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

Abstract

Stoichiometric α-hexacelsianLTA and non-stoichiometric α-hexacelsianFAU 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-hexacelsianLTA phase increases. This is caused by structural inversion to the β-hexacelsianLTA phase. The thermal expansion of the unit cell of the α-hexacelsianFAU phase does not show any discontinuities. The distinct unit cell volume properties observed upon heating at 325 °C, are consistent with results of DSC measurements of α-hexacelsianLTA and α-hexacelsianFAU samples. The crystal structures of the α-(at 25 °C) and β-forms (at 362 °C) of Si, Al ordered hexacelsianLTA and disordered hexacelsianFAU 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: Rwp = 6.19, Rp = 4.29, RBragg = 6.68 for α-hexacelsianLTA; Rwp = 5.11, Rp = 3.53, RBragg = 5.42 for α-hexacelsianFAU; Rwp = 7.02, Rp = 4.98, RBragg = 6.48 for β-hexacelsianLTA; and Rwp = 5.05, Rp = 3.58, RBragg = 4.50 for β-hexacelsianFAU, respectively.