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, HRb(3)(Mo(5)O(15))(SeO(3))(2)(H(2)O)(2)1, α-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)2, β-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)3 and K(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)4 were synthesized by hydrothermal reactions. All of the four compounds feature a zero-dimensional (0D) (Mo(5)O(15))(SeO(3))(2)](4-) anionic unit composed of a five-member MoO(6) octahedral ring capped by two SeO(3)(2-) 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. HRb(3)(Mo(5)O(15))(SeO(3))(2)(H(2)O)(2)1 crystallizes in a polar space group (Pca2(1)). α-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)2 crystallizes in a centrosymmetric (CS) space group (P2(1)/n) whereas β-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)3 and K(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)4 are isomorphous, crystallize in a chiral space group (C2). The chiral structures of 3 and 4 contain two similar polyanions of Mo(5)O(15)(SeO(3))(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 (KH(2)PO(4)) 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.