High-Temperature Phase Transitions in CsH$_{2}$PO$_{4}$

In order to uncover the microscopic origin of the temperature-induced three-order-of-magnitude jump in the proton conductivity of CsH$_{2}$PO$_{4}$ (superprotonic behavior), its crystal structure modifications within the 25\r{ }C -300\r{ }C temperature range under both ambient- and high-pressure conditions have been investigated using synchrotron X-ray diffraction. The high-pressure data show no indication of the thermal decomposition/polymerization at the crystal surface recently proposed as the origin of the enhanced proton conductivity [Phys. Rev B \textbf{79}, 054104 (2004)]. Instead, direct evidence that the superprotonic behavior of the title material is associated with a polymorphic structural transition to a high-temperature cubic phase has been found.