Electrostatic properties in zeolite-type materials from high-resolution x-ray diffraction: The case of natrolite

High-resolution x-ray-diffraction data of natrolite at room temperature give by multipolar refinement an analytical expression of the electron density which permits the calculation of net atomic charges and electrostatic potential. The observed net charges show that the structural formula of natrolite may be written Na₂{Al}₂Si₂{O}₈$(Si${O}₂$),2${H}₂$O. The electrostatic energy of one sodium ion inside the crystal is estimated to -21.6 eV. Calculation of the electrostatic potential excluding one sodium shows that the minimum is precisely at the removed Na position.