High-Temperature Polymorphic Phase Transitions in MH2PO4 (M = K, Rb, Cs) Proton Conductors

We used powder X-ray diffraction (XRD) and ac impedance spectroscopy to investigate the polymorphic phase transitions undergone by the MH2PO4 (M = Cs, K, Rb) proton conductor series upon heating. High-pressure methods used in conjunction with energy dispersive XRD enhanced by synchrotron radiation allowed us to isolate a pure cubic (superprotonic) RbH2PO4 (RDP) polymorph (at P = 1.2 GPa and T = 320 °C) and collect data of enough quality to Rietveld refine its crystal structure. Our data and analysis reveal that cubic RDP (Pm-3m, a = 4.76 ± 0.01 Å) is described by a unit cell where Rb and P atoms are in (0, 0, 0) and (0.5, 0.5, 0.5) positions, respectively, whereas O atoms are in (0.5, 0.234(5) 0.342(6)) positions having a multiplicity of 24 and an occupancy of 0.166. This results in dynamically disordered PO4 tetrahedra that enables the superprotonic conduction in RDP, a mechanism like the one in the high-temperature cubic phase of the Cs-based compound CsH2PO4 (CDP). This is a notable behavior, as RDP and CDP are structurally different at room temperature—RDP is tetragonal (I-42d) and CDP is monoclinic (P21/m)—but following polymorphic modifications upon heating they both end up in superprotonic cubic phases that are isostructural to one another. On the other hand, we found that although the K-based phosphate KH2PO4 (KDP) has the same crystal structures as RDP at room and at intermediate temperatures, further heating does not lead to a cubic KDP phase. Overall, our results are significant from both the fundamental and applied perspective as superprotonic phosphates are promising materials for fuel cell electrolyte applications.