Hysteretic Phonons and Quasielastic Response: A Raman Study of Thermal Memory in Two-Dimensional CuCrP 2 S 6

We present a comprehensive temperature-dependent inelastic light scattering (Raman) study on single crystals of two-dimensional CuCrP2S6, a layered van der Waals material exhibiting coupled magnetic and electric degrees of freedom. Raman measurements were performed from 5 to 300 K to probe the phonon dynamics across multiple structural and magnetic phase transitions. Our analysis reveals pronounced thermal hysteresis in phonon frequency, line width, intensity and dynamic Raman susceptibility, confirming the first-order nature of the antipolar transition near TC1 ∼ 145 K and a second-order transition near TC2 ∼ 190 K. Low-frequency modes associated with Cu+ and Cr3+ ions exhibit softening and anomalous line width behavior, in particular phonon mode P2 (∼37 cm–1), which shows nonmonotonic temperature dependence and intensity enhancement near 60 K, suggesting persistent off-center Cu+ dynamics in the quasi-antipolar phase. The coexistence and coupling of soft phonon modes and central peaks indicate a crossover from displacive to order–disorder type transition mechanisms. Additionally, phonon anomalies below the Néel temperature (TN ∼ 32 K) reflect spin-phonon coupling, linking lattice vibrations to long-range magnetic correlations. Our findings provide critical insight into the lattice instabilities, symmetry evolution, and quasiparticle interactions in CuCrP2S6, offering a deeper understanding of phase transition dynamics in two-dimensional multiferroic systems and guiding the future design of magnetoelectric and spintronic devices.