High temperature phase transformations and Superprotonic conductivity in Cs2(HSeO4)(H2PO4)

The compound Cs₂(HSeO₄)(H₂PO₄) is of interest due to its high conductivity in its superprotonic state. In the present work, in situ X-ray diffraction studies, simultaneous thermal analysis, and AC impedance spectroscopy, each performed under controlled value of steam partial pressure (pH₂O), were carried out to elucidate the crystallographic features of the transformation and resolve the conductivity in the high temperature phase. The studies reveal that the material transforms to a cubic phase at a temperature of approximately 116 °C, that the activation energy for proton transport in the cubic phase is 0.304(2) eV, and the magnitude of the conductivity is comparable to that of Cs₂(HSO₄)(H₂PO₄). Despite differences in the room temperature structures of Cs₂(HSeO₄)(H₂PO₄), Cs₂(HSO₄)(H₂PO₄), and CsH₂PO₄, each has a monoclinic to cubic transformation entropy of approximately 23 J/mol(CsH_xXO₄)/K. Under pH₂O = 0.05 atm, the cubic phase of Cs₂(HSeO₄)(H₂PO₄) is stable to approximately 250 °C. Under elevated pH₂O (0.3 atm), exsolution of a trigonal phase, with structure analogous to that of Cs₃H(SeO₄)₂, was found to accompany the transformation to the cubic phase. While the driver for this transformation is not fully known, the cell volumes of both the exsolved and matrix phases indicate they are chemically distinct, respectively, from Cs₃H(SeO₄)₂ and Cs₂(HSeO₄)(H₂PO₄), suggesting additional chemical levers for control of transformation behavior.