Time—temperature resolved synchrotron XRPD study of the hexacelsian α↔β polymorph inversion

Abstract

Stoichiometric α-hexacelsian LTA and non-stoichiometric α-hexacelsian FAU phases were synthesized by the method of high-temperature induced transformation of Ba-exchanged LTA and FAU zeolites. The unit cell thermal expansion properties were followed using synchrotron X-ray Powder Diffraction (XRPD) data in the temperature range between 25°C and 700°C. Both samples exhibit normal thermal behavior with all unit cell axes expanding upon heating. The expansion properties of the unit cell volumes are similar in the temperature range between 25°C and 325°C. At 325°C the unit cell volume of the α-hexacelsian LTA phase increases. This is caused by structural inversion to the β-hexacelsian LTA phase. The thermal expansion of the unit cell of the α-hexacelsian FAU phase does not show any discontinuities. The distinct unit cell volume properties observed upon heating at 325/dgC, are consistent with results of DSC measurements of α-hexacelsian LTA and α-hexacelsian FAU samples. The crystal structures of the α- (at 25°C) and β-forms (at 362°C) of Si, Al ordered hexacelsian LTA and disordered hexacelsian FAU phases, were refined in the trigonal space group P-3 (147), from synchrotron XRPD data by the Rietveld method. These structures were refined with agreement factors: R wp = 6.19, R p = 4.29, R Bragg = 6.68 for α-hexacelsian LTA; R wp = 5.11, R p = 3.53, R Bragg = 5.42 for α-hexacelsian FAU; R twp = 7.002, R p=4.98, R Bragg = 6.68 for β-hexacelsian LTA; and R wp = 5.05, R p = 3.58, R Bragg = 4.50 for β-hexacelsian FAU, respectively.