High Temperature Phase Transitions in RbH2PO4

MH2PO4 (M=Cs, Rb) solid acids exhibit a several-order-of-magnitude increase in proton conductivity upon heating above a temperature threshold [1, 2]. This behavior allows these solid acids to function as fuel-cell electrolytes at intermediate temperatures [3]; this has attracted much interest. We studied the structure of RDP when heated within the 25-250\r{ }C range. We used temperature- and time-resolved powder XRD on powder RDP. Our XRD data, evidenced a tetragonal to monoclinic transition that occurs within the 90-110\r{ }C interval. Rietveld refinements demonstrate that the newly observed monoclinic structure is a RDP polymorph. This suggests that the superprotonic behavior in RDP might be triggered by a monoclinic-cubic polymorphic transition \textit{similar }to the one observed in CsH2PO4. Further heating of the sample, under ambient pressure and humidity conditions, results in dehydration. A possible method to avoid dehydration in future research is to carry out the heating on samples subjected to high pressures of about 1GPa [1,2,4]. [1] Boysen \textit{et al., }Chem. Mater. \textbf{15, }727(2003). [2] Boysen \textit{et al., }Chem. Mater. \textbf{16, }693(2004). [3] Boysen et al., Science \textbf{303, }68(2004). [4] Botez et al., J. Chem. Phys. \textbf{127, }194701(2007)