High-temperature phase transitions in CsH2PO4 under ambient and high-pressure conditions: a synchrotron x-ray diffraction study.

To clarify the microscopic origin of the temperature-induced three-order-of-magnitude jump in the proton conductivity of CsH(2)PO(4) (superprotonic behavior), we have investigated its crystal structure modifications within the 25-300 degrees C temperature range under both ambient- and high-pressure conditions using synchrotron x-ray diffraction. Our 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 69, 054104 (2004)]. Instead, we found direct evidence that the superprotonic behavior of the title material is associated with a polymorphic structural transition to a high-temperature cubic phase. Our results are in excellent agreement with previous high-pressure ac impedance measurements.