Large nonlinear optical effect in tungsten bronze structures via Li/Na cross-substitutions

A Polar Tetragonal Tungsten Bronze with Colossal Second-Harmonic Generation

A polar tetragonal tungsten bronze, Pb_1.91 K_3.22 □_0.85 Li_2.96 Nb₁₀ O₃₀ (□: vacancies), has been successfully synthesized by a high temperature solid-state reaction. Single crystal and powder X-ray diffraction indicate that the structure of Pb_1.91 K_3.22 □_0.85 Li_2.96 Nb₁₀ O₃₀ crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO₆ , LiO₉ , PbO₁₂ , and (Pb/K)O₁₅ polyhedra. While NCS Pb_1.91 K_3.22 □_0.85 Li_2.96 Nb₁₀ O₃₀ undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb_1.45 K_3.56 Li_3.54 Nb₁₀ O₃₀ (P4/mbm) once heated to 1200 °C. Powder second-harmonic generation (SHG) measurements with 1064 nm radiation indicate that Pb_1.91 K_3.22 □_0.85 Li_2.96 Nb₁₀ O₃₀ exhibits a giant phase-matchable SHG intensity of ≈71.5 times that of KH₂ PO₄ , which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well-aligned NbO₆ octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.