A LARGE VOLUME CONTRACTION ACCOMPANIES THE LOW-TEMPERATURE TO HIGH-TEMPERATURE PHASE-TRANSITION OF ZEOLITE SR-RHO

Upon heating, Sr-exchanged zeolite rho, Sr6Al12Si36O96, changes into a high-temperature form with a unit cell volume which is, surprisingly, about 7% smaller than the low-temperature modification. Both modifications crystallize in space group I43m. At 373 K in vacuum the unit cell constant a of Sr-rho is 14.5578(4) Å; at 473 K it is 14.2293(9) Å. Rietveld-type refinement of powder X-ray diffraction data shows that the two forms differ in the distribution of Sr atoms relative to the double eight-membered rings in the (Si,Al)O2 framework of zeolite rho: in the high-temperature form the Sr atoms are concentrated in the centers of these rings and pull them together, while in the low-temperature form threequarters of the Sr atoms are located in the plane of the single eight-membered ring (on the face of the double eight-membered rings) and thus prevent a collapse of these rings which, nevertheless, occurs in the high-temperature form. The transformation between the two forms is of the reconstructive type, as far as the Sr atom positions are concerned, but is fully reversible. © 1991.