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Published November 2, 2000 | public
Journal Article

Superprotonic behavior of Cs_2(HSO_4)(H_2PO_4) – a new solid acid in the CsHSO_4–CsH_2PO_4 system

Abstract

Investigations into the CsHSO₄–CsH₂PO₄ system have yielded a new solid acid, Cs₂(HSO₄)(H₂PO₄), with a superprotonic phase transition that occurs over the temperature range 61–105°C. In the room temperature structure, the SO₄ and PO₄ groups are randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs₂(HSO₄)(H₂PO₄) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, ΔH=44±2 J/g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with a₀=4.926(5)Å, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO₄ groups (X=P or S) at the center. The conductivity in the high temperature phase at 110°C is 3×10⁻³ Ω⁻¹ cm⁻¹, and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO₄ group reorientations in the cubic phase of Cs₂(HSO₄)(H₂PO₄), as is known to occur in the high temperature, tetragonal phase of CsHSO₄.

Additional Information

Proceedings of the 12th International Conference on Solid State Ionics. Copyright © 2000 Elsevier Science B.V. Available online 30 November 2000. The authors are pleased to acknowledge the financial support the National Science Foundation via a National Young Investigator Award to SMH. Additional funding was provided by the Irvine Foundation. The authors thank Professor George Rossman and Liz Miura at the California Institute of Technology for their kind guidance with infrared analyses and Paul Wagner for his assistance with high temperature powder diffractometry.

Additional details

Created:
August 21, 2023
Modified:
October 25, 2023