Structural modifications in the RbxCs1-xH2PO4 (0$\le $x$\le $1) superprotonic conductor series: a single-crystal x-ray diffraction and impedance spectroscopy study.

We have used single-crystal x-ray diffraction to investigate the structural modifications induced by Rb-doping of the superprotonic conductor CsH2PO4. Our data collected on the RbxCs1-xH2PO4 (0 $\le $ x $\le $ 1) series shows that the monoclinic P21/m CsH2PO4 modification persists upon Rb-doping up to x $=$ 0.8. We found that Rb0.8Cs0.2H2PO4 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 RbH2PO4. The temperature dependence of the proton conductivity determined from impedance spectroscopy data collected within the 160\textdegree C-250\textdegree C range is also markedly different at high Rb-doping levels, x $\ge $ 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 P21/m modification at a significantly lower temperature (\textasciitilde 80 \textdegree C) than its RbH2PO4 counterpart (\textasciitilde 120 \textdegree C).