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

The compound Cs 2 (HSeO 4 )(H 2 PO 4 ) 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 ( p H 2 O), 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 Cs 2 (HSO 4 )(H 2 PO 4 ). Despite differences in the room temperature structures of Cs 2 (HSeO 4 )(H 2 PO 4 ), Cs 2 (HSO 4 )(H 2 PO 4 ), and CsH 2 PO 4 , each has a monoclinic to cubic transformation entropy of approximately 23 J/mol(CsH x XO 4 )/K. Under p H 2 O = 0.05 atm, the cubic phase of Cs 2 (HSeO 4 )(H 2 PO 4 ) is stable to approximately 250 °C. Under elevated p H 2 O (0.3 atm), exsolution of a trigonal phase, with structure analogous to that of Cs 3 H(SeO 4 ) 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 Cs 3 H(SeO 4 ) 2 and Cs 2 (HSeO 4 )(H 2 PO 4 ), suggesting additional chemical levers for control of transformation behavior. • The compound Cs 2 (HSeO 4 )(H 2 PO 4 ) undergoes a superprotonic transition to a cubic phase that initiates at approximately 116 °C. • The rise in conductivity across the transition is moderate, about a factor of thirty, reflecting the high conductivity in the low temperature phase. • The activation energy for proton transport in the superprotonic phase is 0.304(2) eV, and the transport properties are similar to those of superprotonic Cs 2 (HSO 4 )(H 2 PO 4 ). • Dehydration of Cs 2 (HSeO 4 )(H 2 PO 4 ) can be suppressed up to temperatures well above the superprotonic transition by supplying a small steam partial pressure (∼ 0.04 atm). • Elevated steam partial pressures p H 2 O (∼0.3 atm) induce exsolution of a trigonal phase, with structure analogous to that of Cs 3 H(SeO 4 ) 2 , from the main cubic superprotonic phase.