1.4 Protonic Conduction
1.4.3 High temperature proton conduction in solid acids
Although there is agreement in the literature that proton conduction occurs by the Grotthuss mechanism in the low temperature phases of solid acids, in the end there is no definitive proof of the particulars of the mechanism. Hence, there is a question as to which pair is formed: D and L or vacancy/interstitial defect pairs. Similarly, it has not been determined if the structural relaxation necessary for protonic conduction in these low temperature phases occurs by actual rotations of the tetrahedra or simply by dipole reorientations resulting from proton jumps.
In contrast to protonic conduction in the room temperature phases of solid acids, it is then possible for proton conduction through only “normal,” crystallographic proton sites. The combination of fast tetrahedral dynamics and proton translations along hydrogen bonds of a disordered network results in high protonic conductivity.
Superprotonic conduction is therefore a result of the ideal structure rather than intrinsic defects60. In terms of Equation 1-1, this superprotonic conductivity is a product of the increase in the proton’s mobility and the increase in the number of mobile protons (all of them).
The structure proposed by Jirak for CsHSO4 in its superprotonic phase is given in Figure 1.1161. It should be mentioned that there is some disagreement in the literature over the exact position of the oxygen atoms, and hence the protons. This structure was chosen as it gives the most realistic arrangement and length to the hydrogen bonds, as well as its overall fit to experimental data (to be discussed in Section 4.6.1).
a) b)
Cs
H+ SO4
Figure 1.11 Tetragonal structure of CsHSO4 above its superprotonic phase transition projected along the [100], c), and [010], d), directions. Two orientations of the tetrahedra result in partially occupied proton sites and a disordered network of hydrogen bonds (dashed lines).
The room temperature phase of CsHSO4-II is monoclinic, space group P21/c, comprising zigzag chains of hydrogen bonded SO4 tetrahedra alternating with zigzag rows of cesium atoms (Figure1.3). There are four crystallographically distinct oxygens, two of which are involved in asymetric hydrogen bonds with O(1)···O(2) distances of 2.63 Å26. On the contrary, after transforming to the superprotonic tetragonal phase (space group I41/amd), the oxygens become crystallographically identical and all oxygens participate in hydrogen bonds. There are two possible orientations of the tetrahedra, resulting in ½ and ¼ occupancy of the oxygen and proton sites, respectively. Hydrogen bonds of average length 2.78 Å connect the oxygens61. Other proposed structures have a
different number of tetrahedral orientations, hydrogen bond lengths and hydrogen bond orientations. However, regardless of the exact configuration of oxygens and protons in the superprotonic phase of CsHSO4, the method of proton conduction remains the same:
rapid reorientations of the SO4 group forming a dynamically disordered network of hydrogen bonds through which protons can jump from one tetrahedron to the next.
This mechanism of proton transport is responsible for the high conductivity in all superprotonic phases of solid acids, with any differences between their conductive processes attributed largely to the relative symmetry of the specific material. For
example, CsHSO4, being tetragonal, shows a small anisotropy in its conductivity parallel and perpendicular to its 4-fold axis62. In contrast, the compound Cs2(HSO4)(H2PO4), which transforms to a cubic structure (space group Pm3m) exhibits isotropic
conductivity in the superprotonic phase63. Nevertheless, on a very local scale, the process of proton transfer and reorientation is considered to be very similar in all superprontic phases and conclusions reached for one compound should apply at least to structurally related compounds, if not to the whole class of solid acids.
Chapter 2. Experimental Methods
2.1 Synthesis
The solid acids analyzed in these studies were all grown by slow evaporation of an aqueous solution containing high purity metal carbonates and the appropriate mineral acids:
M2CO3 + HnXO4 + H2O →−H2O → single crystals
where M = Cs, Rb, NH4, K, Na, Li and X = S, P. Most crystals were grown at room temperatures, but some compounds were found to grow only at elevated/lowered
temperatures. The compounds discussed in this work are primarily mixed cation sulfates (Chapter 3) and cation sulfate-phosphates (Chapter 4). First attempts at their synthesis were carried out in solutions with total metal to anion (M:XO4) ratios of 1:1. Therefore, unless otherwise noted, it is safe to assume a compound was synthesized at ~25°C with a solution M:XO4 ratio of 1:1.
After the formation of crystal samples, they were collected by either removing individual crystals directly from solution, or by filtration over a porous ceramic (since the solutions are still quite acidic and would eat through normal filters). If necessary, the samples were washed with acetone or isopropanol to remove any excess solution clinging to the crystals. Deliquescent compounds were placed in desiccated containers, while most other samples were stored in ambient conditions.
For large quantities of a desired phase or to force the synthesis of a compound not found to grow by the above method, organic solvents were used to precipitate powder samples. The most common solvents used were acetone, methanol, and isopropanol. The
powders were filtered from solution and washed with the precipitating liquid on ceramic filters. Powder samples were stored in sealed containers to limit surface water absorption.