The nature of chemical interactions in anhydrous sodalites, M-6(AlSiO4)(6), M = Na, Ag, Tl

Maximum. entropy method (MEM) charge density analysis has been carried out on synchrotron X-ray powder diffraction data of a series of anhydrous sodal ites, M-6(AlSiO4)(6), M = Na, Ag, Tl, Analysis of the MEM electron densitie s reveals the nature of the interactions in the structures. The sodium atom s are bonded to the framework with predominantly electrostatic closed shell interactions, whereas the heavy metal ions form partly covalent bonds to t he framework oxygens. At the same time the heavy metal atoms appear almost neutral, while the sodiums, as expected, are positively charged. The differ ences in the atomic charges and in the chemical interactions between the gu est atoms and the framework are used to explain the varying dynamical behav ior of the guest atoms. The guest-host interactions have pronounced effects on the framework atoms with Al, especially, showing reduced density. The r emoval of electron density from the framework bonds upon formation of coval ent guest-host bonds is the cause of the abnormal chemical, shifts and quad rupole coupling constants found in NMR for the heavy metal compounds. The f act that experimental electron densities can be estimated from powder diffr action data gives promise for increasing the understanding of the chemical and physical properties of a wide range of technologically important, mater ials.