K₂[Ge(B₄O₉)]·2H₂O: A unique 3D alternating linkage mode of a B₄O₉ cluster and GeO₄ unit in borogermanate with two pairs of interweaving double helical channels

The first noncentrosymmetric potassium templated borogermanate, K ₂[Ge(B₄O₉)]·2H₂O, has been solvothermally synthesized and characterized by IR, SEM, powder X-ray diffraction (PXRD), TGA, energy dispersive spectroscopy (EDS), single crystal X-ray diffraction, and second harmonic generation (SHG) activity, respectively. This compound crystallized in the monoclinic space group Cc with a = 6.793(1) Å, b = 12.0699(3) Å, c = 11.895(3) Å, β = 91.156(4)°, V = 975.1(4) ų, and Z = 4. Interestingly, the structure possesses a unique 3D alternating linkage mode of B₄O₉ clusters and GeO₄ units and contains two pairs of interweaving double helical channels with a 10-membered ring. In one, the right- and left-handed helices couple each other, and in the other, the left- and right-handed helices couple together. The compound exhibits distinct nonlinear optical (NLO) properties because it lacks a symmetric center. Furthermore, the framework of the title compound not only displays high thermal stability but also can be recoverable to a large degree during a dehydration-hydration cycle.