New nonlinear optical oxyfluoride K₂NbO₂F₃ with strong pseudo-centrosymmetry, obtained by mild hydrothermal synthesis

New polar K2NbO2F3 crystals were obtained under mild hydrothermal conditions, which differ from the higher temperature synthesis of previously known Nb-containing oxyfluorides. X-ray diffraction analysis of the single crystal, performed in the polar space group Pna2(1), is consistent with the existence of the second harmonic generation (SHG) effect in powders. A monotonous increase in SHG power in samples with increasing powder grain size indicates phase matching of neodymium laser radiation and its second harmonic. However, SHG is relatively small and does not exceed a third of the intensity of standard KH2PO4 powders. The weakness of the second-order optical nonlinearity in K2NbO2F is related to the strong pseudo-symmetry of the crystal structure, corresponding to the space group Pnam-D-2h(16), and thus the structure is very close to centrosymmetric. Structural evidence for the polarity of the K2NbO2F3 crystal is the subtle differences in the Nb-O bonds along the c axis in the (NbO3F3) octahedra, together with the presence of similar differences in the K1-O bonds. Extremely weak, close to the detection limit, very small deviations from the centrosymmetry of the entire structure also correlate with the existence of an almost second-order irreversible transformation of K2NbO2F3 into a centrosymmetric phase at temperatures 50-60 degrees C below the decomposition temperature of the substance at 560 degrees C.