Na 12 (NbO) 3 (PO 4 ) 7 : A Congruent Melting Nonlinear Optical Crystal with Large NbO 6 Distortion and High Laser-Induced Damage Threshold.

A new noncentrosymmetric crystal Na 12 (NbO) 3 (PO 4 ) 7 was successfully synthesized in the niobium phosphate system. Its structure characterizes isolated and highly distorted NbO 6 octahedra joining with isolated PO 4 groups to form a unit of a (Nb 6 P 6 O 12 ) hexagonal star by sharing O atoms. In (Nb 6 P 6 O 12 ) hexagonal stars, all Nb and P atoms are in a hexagonal star-like arrangement and all Na atoms are also in a hexagonal star-like arrangement, except for Na(3) and Na(10) atoms. Notably, it exhibits a strong phase-matched second harmonic response: 3 × KDP, which is rare in known niobium phosphate systems. Meanwhile, it also has a wide optically transparent window (0.29-4.44 μm) and a high laser-induced damage threshold (3.5 GW/cm 2 ). More importantly, Na 12 (NbO) 3 (PO 4 ) 7 is a congruent melting compound that has the potential to be grown into large-sized single crystals by the Czochralski method. These excellent properties make it a promising candidate as a mid-infrared nonlinear optical crystal.