Localized Excitons and Exciton-Phonon Coupling in Antiferromagnetic AgCrP2S6

Abstract

AgCrP2S6 combines a low-symmetry thiophosphate framework with an antiferromagnetic Cr sublattice and nonmagnetic Ag sites, where covalency and magnetic localization compete in the low-energy optical response. Using single-crystal x-ray diffraction, Raman spectroscopy, lattice-dynamical calculations, and self-consistent vertex-corrected many-body methods, we determine its structural, electronic, and excitonic properties. The material remains monoclinic from 100 to 300 K, with low-temperature Ag-site disorder, and is dynamically stable. Many-body calculations find a reduced quasiparticle gap relative to Cr trihalides, yet the lowest excitons stay predominantly local: a weakly bright, strongly anisotropic Frenkel exciton near 1.4 eV with dominant onsite d-d character and ligand-assisted d-p admixture. In the C2h setting this state is mainly Bu, with Ag diagonal and Bg bright-dark phonon coupling channels. Stronger p-d hybridization narrows the gap, while antiferromagnetic exchange and Ag dilution suppress intersite coherence. Cation substitution and ligand-ion engineering thus provide a transferable handle for tuning exciton localization in related 2D magnets, with implications for boson coupling and spintronics.