Na₁₂(NbO)₃(PO₄)₇: A Congruent Melting Nonlinear Optical Crystal with Large NbO₆ Distortion and High Laser-Induced Damage Threshold

A new noncentrosymmetric crystal Na-12(NbO)(3)(PO4)(7) was successfully synthesized in the niobium phosphate system. Its structure characterizes isolated and highly distorted NbO6 octahedra joining with isolated PO4 groups to form a unit of a (Nb6P6O12) hexagonal star by sharing O atoms. In (Nb6P6O12) 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 x KDP, which is rare in known niobium phosphate systems. Meanwhile, it also has a wide optically transparent window (0.29-4.44 mu m) and a high laser-induced damage threshold (3.5 GW/cm(2)). More importantly, Na-12(NbO)(3)(PO4)(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.