THE COMPUTER MODELING OF LANTHANUM IN THE FRAMEWORK OF FAUJASITE .2. HYDRATED LANTHANUM ZEOLITE-Y

The methodology of Part 1 was used to compute the lattice energies of various structures for hydrated La-ZY. The structure deduced by Cheetham et al. from their 5 K neutron diffraction measurements is shown to be compatible with the potentials used to a high degree of consistency, except for framework hydrogen. However, the modeling studies suggest this is not the only feasible or even the most stable structure type. One variant is the same as the diffraction structure except that the H's of the OH-'s relate differently to the framework. Much more common among the simulated structures, however, is an arrangement in which one of the extraframework H's is on an Sll' site, where it interacts closely with three O2's, and the other is more distant from the framework. The La3+ ions remain on Sl' sites. An examination of the Madelung and short-range energies of all the species composing the various structures shows that their relative stabilities cannot be understood in terms of a few predominant local interactions. This study also indicates that the conclusions of Cheetham et al. concerning framework hydrogen need to be reviewed.