Synthesis, characterization, and theoretical analysis of three new nonlinear optical materials K 7 MRE 2 B 15 O 30 (M= Ca and Ba, RE= La and Bi)

Three new complex borate compounds K 7 CaBi 2 B 15 O 30 , K 7 CaLa 2 B 15 O 30 and K 7 BaBi 2 B 15 O 30 have been synthesized by the high-temperature solution method. K 7 CaLa 2 B 15 O 30 and K 7 CaBi 2 B 15 O 30 crystallize in the chiral trigonal space group R 32, while K 7 BaBi 2 B 15 O 30 crystallizes in the noncentrosymmetric orthorhombic polar space group Pca 2 1 . All of the title compounds have similar three-dimensional crystal structures, which are composed of isolated B 5 O 10 groups and LaO 6 or BiO 6 octahedra, and K + , Ca 2+ , and Ba 2+ cations fill into the cavities to keep charge balance. Based on our research, in the system of K 7 M II RE 2 B 15 O 30 (M II = Ca, Sr, Ba, Zn, Cd, Pb, K/RE 0.5 ; RE = Sc, Y, La, Gd, Lu, Bi), K 7 BaBi 2 B 15 O 30 is unique and crystallizes in a different space group, which enriches the structural chemistry of borate. Detailed structural analyses indicate that the structural variation is due to the difference in size and coordination number of the alkaline-earth metal cations. Besides, UV-Vis-NIR spectroscopy analysis and the second-harmonic generation (SHG) measurement on the powder samples show that K 7 CaBi 2 B 15 O 30 exhibits a UV cutoff edge (about 282 nm) and a moderate SHG response (about 0.6 × KDP). In addition, thermal analysis and infrared spectroscopy were also presented. To better understand the structure-property relationships of the title compounds, the first-principles calculations have been performed.