Cd-exchanged heulandite: symmetry lowering and site preference

Fully Cd-exchanged heulandite was obtained from a Na-exchanged sample by tr eatment with 1 M Cd acetate solution at 373 K. Subsequent electron micropro be analyses revealed 4.0 Cd2+ per formula unit (pfu), that was in good agre ement with 4.11 Cd pfu refined from single-crystal X-ray data collected at 293 K. Crystal structure refinements were performed in the space groups C2/ m, C2, Cm, C (1) over bar, and C1, of which only the Cm model yielded reaso nable agreement with the observed diffraction data. Cadmium preferentially occupied the center of the A channel, where it formed a distorted octahedra l Cd2+(H2O)(6) complex, and a central position in the B channel that was se venfold coordinated by oxygen atoms of the framework and channel H2O molecu les. Seven additional low-populated Cd sites were located in the channels. Cd-exchanged heulandite was refined in Cm symmetry in contrast to most othe r natural and cation-exchanged heulandites that commonly show C2/m symmetry . Symmetry lowering is attributed to Si,AI ordering in the tetrahedral fram ework as well as to the asymmetrical distribution of Cd2+ ions due to prefe rred Cd bonding of oxygen atoms sharing Al-enriched tetrahedra and cation-c ation repulsion. (C) 2000 Elsevier Science B.V. All rights reserved.