Heating-induced phase transformation in zeolite brewsterite: new 4- and 5-coordinated (Si,Al) sites

The dehydration-rehydration processes and phase transformations of brewster ite (space group P2(1)/ m, ideal formula (Sr,Ba)Al2Si6O16 . 5H(2)O) were st udied by single crystal X-ray diffraction on two samples heated in an evacu ated capillary for 24 h at 280 degrees C and 330 degrees C, respectively, a nd then cooled to room temperature. After the data collection, the capillar y of the crystal heated at 280 degrees C was broken, the sample was kept at room conditions for one month, and diffraction data were recollected. Brew sterite heated in vacuum at 280 degrees C shows a strong decrease of the un it cell volume (about 10%) and the statistical breaking of the T1-O7-T2 bri dge of the 4-ring of the 4(2)5(4) PBU. T1 cation, in 31% of cases, migrates to a new tetrahedral site, which shares three vertices with the previously occupied one; the fourth vertex is on the mirror plane, and joins two adja cent layers. T2 cation, in 20% of cases, migrates to a new site, which is c haracterized by a 5-fold coordination. This coordination polyhedron shares three vertices with the previously occupied tetrahedron; the fourth vertex is on the mirror plane, and joins two adjacent layers; the fifth one is uns hared. The formation of these new T-O-T bridges prevents the re-expansion a nd rehydration of the heated brewsterite; as a consequence, the crystal bro ught back to room conditions does not show any noticeable structural differ ence with respect to that heated at 280 degrees C. The exchangeable cations , which occupy only one extraframework site in the untreated brewsterite, s pread over several sites. The sample heated at 330 degrees C shows a furthe r decrease in the unit cell volume of about 2% with respect to the sample h eated at 280 degrees C, and an increase in the migration percentage of T1 a nd T2 cations to the new sites.