High-temperature crystal structures and chemical modifications in RbH$_{2}$PO$_{4}$

We have used labortatory and synchrotron x-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline RbH$_{2}$PO$_{4}$ upon heating within the 30-250 $^{\circ}$C temperature regime. We report no evidence of the previously reported onset of partial polymerization at $T = 96 ^{\circ}$C which was proposed as an explanation for the observed high-temperature several-order-of-magnitude increase in proton conductivity observed in phosphate-based solid acids. Instead, we found that a tetragonal-to-monoclinic polymorphic transition initiates at $T \sim 90 ^{\circ}$C. The transition is complete at $T \sim 130 ^{\circ}$C, and the new monoclinic RbH$_{2}$PO$_{4}$ polymorph is stable upon further heating to $T = 200 ^{\circ}$C. Moreover, this reported monoclinic phase is {\it isomorphic} to that of monoclinic CsH$_{2}$PO$_{4}$. This remarkable similarity suggests that the microscopic structures and dynamics responsible for the high-temperature superprotonic behavior of RbH$_{2}$PO$_{4}$ could be the same as those of its Cs-based counterpart.