Structural Modifications in the RbxCs1-xPO 4 (0 ≤ x ≤ 1) Superprotonic Conductor Series: A Single-Crystal X-ray Diffraction and Impedance Spectroscopy Study

We have used single-crystal X-ray diffraction measurements to investigate the structural modifications induced by Rb-doping of the protonic conductor CsH2PO4. Data collected on the RbxCs 1-xH2PO4 (0 ≤ x ≤ 1) series shows that the monoclinic P21/m CsH2PO4 presentation persists upon Rb-doping up until x = 0.8. Rb0.8Cs0.2H2PO 4 exhibits a previously unreported P21/c monoclinic structure, 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 RbH2PO 4. The temperature dependence of the proton conductivity determined from impedance spectroscopy data collected within the 160°C--250°C range is also markedly different at high Rb-doping levels, x = 0.8. Finally, Rb0.9Cs0.1H2PO4 undergoes a transition from its room-temperature tetragonal I-42d phase to an intermediate-temperature monoclinic P21/m modification at a significantly lower temperature (~80 °C) than pure RbH2PO4 (~120 °C).