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The goal of this work is the uncovering of the microscopic structures and mechanisms responsible for the enhancement in proton conduction observed in CsH₂PO₄ (CDP) and RbH₂PO₄ (RDP) upon heating. Understanding this superprotonic behavior is significant not only for its potential applications in hydrogen fuel cell technology, but also from a fundamental perspective, as highly efficient proton conduction mechanisms are likely to play an important role in processes that occur in a wide range of physical, chemical, and biological systems. 

Throughout the initial reporting period, we obtained several important results, including the clarification of the monoclinic-to-cubic phase transition responsible for the superprotonic behavior of CDP. In addition, this preliminary data was used to successfully secure further funding for the development of our research program, clearly demonstrating the positive impact of this GB-type (starter) grant. Also, several undergraduate and M.S. level students have been actively involved in the project.

During the current reporting period, we have continued to produce significant results towards the end goal of understanding the microscopic mechanisms responsible for the increase in proton conductivity upon heating in the title materials. For example, using temperature-resolved synchrotron x-ray diffraction, we have been able to observe a polymorphic phase transition in RDP from its tetragonal room temperature phase to a new intermediate-temperature (T~150ºC) monoclinic modification that is isomorphic to that of the room-temperature CDP.

This finding, which was recently published in the Journal of Physics: Condensed Matter, is important as it strongly suggests that the proton conductivity enhancement observed in RDP might have a microscopic origin similar to that of its Cs-based counterpart, i.e. a polymorphic structural transition to a high-symmetry disordered phase. Very recently, we have also observed the superprotonic CDP phase using high-pressure neutron diffraction, another significant result as neutrons can reveal information about the proton positions in the crystal lattice. Finally, we have performed preliminary neutron spectroscopy experiments in order to investigate the proton diffusion through these structures.

There has also been continuing student involvement in the project, both at the graduate and undergraduate levels. In this context, we are pleased to report that two students were awarded M.S. degrees in physics based on work carried out within my research group. Significantly, one of them started as an undergraduate, benefiting from an ACS-PRF Supplement for Underrepresented Minorities (SUMR) scholarship, and, upon graduating from UT El Paso he was admitted to the Ph.D. program at Rice University.

We plan future high-pressure synchrotron x-ray diffraction, as well as neutron spectroscopy experiments to further investigate the high temperature behavior of these materials. We hope to prove the existence of the elusive high-temperature cubic RDP polymorph, and to get further insight into the highly-efficient proton diffusion mechanisms in the superprotonic phases. In addition, as we progress towards the achievement of our scientific goals, we plan to continue, and possibly increase the student participation in our research. <