6/LiNbO3) = 0.016 and ρ­(CsTaMoO6/LiNbO3) = 0.022 and ρ­(CsNbMoO6/KDP) = 0.39 and ρ­(CsTaMoO6/KDP) = 0.52. The symmetry of the sublattices of disordered atoms significantly affects the SHG properties of the crystals, and the values of the calculated properties for sublattices of different symmetries can vary by an order of magnitude