| Recent studies have shown that the proton conductivity of MH2PO 4 (M=Cs, Rb) solid-acids exhibits a sharp, several-order-of-magnitude increase upon heating above a certain temperature threshold [Boysen et al., Chem. Mater. 15, 727(2003), Boysen et al., Chem. Mater. 16, 693(2004)]. This so-called superprotonic behavior allows the above-mentioned compounds to function as fuel-cell electrolytes at intermediate temperatures [Boysen et al., Science 303, 68(2004)], a remarkable application that has attracted much interest. Yet, the crystal structures and microscopic mechanisms responsible for this heating-induced proton conductivity enhancement are not fully understood.;Our group has previously demonstrated [Botez et al., J. Chem. Phys. 127, 194701(2007)] that the superprotonic behavior in CsH2PO4 is due to the transformation of its room-temperature monoclinic (P 21/m) phase into a high-temperature cubic (P m 3 m) polymorph at 237°C. Although a similar jump in the proton conductivity upon heating has been reported for RbH2PO4 [Boysen et al. , Chem. Mater. 16, 693(2004)], recent thermal analysis and qualitative X-ray diffraction (XRD) studies [Ortiz et al., J. Phys. Chem. Solids 59, 1111(1997), J.-H. Park et al. , J. Phys. Cond. Mat. 13, 9411(2001)] have suggested that the room temperature tetragonal (I -4 2 d) RbH2PO4 phase actually decomposes via dehydration at temperatures as low as 96°C. Surprisingly, this implies that RbH2PO4's superprotonic behavior cannot be due to a polymorphic phase transition.;In our present studies we attempt to clarify the structural and/or chemical modifications of RbH2PO4 upon heating within the 25-250°C temperature range. We use temperature- and time-resolved powder x-ray diffraction (XRD) methods on polycrystalline samples obtained by crushing single RbH 2PO4 crystals previously prepared by slow evaporation. Our XRD data, collected in the reflectivity geometry over the 1.5-3.5A d-spacing range, evidence a tetragonal-to-monoclinic transition that occurs within the 90-110°C temperature interval. We indexed the high temperature monoclinic phase to space group P21/m and lattice parameters a=7.728A, b=6.187A, c=4.810A, and beta=109.15°. We have also carried out Rietveld refinements that conclusively demonstrate that the newly observed monoclinic structure is a RbH2PO4 polymorph. These results are significant as they unambiguously establish that prior to any temperature induced chemical changes, RbH2PO4 transforms into a stable polymorph, whose crystal structure is isomorphic to the monoclinic structure observed in room temperature CsH 2PO4. This strongly suggests that the superprotonic behavior in RbH2PO4 is triggered by a monoclinic-cubic polymorphic phase transition similar to the one observed in CsH2PO4. Unfortunately, our current experiments could not reveal such a transition as, upon further heating to 210°C under ambient pressure and humidity conditions, the monoclinic phase starts dehydrating via the reaction 2RbH2PO4 = Rb2H2P 2O7 + 2H2O. A possible method to avoid dehydration in future experiments involves the heating of samples subjected to high pressures of about 1GPa [Boysen et al., Chem. Mater. 15, 727(2003), Boysen et al., Chem. Mater. 16, 693(2004), Botez et al., J. Chem. Phys. 127, 194701(2007)]... |
