The nature of the microscopic modifications that accompany the heating-induced three-order-of-magnitude proton conductivity jump in phosphate-based solid acids has been under dispute for almost two decades. Some attribute this behavior to polymorphic structural transitions to so-called superprotonic phases, while others associate it with dehydration and chemical decomposition. Clarifying the chemical composition and crystal structures of the phases present in these materials at high temperatures is important in order to propose realistic scenarios for proton migration, and, eventually, uncover the microscopic mechanisms responsible for their superprotonic behavior.
We carried out laboratory and synchrotron x-ray diffraction experiments aimed at investigating the structural and chemical modifications undergone by CsH2PO4 (CDP) and RbH2PO4 (RDP) upon heating. Our data demonstrate that the room-temperature (RT) monoclinic (P 21/m) CDP phase transforms into a cubic (P m 3 m) polymorph at exactly the same temperature where the superprotonic jump was observed, and prior to the initiation of dehydration. Using high-pressure methods we managed to stabilize the superprotonic CDP phase and uncover details of its crystal structure. For RDP we found a transition from the RT orthorhombic (I -4 2 d) structure to an intermediate-temperature monoclinic (P 21/m) phase, whose crystal structure is almost identical to that of monoclinic CDP. This remarkable similarity opens the possibility that that the dynamics responsible for the high-temperature superprotonic behavior of these compounds does not depend on the cation type, but on the symmetry of the crystal structure.
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