Energy of migration of monovalent ions in NaCl: an experimental and theoretical study

Experimental diffusion measurements show that migration enthalpies of Cl-, Br- and I- in NaCl are comparable, while that of F- is considerably lower. Earlier studies had shown that migration enthalpies of Na+, K+, Rb+ and Cs+ in NaCl were similar. The polarised point ion model predicts migration energies of ions (by vacancy mechanism) to monotonically increase with ion size, contrary to experiment. Inversely, the shell model calculations rightly predict the variation of migration energies with ionic size. Thus, migration energies by vacancy mechanism do not vary significantly for ions larger than the host ions. However, in the case of the small ions, Li+ and F-, the migration energies by vacancy mechanism are much lower and in good agreement with experiment for F-