High-temperature crystal structures and chemical modifications in RbH2PO4

We have used laboratory and synchrotron x-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline RbH2PO4 upon heating within the 30-250 deg. C temperature interval. Our data show no evidence of the previously reported onset of partial polymerization at T = 96 deg. C (Park et al 2001 J. Phys.: Condens. Matter 13 9411) which was proposed as an explanation for the high-temperature proton conductivity enhancement in phosphate-based solid acids. Instead, we found that a tetragonal → monoclinic polymorphic transition initiates at T∼90 deg. C. The transition is complete at T∼130 deg. C, and the new monoclinic RbH2PO4 polymorph is stable upon further heating to T = 200 deg. C. Moreover, its crystal structure is isomorphic to that of monoclinic CsH2PO4. This remarkable similarity suggests that the microscopic structures and dynamics responsible for the high-temperature superprotonic behavior of RbH2PO4 could be the same as those of its Cs-based counterpart.