  Diffusion is the underlying mechanism for many complicated materials
phenomena, and understanding it is basic to the discovery of novel materials
with desired physical and mechanical properties. Certain groups of solid
phases, such as the bcc phase of IIIB and IVB metals and their alloys, which
are only stable when they reach high enough temperatures and experience
anharmonic vibration entropic effects, exhibit "anomalously fast diffusion".
However, the underlying reason for the observed extraordinary fast diffusion is
poorly understood and due to the existence of harmonic vibration instabilities
in these phases the standard models fail to predict their diffusivity. Here, we
indicate that the anharmonic phonon-phonon coupling effects can accurately
describe the anomalously large macroscopic diffusion coefficients in the bcc
phase of IVB metals, and therefore yield a new understanding of the underlying
mechanism for diffusion in these phases. We utilize temperature-dependent
phonon analysis by combining ab initio molecular dynamics with lattice dynamics
calculations to provide a new approach to use the transition state theory
beyond the harmonic approximation. We validate the diffusivity predictions for
the bcc phase of titanium and zirconium with available experimental
measurements, while we show that predictions based on harmonic transition state
theory severely underestimates diffusivity in these phases.
