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

Abstract

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 Pna2₁, 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_2h¹⁶, 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 °C below the decomposition temperature of the substance at 560 °C.