Phase Behavior of Complex Superprotonic Solid Acids

Citation

Panithipongwut, Chatr (2013) Phase Behavior of Complex Superprotonic Solid Acids. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/NXG8-TY79. https://resolver.caltech.edu/CaltechTHESIS:05302013-183134007

Abstract

Superprotonic phase transitions and thermal behaviors of three complex solid acid systems are presented, namely Rb₃H(SO₄)₂-RbHSO₄ system, Rb₃H(SeO₄)₂2-Cs₃H(SeO₄)₂ solid solution system, and Cs₆(H₂SO₄)₃(H_1.5PO₄)₄. These material systems present a rich set of phase transition characteristics that set them apart from other, simpler solid acids. A.C. impedance spectroscopy, high-temperature X-ray powder diffraction, and thermal analysis, as well as other characterization techniques, were employed to investigate the phase behavior of these systems.

Rb₃H(SO₄)₂ is an atypical member of the M₃H(XO₄)₂ class of compounds (M = alkali metal or NH₄⁺ and X = S or Se) in that a transition to a high-conductivity state involves disproportionation into two phases rather than a simple polymorphic transition [1]. In the present work, investigations of the Rb₃H(SO₄)₂-RbHSO₄ system have revealed the disproportionation products to be Rb₂SO₄ and the previously unknown compound Rb₅H₃(SO₄)₄. The new compound becomes stable at a temperature between 25 and 140 °C and is isostructural to a recently reported trigonal phase with space group P3̅m of Cs₅H₃(SO₄)₄ [2]. At 185 °C the compound undergoes an apparently polymorphic transformation with a heat of transition of 23.8 kJ/mol and a slight additional increase in conductivity.

The compounds Rb₃H(SeO₄)₂ and Cs₃H(SeO₄)₂, though not isomorphous at ambient temperatures, are quintessential examples of superprotonic materials. Both adopt monoclinic structures at ambient temperatures and ultimately transform to a trigonal (R3̅m) superprotonic structure at slightly elevated temperatures, 178 and 183 °C, respectively. The compounds are completely miscible above the superprotonic transition and show extensive solubility below it. Beyond a careful determination of the phase boundaries, we find a remarkable 40-fold increase in the superprotonic conductivity in intermediate compositions rich in Rb as compared to either end-member.

The compound Cs₆(H₂SO₄)₃(H_1.5PO₄)₄ is unusual amongst solid acid compounds in that it has a complex cubic structure at ambient temperature and apparently transforms to a simpler cubic structure of the CsCl-type (isostructural with CsH₂PO₄) at its transition temperature of 100-120 °C [3]. Here it is found that, depending on the level of humidification, the superprotonic transition of this material is superimposed with a decomposition reaction, which involves both exsolution of (liquid) acid and loss of H₂O. This reaction can be suppressed by application of sufficiently high humidity, in which case Cs₆(H₂SO₄)₃(H_1.5PO₄)₄ undergoes a true superprotonic transition. It is proposed that, under conditions of low humidity, the decomposition/dehydration reaction transforms the compound to Cs₆(H_2-0.5xSO₄)₃(H_1.51.5PO₄)_4-x, also of the CsCl structure type at the temperatures of interest, but with a smaller unit cell. With increasing temperature, the decomposition/dehydration proceeds to greater and greater extent and unit cell of the solid phase decreases. This is identified to be the source of the apparent negative thermal expansion behavior.

References:
[1] L.A. Cowan, R.M. Morcos, N. Hatada, A. Navrotsky, S.M. Haile, Solid State Ionics 179 (2008) (9-10) 305.
[2] M. Sakashita, H. Fujihisa, K.I. Suzuki, S. Hayashi, K. Honda, Solid State Ionics 178 (2007) (21-22) 1262.
[3] C.R.I. Chisholm, Superprotonic Phase Transitions in Solid Acids: Parameters affecting the presence and stability of superprotonic transitions in the MHnXO4 family of compounds (X=S, Se, P, As; M=Li, Na, K, NH4, Rb, Cs), Materials Science, California Institute of Technology, Pasadena, California (2003).

Thesis Files