Anomalous isotope effect for hydrogen diffusion in fcc metals

Most available data for diffusion of hydrogen isotopes in fcc metals show Arrhenius behavior with preexponential factors which exhibit the classical inverse-square-root mass dependence but with activation energies $U$ which show the anomalous behavior $U_{\mathrm{H}}>U_{\mathrm{D}}>U_{\mathrm{T}}$. This behavior may be understood in terms of a quantum rate theory based on Gaussian wave-packet ensembles in which the stright-line Arrhenius behavior arises from the opposing effects of quantum statistics and tunneling.