Stoichiometric alpha-hexacelsian(LTA) and non-stoichiometric alpha-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 degrees C and 700 degrees 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 degrees C and 325 degrees C. At 325 degrees C the unit cell volume of the alpha-hexacelsian(LTA) phase increases. This is caused by structural inversion to the beta-hexacelsian(LTA) phase. The thermal expansion of the unit cell of the alpha-hexacelsian(FAU) phase does not show any discontinuities. The distinct unit cell volume properties observed upon heating at 325 degrees C, are consistent with results of DSC measurements of alpha-hexacelsian(LTA) and alpha-hexacelsian(LTA) samples. The crystal structures of the alpha- (at 25 degrees C) and beta-forms (at 362 degrees 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 alpha-hexacelsian(LTA); R-wp = 5.11, R-p = 3.53, R-Bragg = 5.42 for alpha-hexacelsian(FAU); R-wp = 7.02, R-p = 4.98,R-Bragg = 6.48 for beta-hexacelsian(LTA); and R-wp = 5.05, R-p = 3.58, R-Bragg = 4.50 for beta-hexacelsian(FAU), respectively.