The heat of transport Q* of tritium in the α phase of vanadium has been measured at average temperatures of 343, 423 and 523 K, and an isotope dependence of Q* has been ascertained by comparing the present data with those of protium and deuterium reported by other investigators. The following characteristics of Q* common to the group V metals have been pointed out by summarizing the experimental data of Q* of hydrogen isotopes in vanadium, niobium and tantalum reported by the present authors and other investigators, 1. (i) Q*(Ta) > Q*(Nb) > Q*(V) for each hydrogen isotope. 2. (ii) QT* > QD* > QH* in each metal. 3. (iii) There is a definite relation between the heat of transport Q* and the activation energy of diffusion ΔHm concerning their element and isotope dependence. 4. (iv) The bias effect of the biased-jump diffusion model approaches zero with temperature. The atomic origin of the bias effect has been ascribed to the lattice distortion energy and electronic polarization energy localized in the neighbourhood of the hydrogen isotopes and the isotope dependence of the heat of transport Q* has been interpreted in terms of the isotope dependence of the lattice distortion energy.
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