Using X-ray diffraction to study thermal phase transitions in Cs5H3(SO4)4·xH2O

The crystal structures of three phases of Cs5H3(SO4)4·xH2O–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 SO4 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 SO4 reorientation is the rate-determining step.