Single-crystal x-ray diffraction and impedance spectroscopy investigations of the RbxCs1-xH2PO4 (0 ≤ x ≤ 1) proton conductor series

We have used single-crystal x-ray diffraction to investigate the structural modifications induced by Rb-doping of the superprotonic conductor CsH2PO4. We found that the monoclinic P2(1)/m CsH2PO4 modification persists within the RbxCs1-xH2PO4 (0 <= x <= 1) series upon Rb-doping from x = 0.1 to x = 0.7. Rb0.8Cs0.2H2PO4 (x = 0.8), however, exhibits a previously unreported P2(1)/c monoclinic phase, where the mirror plane is lost and disorder is present in the PO4 tetrahedra even at room temperature. Higher levels of x display a tetragonal I.42d unit cell isomorphic with the known structure of RbH2PO4. The temperature dependence of the proton conductivity determined from impedance spectroscopy data collected within the 160 degrees C-250 degrees C range is also markedly different at high Rb-doping levels, x >= 0.8. Finally, we found that Rb0.9Cs0.1H2PO4 undergoes a transition from its room-temperature tetragonal I-42d phase to an intermediate-temperature monoclinic P2(1)/m modification at a significantly lower temperature (similar to 80 degrees C) than its RbH2PO4 counterpart (similar to 120 degrees C).