Av. Bochkarev et al., Thermodynamic isotope effect in the system hydrogen-solid intermetallic compound hydride, RUSS J PH C, 73(12), 1999, pp. 1968-1972
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.