Structural isotopic effect of the alpha/beta phase transition in vanadium hydride and its influence on the equilibrium coefficient of separation of hydrogen isotopes in the gas-solid system

The equilibrium separation coefficient for hydrogen isotopes (alpha(H-D)) i n the vanadium dihydride VHn(s, n approximate to 0.7)-H-2(g) system was mea sured by equilibration in a circulation unit and laser desorption at 298, 3 73, and 473 g. The alpha(H-D) distribution coefficients determined by the t wo methods were in close agreement with each other. The literature neutron diffraction data were used to calculate the isotopic ratios of the reduced partition functions and alpha(H-D) for the alpha' and beta' phases of the V 2H and V2D hydrides by direct summation over vibrational energy levels. The calculated isotopic effects (both structural isotopic effects in phase dia grams and the separation coefficient for hydrogen isotopes) were fairly clo se to the experimental values. This result substantiates the suggestion tha t the reason for the unusual behavior of the separation coefficient for hyd rogen isotopes and for the difference in the crystal structures of vanadium hydride and its deuterated analogue is a combination of a strongly anharmo nic potential in octahedral lattice sites and a small difference in the ene rgies of coordination of hydrogen atoms in tetrahedral and octahedral sites .