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 Nb 10 O 30 (□: 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 Nb 10 O 30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO 6 , LiO 9 , PbO 12 , and (Pb/K)O 15 polyhedra. While NCS Pb 1.91 K 3.22 □ 0.85 Li 2.96 Nb 10 O 30 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 Nb 10 O 30 (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 Nb 10 O 30 exhibits a giant phase-matchable SHG intensity of ≈71.5 times that of KH 2 PO 4 , 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 NbO 6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.