Concerted mass transport governs anomalous diffusion in anharmonic bcc metals

Understanding the microscopic origin of anomalous self-diffusion near melting remains a central challenge in bcc metals. Here we combine ab initio and machine-learning interatomic-potential molecular dynamics simulations (AIMD, MLIP-MD) to elucidate the mechanisms governing high-temperature mass transport in group-3–6 metals. Comparative AIMD simulations near melting reveal a clear distinction between dynamically-stabilized group-3 and group-4 bcc metals and the quasiharmonic group-5 and group-6. In the former, characterized by strongly anharmonic energy landscapes, sufficiently large thermal displacements of individual atoms trigger cascades of cooperative transport, including ring exchanges and spontaneous formation and migration of vacancy–crowdion-like Frenkel pairs. In contrast, diffusion in quasiharmonic bcc metals remains dominated by conventional vacancy-mediated mechanisms. Temperature-dependent MLIP-MD simulations of bcc Ti, taken as a representative dynamically-stabilized system, further show that migration rates of isolated point defects remain Arrhenius-like upon cooling toward dynamical instability. The curvature in self-diffusivity near melting instead originates from thermally activated collective reactions within nominally pristine lattice regions. Structural analysis reveals transient omega-like local environments as fingerprints of lattice softening accompanying these correlated dynamics. Together, these results identify anharmonic energy landscapes as the origin of anomalous diffusion in dynamically-stabilized bcc metals and suggest a general framework for high-temperature transport in crystalline solids.