Influence of Silica Nanoparticles on the Crystallization Behavior of and Proton Relaxation in Cesium Hydrogen Sulfate

The influence of nanoparticulate SiO_2 on the crystallization behavior of CsHSO_4 from aqueous solution has been quantitatively evaluated using powder X-ray diffraction (XRD) and ^1H magic angle spinning nuclear magnetic resonance (NMR) spectroscopy. It is shown that SiO_2 induces amorphization of a portion of CsHSO_4 and crystallization of the otherwise metastable phase II form of CsHSO_4. The fraction of amorphized CsHSO_4 (as determined from an evaluation of the XRD peak intensity) was found to increase from 0% in the absence of SiO_2 to fully amorphized in the presence of 90 mol % (~70 wt %) SiO_2. Within the crystalline portion of the composites, the weight fraction of CsHSO_4 phase III was observed to fall almost monotonically from 100% in the absence of SiO_2 to about 40% in the presence of 70 mol % SiO_2 (from both XRD and NMR analysis). These results suggest a crystallization pathway in which SiO_2 particles incorporate an amorphous coating of CsHSO_(4-)like material and are covered by nanoparticulate CsHSO_(4-II), which coexists with independently nucleated particles of CsHSO_(4-III). In composites with small molar fractions of CsHSO_4, the entirety of the acid salt is consumed in the amorphous region. At high CsHSO_4 content, the extent of amorphization becomes negligible, as does the extent of crystallization in metastable phase II. The phase distribution was found to be stable for over 1 year, indicating the strength of the stabilization effect that SiO_2 has on phase II of CsHSO_4

Materials Chemistry of Superprotonic Solid Acids

Solid acid is a class of materials that shows potential as a fuel cell electrolyte. Understanding the phase and mechanical stability are required for further development of this technology. We addressed both issues in this work. We expanded the use of the crystallographic theory of the phase transformation to three major classes of solid acids. That allowed us to relate material properties hysteresis to fundamental crystallographic and thermodynamic parameters. The understanding of the mechanism of the transformation can guide the effort to create materials with desired hysteresis. Careful investigation of the thermal and phase behavior of CsHSO₄, CsH₂PO₄, Rb₃H(SeO₄)₂ and in Cs1-xRbxH₂PO₄ solid solution series for both low and high temperature phases was performed and crystal symmetry and lattice parameters for Cs0.75Rb0.25H₂PO₄, T=240°C phase were found for the first time. Consistency between predicted and measured properties was shown for all three different classes of solid acids as well as for the isostructural solid solution series. Nanocomposite materials based on cesium hydrogen sulfate and nanometer size silica were characterized. We observed 30-40 nm size surface stabilization of our material at the high temperature phase, otherwise metastable at room temperature. We developed methods to quantitatively study interface phases and its effect on ion mobility. The method allowed us to quantitatively find crystalline and amorphous amounts in the composites. We observed 3-4 order decrease in spin-lattice relaxation values of the metastable phase in the composite. Solid state NMR allowed surface interactions directly and suggest high ion mobility. Strong effect on superprotonic transition temperature in composites was observed. Superprotonic phase was stable in composites at temperatures up to 70°C below phase transition compared with pure phase CsHSO₄. The mechanism and activation energy of the creep plastic deformation in CsHSO4 were found. Based on that, a method to reduce creep by 1-2 orders of magnitude was developed and creep-resistant material was synthesized.</p

Phase transformation and hysteresis behavior in Cs_(1-x)Rb_xH_2PO_4

A new theory on the origin of hysteresis in first order phase transformations was evaluated for its applicability to the phase transformation behavior in the Cs_(1 − x)Rb_xH_2PO_4 solid solution system. Specifically, the correlation between λ_2, the middle eigenvalue of the transformation matrix describing the cubic-to-monoclinic superprotonic transition, and the transformation hysteresis was examined. The value of λ_2 was estimated from a combination of room temperature diffraction data obtained for compositions in the solid solution system and high temperature diffraction data obtained for the CsH_2PO_4 end-member. The transformation hysteresis was determined for Cs_(1 − x)Rb_xH_2PO_4 compositions (x = 0, 0.25, 0.50 and 0.75) by single-frequency electrical impedance measurements. It was found that the transition temperature increases monotonically with increasing Rb content, from 227.6 ± 0.4 °C for the end-member CsH_2PO_4 to 256.1 ± 0.3 °C for Cs_(25)Rb_(75)H_2PO_4, as does the hysteresis in the phase transition, from 13.4 °C to 17.4 °C. Analysis of the transformation matrix reveals that, for this system, λ_2 depends only on the b lattice parameter of the paraelectric phase and the α_0 lattice parameter of the cubic phase. The computed values of λ_2, based on extrapolations accounting for chemical contraction with increasing Rb substitution and thermal expansion on heating, were far from 1, ranging from 0.9318 to 0.9354. The observation of λ_2 increasing with Rb content is attributed to the relatively large thermal expansion in the b-axis of the low temperature monoclinic phase in combination with an increase in transition temperature with increasing x. That the hysteresis does not decrease as λ_2 approaches 1, counter to the theoretical expectations, may reflect uncertainties in the method of estimating λ_2 for Rb substituted compositions, or the discovery of a system in which hysteresis is not dominated by considerations of crystallographic compatibility