Rb₂ZnGe₃Se₈: A Large-Birefringence Layered Chalcogenide Enabled by Anisotropic Electrons in Tetrahedral Units

Birefringence, as a manifestation of optical anisotropy in crystalline materials, is a fundamental physical property that plays a crucial role in nonlinear optics, polarization control, and photonic device engineering. Traditionally, birefringence has been primarily ascribed to geometric anisotropy in crystal structures. However, recent research has demonstrated that anisotropic electronic distributions, even in geometrically symmetric frameworks, can significantly enhance optical anisotropy. In this work, we report on the synthesis, structural characterization, and optical property evaluation of Rb₂ZnGe₃Se₈, a newly identified member of the A^I₂B^IIM^IV₃Q₈ chalcogenide family, which exhibits a large theoretical birefringence of 0.257 at the wavelength of 1064 nm. First-principles calculations reveal that the large birefringence arises from spatially anisotropic electron distributions within the [GeSe₄] tetrahedra, rather than from overt structural distortion. The results highlight a distinct electronic origin of birefringence and expand the current understanding of how directionally polarized bonding interactions can induce significant optical anisotropy in nominally symmetric frameworks. This work deepens the understanding of structure-electronic-optical coupling mechanisms in layered chalcogenide frameworks and supports the rational design of materials with large birefringence.