LAUSR

Abstract We have used single-crystal x-ray diffraction to investigate the structural modifications induced by Rb-doping of the superprotonic conductor CsH 2 PO 4 . We found that the monoclinic P 2 1 / m CsH 2 PO 4 modification persists within the Rb x Cs 1-x H 2 PO 4 (0 ≤  x  ≤ 1) series upon Rb-doping from x = 0.1 to x = 0.7. Rb 0.8 Cs 0.2 H 2 PO 4 (x = 0.8), however, exhibits a previously unreported P 2 1 / c monoclinic phase, where the mirror plane is lost and disorder is present in the PO 4 tetrahedra even at room temperature. Higher levels of x display a tetragonal I-42d unit cell isomorphic with the known structure of RbH 2 PO 4 . The temperature dependence of the proton conductivity determined from impedance spectroscopy data collected within the 160⁰C-250 °C range is also markedly different at high Rb-doping levels, x ≥ 0.8. Finally, we found that Rb 0.9 Cs 0.1 H 2 PO 4 undergoes a transition from its room-temperature tetragonal I-42d phase to an intermediate-temperature monoclinic P 2 1 / m modification at a significantly lower temperature (∼80 °C) than its RbH 2 PO 4 counterpart (∼120 °C).