When Cubic is Not Isotropic: Phonon–Exciton Decoupling in CuInSnS ₄ Single Crystals

ABSTRACT Atomic‐scale disorder can create hidden optical anisotropy even in crystals that are structurally cubic on average. Here, we show that single crystals host locally symmetry‐broken environments arising from intrinsic In/Sn cation disorder, which affect vibrational and excitonic properties in markedly different ways. Combining polarization‐ and temperature‐dependent Raman spectroscopy, infrared near‐field microscopy, steady‐state and time‐resolved photoluminescence, and first‐principles calculations, we find that phonons remain largely symmetry‐averaged and locally homogeneous on the nanoscale. In contrast, photoluminescence reveals a lower‐energy band‐tail emission with pronounced polarization anisotropy following a well‐defined angular symmetry, highlighting the strong sensitivity of excitonic states to local symmetry breaking. This phonon–exciton decoupling reveals that intrinsic disorder can localize excitons while preserving vibrational coherence and dielectric homogeneity, thereby opening new opportunities for polarization‐sensitive light sources, anisotropic photodetectors, and exciton‐based optical functionalities even in nominally cubic multinary semiconductors.