Synthesis, Structure Determination, and Characterizations of a Polar Salt-Inclusion Scandium Germanate, Rb 10 Li 3 Sc 4 Ge 12 O 36 F.

A noncentrosymmetric salt-inclusion germanate, Rb 10 Li 3 Sc 4 Ge 12 O 36 F, was grown through spontaneous crystallization from a LiF-RbF flux. It crystallizes in the polar space group P 31 c 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 [Ge 4 O 12 ] chains from condensed [GeO 4 ] tetrahedra running along the c axis, which are interconnected by the [ScO 6 ] octahedra. Voids of the 3D net are filled with Rb + ions, Li + ions, and isolated trigonal-bipyramidal [Rb 3 Li 2 F] 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.