Dehydration dynamics of barrerite: an in-situ synchrotron XRPD study.

The thermally induced structural modifications of the natural zeolite barrerite [Na16Al16Si56O144\ub752H2O, a = 13.6239(4) \uc5, b = 18.2033(5) \uc5, c = 17.8317(7) \uc5, V = 4422.3(3) \uc53, space group Amma, framework type STI] were studied in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation, in the temperature range 339\u2013973 K. In the initial stage of heating, up to 508 K, barrerite Phase A (space group Amma) is stable, the unit-cell volume decreases by about 4% and a water release of about 66% is observed. Between 521 and 598 K, a phase transition to the collapsed so-called barrerite Phase B (space group Amma) is observed. During the transition, the rotation of the 4254 secondary building units causes a large decrease in cell volume and deformation of the channel system. Phase B, at 611 K, shows the statistical breaking of T-O-T bridges in the 4-rings and the migration of the involved tetrahedral atoms to new \u201cface-sharing\u201d tetrahedra, with a consequent reduction of the free volume of the channels parallel to [100]. The new structure is stable up to 741 K and the total volume decrease is about 16%. A new phase appears from 754 K with cell parameters similar to those reported for the highly deformed barrerite Phase D and is stable up to 910 K, which is the temperature at which the total volume decrease is 22.5%. The material does not undergo amorphization up to the highest temperature investigated