Directional Te3 Polyanion Connectivity in Layered In2Te5: Linear Dichroism and Polarization-Dependent Effective Second-Harmonic Generation

Directional polyanion connectivity provides a structural perspective on optical anisotropy in layered materials. Here, we investigate the structural, electronic, and optical properties of In2Te5. Crystallographic analysis identifies distinct Type-A and Type-B Te3 motifs with preferential connectivity along the b-axis. First-principles calculations show that Te3-associated sites contribute 70.24% of the integrated projected density of states within 0.2 eV below the valence-band maximum, with per-atom enrichment factors of 1.86 and 2.59 for Type-A and Type-B sites, respectively, relative to the framework Te. Polarized Raman spectroscopy and optical measurements establish pronounced anisotropy, including broadband linear dichroism with an effective-absorptance ratio of 1.47–1.49 over 350–808 nm. Calculated absorption spectra reproduce the preferred b-axis polarization direction, although quantitative discrepancies remain. Furthermore, effective second-harmonic generation (SHG) measurements on four flakes (13–75 nm) reveal parallel-channel maxima close to the b-axis. These findings support a structure–electronic–optical correlation underlying linear dichroism and reveal a related crystallographic preference for effective SHG.