| Abstract
| - Cesium dihydrogen phosphate, CsH2PO4, undergoes a superprotonic transition at 228 °C, which competes with dehydration under ambient pressure. Under high pressures (1.0 ± 0.2 GPa), the transition occurs at 260 °C, greatly precedes dehydration, and is accompanied by a sharp jump in conductivity of almost three orders in magnitude. The existence of such a transition in CsH2PO4 indicates that the details of room-temperature hydrogen bonded networks are of secondary importance in determining high-temperature properties.
- The high-temperature behavior of CsH2PO4 has been carefully examined under bothambient and high pressure (1.0 ± 0.2 GPa) conditions. Ambient pressure experimentsencompassed thermal analysis, AC impedance spectroscopy, 1H NMR spectroscopy, andpolarized light microscopy. Simultaneous thermogravimetric analysis, differential scanningcalorimetry, and evolved gas analysis by mass spectroscopy demonstrated that a structuraltransition with an enthalpy of 49.0 ± 2.5 J/g occurred at 228 ± 2 °C, just prior to thermaldecomposition. The details of the decomposition pathway were highly dependent on samplesurface area, however the structural transformation, a superprotonic transition, was not.Polarized light microscopy showed the high temperature phase to be optically isotropic innature, 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 transitiontemperature, from 1.2 × 10-5 to 9.0 × 10-3 Ω1-cm-1, followed by a rapid decline due todehydration. In addition, chemically adsorbed surface water was shown to increase theconductivity of polycrystalline CsH2PO4 over well-dried samples, even at mildly elevatedtemperatures (>200 °C). At high pressure an apparent irreversible phase transition at 150°C and a reversible superprotonic phase transition at 260 °C were observed by impedancespectroscopy. At the superprotonic transition, the conductivity increased sharply by ∼3 ordersof magnitude to 3.5 × 10-2 Ω1-cm-1 at 275 °C. This high conductivity phase was stable tothe highest temperature examined, 375 °C, and exhibited reproducible and highly Arrheniusconduction 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 °C.
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