New nonlinear optical potassium iodate K[IO3] and borates K3[B6O10]Br, KTa[B4O6(OH)4](OH)2·1.33H2O—Synthesis, structures and relation to the properties

Three optically uniaxial non-centrosymmetric potassium compounds, iodate K[IO3] and two borates K3[B6O10]Br, KTa[B4O6(OH)4](OH)2·1.33H2O have been synthesized and characterized by single-crystal X-ray diffraction. The materials were synthesized through hydrothermal techniques using initial reagents and mineralizers. All the compounds are trigonal-hexagonal: the space groups are R3 (K[IO3]), R3m (K[B6O10]Br) and P-62m (KTa[B4O6(OH)4](OH)2·1.33H2O). Powder second-harmonic generation (SHG) measurements on crystals, using 1064 nm Nd:YAG laser radiation, indicate the materials are all phase-matchable and have strong second-order nonlinearities. The correlation with the perovskite structure has been found and described for K,Br-borate and K-iodate. Structure-properties relation is discussed and attributed to stereo-active lone-pair on I5+ and asymmetrical bonds in the compounds. The role of K atoms is pronounced from crystal chemistry point of view, contributing optical nonlinearities. - Graphical abstract: Structure-properties relation is attributed to stereo-active lone-pair on I5+ and asymmetrical bonds in the compounds with the crucial role of K atoms, K[IO3] structure along 3-fold axis with umbrella-like IO3-group and with octahedral coordination—analog with perovskite. Highlights: ► Syntheses in hydrothermal conditions trigonal KIO3, K3[B6O10]Br and KTa[B4O6(OH)4](OH)2. ► Correlation with the perovskite, found for K-iodate and K,Br-borate. ► Structure-properties relation, attributed to I5+ lone-pair and asymmetric bonds including K-atoms. ► The compounds are phase matchable and strongly nonlinear from powder SHG measurements. ► Averaged second-order nonlinear coefficients 〈d〉 are 24, 2.8 and 1.9 pm/V.