New tartratoborates: Synthesis, structure, and characterization of non-centrosymmetric ASr[C₄H₂O₆B(OH) ₂]·4H₂O (A = K⁺, Rb⁺)

Two new tartratoborate nonlinear optical materials, KSr[C₄H ₂O₆B(OH)₂]·4H₂O (KSCB) and RbSr[C₄H₂O₆B(OH)₂]·4H ₂O (RSCB), have been prepared through slow evaporation at room temperature. Single-crystal X-ray diffraction data reveal that KSCB crystallizes in the triclinic system, space group P1 (no. 1). It consists of KO₈ polyhedra, SrO₉ polyhedra, BO₄ tetrahedra, tartrate molecules and H₂O molecules. The isolated tartratoborate anions are connected to form a three-dimensional network structure by sharing oxygen atoms with K⁺ and Sr²⁺. The structural data show that KSCB and RSCB are isostructural and noncentrosymmetric. KSCB and RSCB produce second-harmonic generation intensities of 1.5 × KH₂PO ₄ and 1.7 × KH₂PO₄, respectively, and they are both type I phase-matchable. The UV-Vis diffuse reflectance spectra of these compounds indicate that they are both transparent from 250 to 850 nm. Related infrared spectra and thermal analyses were also performed.