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

Abstract

Four new quaternary molybdenum selenites, namely, HRb₃(Mo₅O₁₅)(SeO₃)₂(H₂O)₂1, α-Rb₄Mo₅O₁₅(SeO₃)₂(H₂O)₂2, β-Rb₄Mo₅O₁₅(SeO₃)₂(H₂O)₂3 and K₄Mo₅O₁₅(SeO₃)₂(H₂O)₂4 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)₂1 crystallizes in a polar space group (Pca2₁). α-Rb₄Mo₅O₁₅(SeO₃)₂(H₂O)₂2 crystallizes in a centrosymmetric (CS) space group (P2₁/n) whereas β-Rb₄Mo₅O₁₅(SeO₃)₂(H₂O)₂3 and K₄Mo₅O₁₅(SeO₃)₂(H₂O)₂4 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.