Tailored Ordered Structures with Functional Units of Distorted [NbO₆] and Antiparallel [GeO₄] for Enhanced Birefringence in Germanate Crystal

Abstract Germanate oxides have been scarcely studied as promising birefringent crystals due to the inherently weak optical anisotropy. Here, a property improvement strategy with synergetic electron distribution and lattice vibration optimization is proposed by introducing tailored functional units of distorted [NbO₆] octahedra and antiparallel [GeO₄] motifs in germanates. A prospective Cs₃Nb₅GeO₁₆ (CNGO) crystal exhibits a breakthrough of experimental birefringence (0.19 at 532 nm), the largest value among the available germanate oxides, as well as a wide transparent window and excellent thermal stability up to 1265 °C. Combined density functional theory (DFT) and dipole moment calculations are performed to unveil the underlying physical mechanism of structure–property relationship. These attributes forebode the great potential of CNGO for manipulating polarized light and open a new avenue for innovating birefringent materials with desirable performances.