Cd-exchanged heulandite: symmetry lowering and site preference

Abstract

Fully Cd-exchanged heulandite was obtained from a Na-exchanged sample by treatment with 1 M Cd acetate solution at 373 K. Subsequent electron microprobe analyses revealed 4.0 Cd 2+ per formula unit (pfu), that was in good agreement 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 ̄ , and C1, of which only the Cm model yielded reasonable agreement with the observed diffraction data. Cadmium preferentially occupied the center of the A channel, where it formed a distorted octahedral Cd 2+(H 2O) 6 complex, and a central position in the B channel that was sevenfold coordinated by oxygen atoms of the framework and channel H 2O molecules. Seven additional low-populated Cd sites were located in the channels. Cd-exchanged heulandite was refined in Cm symmetry in contrast to most other natural and cation-exchanged heulandites that commonly show C2/ m symmetry. Symmetry lowering is attributed to Si,Al ordering in the tetrahedral framework as well as to the asymmetrical distribution of Cd 2+ ions due to preferred Cd bonding of oxygen atoms sharing Al-enriched tetrahedra and cation–cation repulsion.