Quantum-thermal crossover of hydrogen and tritium diffusion in -iron

Yoshikawa, Takehiro*; Takayanagi, Toshiyuki*; Kimizuka, Hajime*; Shiga, Motoyuki   

The diffusion coefficients of hydrogen (H) and tritium (T) in -Fe have been computed using two approximate quantum dynamical techniques, i.e. centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD), in the temperature range of T = 100-1000 K using the embedded-atom-method (EAM) potential. It has been found that the RPMD and CMD methods give very similar results. From a further analysis based on quantum transition state theory (centroid density QTST) combined with path integral molecular dynamics (PIMD), it has been clear that there is a crossover between thermal and quantum mechanisms at about T = 500 K and 300 K for H and T diffusions, respectively. The importance of nuclear quantum effects at low temperatures has been illustrated in terms of the effective free energy surface map.