Quantum and isotope effects on hydrogen diffusion, trapping and escape in iron

We calculate the rate coefficient as a function of temperature for lattice diffusion of hydrogen and its isotopes in a-iron; and also for trapping and escape from a vacancy. We employ Monte-Carlo and molecular dynamics methods based around the Feynman path integral formulation of the quantum partition function. We find large quantum effects including tunnelling at low temperature and recrossing at high temperature due to the finite extent of the particle probability density. In particular these serve to increase the rate of trapping and to decrease the rate of escape at low temperature. Our results also show very clear non classical isotope effects.