Activation energy barriers for Na migration in Na12A zeolite: The main contribution to ionic current via doubly occupied NaII site?

The activation barriers for Na jumps between the cationic sites NaI, NaII, and NaIII are calculated using VASP in the presence of anions (Cl−, Br−) or H2O as well as for the empty Na12A zeolite model. The inter-cage cationic NaIII→NaII'→NaIII′ path, where NaIII′ is located in the neighbor α–cage, was modeled via an intermediate state (NaII′) corresponding to two NaII cations in one 8R window. This NaII′ point can also serve as an intermediate for the second type of intra-cage Na drift, i.e., to a NaIII site in the same cage. Reasonable agreement with experimental activation energy was achieved for NaII'→NaIII′, NaII→NaIII, and NaIII→NaIII jumps. The lower activation energy is obtained for the NaII'→NaIII' jump than for NaII→NaIII, which indicates the possible role of the doubly occupied 8R window as an intermediate state in the Na transfer with high frequency and lower activation energy related to ionic current (inter-cage transfer) in Na12A.