Thermodynamic isotope effect in the system hydrogen-solid intermetallic compound hydride

Equilibrium hydrogen isotope exchange in the system gas-solid intermetallic compound hydride was analyzed theoretically for crystals in which hydrogen can occupy lattice vacancies of several types. A method for calculating se paration coefficients in the absence of mutual influence of vacancies was s uggested, and the dependence of this coefficient on the concentration (pres sure) of hydrogen in the system and the relative numbers of different vacan cies in the lattice was theoretically analyzed. The existence of a pressure dependence of separation coefficient alpha(H-D) at a constant isotope comp osition may be evidence that vacancies of different types participate in hy drogen absorption. Hydrogen atom vibrational frequencies in intermetallic c ompound lattices estimated from neutron diffraction data allow averaged eff ective frequencies for sites of each type to be determined; these frequenci es satisfactorily describe experimental isotope effects. The conclusion is drawn that the method of averaged effective frequencies can be recommended for theoretically analyzing isotope effects in systems with intermetallic c ompounds containing hydrogen substitution sites of different types.