Using X-ray diffraction to study thermal phase transitions in Cs 5H 3(SO 4) 4· xH 2O

Abstract

The crystal structures of three phases of Cs 5H 3(SO 4) 4· xH 2O–a low-temperature phase (LT phase), a room-temperature phase (RT2 phase), and an intermediate phase (RH phase)–have been determined from powder X-ray diffraction data measured at 90 – 450 K. The structures of the LT, RT2, and RH phases were orthorhombic Cmcm ( a = 6.304 Å; b = 10.388 Å; c = 29.499 Å), triclinic P-1 ( a = 6.248 Å; b = 6.187 Å; c = 14.833 Å; α = 92.08°; β = 88.86°; γ = 121.15°), and trigonal P3¯ m1 ( a = 6.233 Å; c = 14.788 Å), respectively. The crystal structures of the hydrated and anhydrous forms in the usual room-temperature phase (RT phase) were also analyzed; reduction in the length of the a-axis was observed upon the loss of crystalline water, whereas the length of the c-axis remained constant. These results suggest that shrinkage in the a direction of the anhydrous form prevented reorientation of its SO 4 tetrahedra, resulting in slower proton diffusion [K. Suzuki and S. Hayashi, Phys. Rev. B 74 (2006) 134303] than that in the hydrated form. These findings are consistent with a proton transport mechanism in which SO 4 reorientation is the rate-determining step.