Evidence for a Structural Transition to a Superprotonic CsH_2PO_4 Phase Under High Pressure

摘要

We have used synchrotron X-ray powder diffraction (SXRPD) to investigate the structural behavior of CsH_2PO_4 upon heating. Temperature-resolved data collected at ambient-pressure demonstrate that a transition from the room-temperature monoclinic phase (P2_l/m, a=7.90A, b=6.39A, c=4.87A, and β = 107.64°) to a high-temperature cubic phase (Pm3m; a=4.96A) occurs at T=237℃. The high-temperature phase is not stable under ambient-pressure conditions, even in the absence of further heating. On the other hand, SXRPD measurements carried out under high-pressure (～1GPa) evidence a transition from monoclinic to a stable cubic phase (Pm3m, a=4.88A) at a temperature within the 255℃-275℃ range. A 1000-fold increase in the proton conductivity (indicating the transition to the superprotonic phase) was previously observed under the same non-ambient conditions. Therefore, our results represent strong evidence that the superprotonic behavior in CsH_2PO_4 is associated with a monoclinic-to-cubic polymorphic structural transition and not with chemical modifications.