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

Periodic density functional calculations on zeolite Na-MAP are presented. T hese calculations used a preliminary model obtained from X-ray powder diffr action as starting point and fully optimized the atom positions within the unit cell. It is shown that the structure obtained in the calculation repro duces the observed powder diffraction pattern better than the preliminary m odel and that internal coordinates are closer to values known from other ze olites. The structural model predicted proved useful for further refinement of the experimental data. The framework of the anhydrous structure is 43 k J (mol NaSiAlO4)(-1) more stable than the framework of the hydrated structu re as long as there are no water molecules present.