High-temperature behavior of CsH2PO4 under both ambient and high pressure conditions

The high-temperature behavior of CsH2PO4 has been carefully examined under both ambient and high pressure (1.0 +/- 0.2 GPa) conditions. Ambient pressure experiments encompassed thermal analysis, AC impedance spectroscopy, H-1 NMR spectroscopy, and polarized light microscopy. Simultaneous thermogravimetric analysis, differential scanning calorimetry, and evolved gas analysis by mass spectroscopy demonstrated that a structural transition with an enthalpy of 49.0 +/- 2.5 J/g occurred at 228 +/- 2 degreesC, just prior to thermal decomposition. The details of the decomposition pathway were highly dependent on sample surface area, however the structural transformation, a superprotonic transition, was not. Polarized light microscopy showed the high temperature phase to be optically isotropic in nature, consistent with earlier suggestions that this phase is cubic. The conductivity of CSH2PO4, as revealed by the impedance measurements, exhibited a sharp increase at the transition temperature, from 1.2 x 10(-5) to 9.0 x 10(-3) Omega(1-)cm(-1), followed by a rapid decline due to dehydration. In addition, chemically adsorbed surface water was shown to increase the conductivity of polycrystalline CsH2PO4 over well-dried samples, even at mildly elevated temperatures (>200 degreesC). At high pressure an apparent irreversible phase transition at 150 degreesC and a reversible superprotonic phase transition at 260 degreesC were observed by impedance spectroscopy. At the superprotonic transition, the conductivity increased sharply by similar to3 orders of magnitude to 3.5 x 10(-2) Omega(1-)cm(-1) at 275 degreesC. This high conductivity phase was stable to the highest temperature examined, 375 degreesC, and exhibited reproducible and highly Arrhenius conduction behavior, with an activation energy for charge transport of 0.35 eV. Upon cooling, CsH2PO4 remained in the high-temperature phase to a temperature of 240 degreesC.