Lattice softening and diffusive dynamics in the polar metal LiReO₃

Polar metals, characterized by the nontrivial coexistence of metallicity and polar structural order, define an emerging frontier in quantum materials research. However, the interplay between their structural phase transitions and fluctuation dynamics remains poorly understood. Here, we reveal distinct diffusive dynamics in metallic lithium rhenium trioxide (LiReO₃) associated with its polar-to-nonpolar transition. Unlike isostructural lithium niobate (LiOsO₃) and related systems, LiReO₃ exhibits pronounced phase fluctuations both above and below T_s. Thermoelectric, Raman, and ultrasound measurements demonstrate a probe-dependent thermal hysteresis, while ultrasound data further show lattice softening and persistent resonant absorption at low temperatures across a broad timescale (1 to 100 microseconds). These observations indicate a multiscale spatiotemporal dynamics governed by a shallow anharmonic potential stabilized by itinerant electrons, as supported by finite-temperature first-principles calculations. By mapping the fluctuation landscape shaped by itinerant electrons, this work offers a previously unexplored perspective for exploiting fluctuation-driven phenomena in polar metals.