The structure of zeolite Na-MAP: Aiding Rietveld refinement by density functional calculations

Periodic density functional calculations on zeolite Na-MAP are presented. These calculations used a preliminary model obtained from X-ray powder diffraction as starting point and fully optimized the atom positions within the unit cell. It is shown that the structure obtained in the calculation reproduces the observed powder diffraction pattern better than the preliminary model and that internal coordinates are closer to values known from other zeolites. The structural model predicted proved useful for further reÐnement of the experimental data. The framework of the anhydrous structure is 43 kJ (mol NaSiAlO₄)ˉ¹ more stable than the framework of the hydrated structure as long as there are no water molecules present.