Syntheses, crystal structures and SHG properties of a series of polar alkali-metal molybdenum(VI) selenites based on strandberg-type [Mo5O15(SeO3)2]4- polyanion

Four new quaternary molybdenum selenites, namely, HRb3(Mo5O15)(SeO3)(2)(H2O)(2) 1, alpha-Rb4Mo5O15(SeO3)(2)(H2O)(2) 2, beta-Rb4Mo5O15(SeO3)(2)(H2O)(2) 3 and K4Mo5O15(SeO3)(2)(H2O)(2) 4 were synthesized by hydrothermal reactions. All of the four compounds feature a zero-dimensional (0D) [(Mo5O15)(SeO3)(2)](4-) anionic unit composed of a five-member MoO6 octahedral ring capped by two SeO32- trigonal pyramids, with the Rb+/K+ or/and H+ cations and water molecules acting as spacers and keeping charge balance. Although these compounds exhibit similar chemical formula, their structures are slightly different. HRb3(Mo5O15)(SeO3)(2)(H2O)(2) 1 crystallizes in a polar space group (Pca2(1)). alpha-Rb4Mo5O15(SeO3)(2)(H2O)(2) 2 crystallizes in a centrosymmetric (CS) space group (P2(1)/n) whereas beta-Rb4Mo5O15(SeO3)(2)(H2O)(2) 3 and K4Mo5O15(SeO3)(2)(H2O)(2) 4 are isomorphous, crystallize in a chiral space group (C2). The chiral structures of 3 and 4 contain two similar polyanions of [Mo5O15(SeO3)(2)](4-) with opposite handedness. Second-harmonic-generation (SHG) measurements indicate that 1, 3 and 4 are all SHG-active. Compound 1 displays a weak SHG response of about 20% of that of KDP (KH2PO4) and is phase-matchable whereas the SHG responses of 3 and 4 are very weak (less than 5% of that of KDP). Thermal analyses and optical property measurements have also been performed.