Synthesis, Structure Determination, and Characterizations of a Polar Salt-Inclusion Scandium Germanate, Rb10Li3Sc4Ge12O36F

A noncentrosymmetric salt-inclusion germanate, Rb10Li3Sc4Ge12O36F, was grown through spontaneous crystallization from a LiF–RbF flux. It crystallizes in the polar space group P31c with cell parameters of a = 10.7587(3) Å, c = 21.6691(10) Å, and Z = 2. Its structure features a complex 3D framework composed of helical [Ge4O12] chains from condensed [GeO4] tetrahedra running along the c axis, which are interconnected by the [ScO6] octahedra. Voids of the 3D net are filled with Rb+ ions, Li+ ions, and isolated trigonal-bipyramidal [Rb3Li2F] superalkali clusters. The title compound has a large band gap of 5.6 eV, a moderate powder second-harmonic-generation response of 0.9KDP, and an extremely small birefringence of 0.001, as was further unraveled by theoretical calculations.