An ab Initio Periodic Hartree-Fock Study of Group IA Cations in ANA-Type Zeolites

Abstract

This study investigates the electronic structure of Group IA cations intercalated into zeolites with the analcime (ANA) framework using ab initio periodic HartreeFock theory. The purpose of the study is to gain a better understanding of the role played by electron-donating species in zeolites in general, with specific applications to materials that have been suggested as storagematrices for radioactive materials. The effect of the intercalated species (Na, K, Rb, and Cs) on the electronic structure of the zeolite is presented on the basis of an analysis of the total and projected density of states, Mulliken charges, and charge density differences. The results of those analyses indicate that, relative to a charge neutral atomic state, the Group IA species donate an electron to the zeolite lattice and interact most strongly with the s and p atomic states of oxygen as the species are moved through the lattice. In addition, estimates of the self-diffusion constants of Na, K, Rb, and Cs based upon a one-dimensional diffusion model parameterized from the ab initio total energy data will be presented. We find that the estimated self-diffusion rate decreased markedly with increasing cation size, and that the diffusion rate of Cs was essentially zero in bulk pollucite. The cationic charge is unchanged as the species moves through the lattice. This indicates that model potentials based upon an ionic interpretation of the Group IA species may be reasonable.

Determination of Transmutation Effects in Crystalline Waste Forms

“…To obtain a better agreement with experiment, we have found it necessary to explore different core-valence radius and interstitial charge densities, which in turn affect the phase of the calculated XANES features. The results from L-DOS calculations are in qualitative agreement with calculations done by Anchell et al [9] using Hartree-Fock theory. The ability to calculate both the L-DOS and the XANES makes possible to correlate features in the XANES and electronic properties of these systems.…”

“…The studies at the APS made use of new instrumentation that became available at the facilities of the Materials Research Collaborative Access Team (MRCAT). The suite of spectroscopic tools used to characterize the radiation effects on a range of length scales included x-ray diffraction (XRD with Rietveld analysis) [7], real-space analysis of the diffraction pattern or pair distribution function (PDF) analysis [8], x-ray absorption near edge structure (XANES) [9], and extended x-ray absorption fine structure spectroscopy (EXAFS) [10]. The combination of these techniques provides structural information at the atomic to next-nearest-neighbor length scale (EXAFS), from the nearest neighbor to the ~1.0 nm length scale (PDF), and at the bulk length scale (XRD-Rietveld analysis).…”

Microwave vs. conventional synthesis of analcime from clear solutions

“…Small pore zeolites, such as analcime may be important for some possible future technical innovations in selective adsorption and heterogeneous catalysis. An example is the possibility of its use in matrix storage for radioactive materials [9]. This zeolite is generally synthesized from aluminosilicate clear solutions or gels at temperatures of about 100-200 1C.…”

Determination of transmutation effects in crystalline waste forms. 1998 annual progress report

“…Of these candidates, pollucite offers the advantage of 40 mass% Cs content in a relatively dense mineral. The measured and calculated solubility of pollucite are approximately three orders of magnitude less than that measured for the silicate glass (STRACHAN andSCHULZ 1979 andANCHELL et al 1993). Pollucite can be produced from aqueous solutions with readily available materials (STRACHAN and SCHULZ 1979) or by low temperature methods (HOGAN and RISUD 1991).…”