Thermal behavior of zeolites. Single crystal x-ray study of dehydration and rehydration mechanism in yugawaralite

Dehydration and rehydration processes and phase transformations of yugawaralite (I) were studied by single crystal x-ray diffraction. I kept in vacuum at room temp. does not undergo any noticeable structural modifications. When I is heated at 100° in vacuum for 24 h, only the water mol. not coordinated with the Ca atom is lost (≈ 6%), while the 2 water mols. occupying alternative positions in the hydrated sample move into only one fully occupied site placed in an intermediate position: the Ca coordination remains virtually unchanged. At 150° a water loss of 53% is indicated, and the residual water mols. are localized in only 2 fully occupied sites. The Ca atoms are 6-coordinated by 4 framework O and the 2 residual water mols. The most relevant modifications of the framework are the increased difference between the Al-OCa-bonded and AI-OCa-nonbonded distances, and a lengthening (by ≈ 0.04 Å) and a narrowing (by ≈ 0.04 Å) of the structural units of I formed by a pair of 5-membered rings sharing an edge when compared with hydrated samples. At 180°, a sudden decrease of the I unit cell vol. occurs (≈ 12%), the cell parameter a is doubled, and the space group changes from Pc to Pn. The structure refinement of a crystal heated at 200° shows a further 50% loss of the residual water present at 150°, a collapsing of the channel system, and strong squashing of the framework. Owing to the doubling of parameter a there are 2 sym. independent Ca sites, both 6-coordinated with five framework O and 1 water mol. At 250°, I does not cause any significant modifications with respect to the structure at 200°. When I heated at 150° is brought back to room conditions for 2 wk, complete rehydration occurs; the water mols. are in the same positions and have the same occupancies as in the hydrated sample