PHASE-TRANSITION OF ZEOLITE NH4-RHO TO A HIGH-TEMPERATURE BUT LOW-VOLUME FORM

An in situ X-ray powder diffraction experiment evaluated by a Rietveld-type refinement shows that NH4-Rho zeolite, Cs(NH4)11Al12Si36O96, exists in two forms when heated in vacuum. The high-temperature form has at 573 K: a = 14.4803 (1) angstrom, D(x) = 1.688 g cm-3, and when quenched at 295 K: a = 14.4024 (6) angstrom, D(x) = 1. 716 g cm-3; the low-temperature form after prolonged cooling in vacuum at 295 K gives a = 14.8168 (2) angstrom, D(x) = 1.576 g cm-3. For all three cases: space group I43mBAR, Z = 1, M(r) = 3087.48, Cu Kalpha, lambda = 1.54056 angstrom. Thus the high-temperature form at 573 K has a volume 6.7% smaller than the low-temperature form at 295 K. At higher temperatures the framework contracts because the NH4 groups which at lower temperatures block the collapse of the single-eight rings Of TO4 (T = Si, Al) tetrahedra assume new positions located in the larger pores of the framework. The transformation between the two forms is reversible and is of the reconstructive type.