3.2 Mixed Cation Sulfate Systems
3.2.2 Resulting phases of the mixed system investigations
The above synthesis route resulted in the compounds listed in Table 3.2. At very high cesium percentages, slight modifications to CsHSO4-III, the meta-stable phase of CsHSO4 that grows out of aqueous solutions, were discovered for all three systems. The evidence for incorporation of the smaller cations into CsHSO4-III was first seen in the SCXD measurements. The lattice constants of the modified structures were nearly identical to that of CsHSO4-III, in an alternative primitive cell, but with the length of the c-axis tripled compared to the pure compound. This primitive cell is transformed into the crystallographically correct cell of undoped CsHSO4-III by the transformation: a′ = a –
½*c, b′ = b, c′ = 2*c. The amount of K, Na , and Li incorporated into CsHSO4-III’s structure is quite small as full data collections were not able to locate the ions although they did confirm the tripling of the c-axis. It would appear that the smaller cations are substituted on the Cs sites where they are hidden by cesium’s much larger scattering factor for X-rays108. In a similar manner, electron microprobe measurements were unable to observe the lighter cations.
For the Na compound, both Na+ and H+ NMR measurements were performed. A very small peak in the Na+ NMR measurement was observed, but it was impossible to rule out small amounts of Na contaminants as the cause of this peak. The proton NMR measurements were more conclusive, in that two distinct peaks of significant magnitude were observed for the doped sample, whereas the scan of the reference, undoped, CsHSO4 sample showed only one peak (see appendix A). The sodium ions then again
Table 3.2 Compounds synthesized in the mixed Cs-K/Na/Li systems. The average cation radius was calculated using both the ratio and particular coordination of the cations in a compound.
System Solution Composition
% M2CO3
Compound
Obtained Space Group or Symmetry
Lattice
Parameters Average
Cation Radius
Phase Transitions
Above RT Comments and References All Systems 0 CsHSO4-III P21/n a = 8.229(2) Å, b = 5.8163(9) Å
c = 9.996(3) Å, β = 106.46(2)° 1.81 Å ~ 62°C→ CsHSO4-II;
142 °C→ supeprotonic a 109
CsHSO4−KHSO4 10-30 α-CsHSO4-III P21(?) a = 7.311(5) Å, b = 5.818(4) Å c = 16.52(2) Å, β = 101.55(4)º
~ 1.81 Å ~ 67°C→ CsHSO4-II?;
140 °C→ supeprotonic
New modification of CsHSO4–III
40 α-CsHSO4-III &
K3H(SO4)2
50-100 K3H(SO4)2 A2/a a = 9.790(4) Å, b = 5.682(2) Å c = 14.702(4) Å, β = 103.02(5)º
1.51 Å 190 °C→ supeprotonic 190 °C→ supeprotonic
a; b
23,110
CsHSO4−NaHSO4 5-10 β-CsHSO4-III P21/m a = 7.329(5) Å, b = 5.829(4) Å c = 16.52(1) Å, β = 101.55(3)º
~ 1.81 Å ~73°C→ CsHSO4-II?;
141 °C→ supeprotonic
New modification of CsHSO4–III 15-35 Cs2Na(HSO4)3 P63/m a = 8.572(2) Å
c = 9.982(2) Å 1.55 Å 139°C→ melt new compound
25,111
40 Cs2Na(HSO4)3
&
CsNa2(HSO4)3
45-55 CsNa2(HSO4)3 P213 a = 10.568(2) Å 1.28 Å 125°C→ melt new compound
25,111
60-100 NaHSO4·H2O a
CsHSO4−LiHSO4 10 γ-CsHSO4-III P21(?) a = 7.316(10) Å, b = 5.818(7) Å
c = 16.50(2) Å, β = 101.54(5)º ~ 1.81 Å ~108°C→ CsHSO4-II?;
141 °C→ supeprotonic
New modification of CsHSO4–III 20-80 Cs2Li3H(SO4)3
•H2O
Pbn21 a = 12.945(3) b = 19.881(4) c = 5.111(1)
1.08 Å 105°C→ slow
decomposition new compound
90-100 Li2SO4 •H2O a
a) Compound previously known.
b) High temperature properties not previously investigated.
revealed their presence indirectly through their effect on the surrounding structure, in this case, the environment of the protons.
The incorporation of the K, Na and Li ions also showed up in the DSC measurements. Upon heating the modified forms of CsHSO4-III, the transition to
CsHSO4-II (another monoclinic form) was observed to be systematically shifted to higher temperatures as the size of the secondary cation decreased, Figure 3.1 a. Also, for the Cs/Na compound, β-CsHSO4-III, two exothermic transitions instead of only one where observed upon cooling, Figure 3.1 b. For this reason, the β-CsHSO4-III compound was more extensively studied than the others. Conductivity measurements along the b-axis revealed three, rather than two transitions, Figure 3.1 c. This discrepancy between the DSC and conductivity results is probably due to sample size, i.e., very small crystals and very large crystals were used in the DSC and conductivity measurements, respectively.
Low temperature DSC measurements also revealed an apparently second order transition at -123.25°C not found in CsHSO4, Figure 3.1d112.
The temperature of the superprotonic phase transition, however, was not significantly effected by the small amounts of K, Na, and Li present, Figure 3.1 a, although the transition enthalpy was consistently lower for the mixed CsHSO4-III compounds (see appendix A). These compounds, as was the case with Cs0.9Rb0.1HSO4, do little to illuminate the cation size effect: their superprotonic phase transitions and structures being essentially identical to those of CsHSO4. On the other hand, they do reveal how sensitive these solid acids are to the addition of a secondary cation. In fact, trace levels would appear to be the upper solubility limit for K, Na, and Li in CsHSO4
(and vice versa), the rest of the crystals synthesized being either line compounds or compounds with a single type of cation, Table 3.2.
40 60 80 100 120 140 160 -0.6
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
40 60 80 100 120 140 160 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Heat/Cool rate = 5OC/min under flowing N2
*
a) b)
Cs/Li
Cs/Na
Cs/K
Cs
Heat Flow (W/g)
Temperature (OC)
endoexo
Cs/Na
*
*
*
*
Cs/Li
Cs Cs/K
Heat Flow (W/g)
Temperature (OC)
Figure 3.1 (See figure caption on next page.)
-125 -124 -123 -122 -121
0.0 0.1 0.2 0.3 0.4
2.2 2.4 2.6 2.8 3.0 3.2
-6 -5 -4 -3 -2 -1 0 1
180 160 140 120 100 80 60 40
Scan rate 0.1OC/min Helium atmosphere
c) d)
H e a t F l o w ( m c a l / s e c )
Temperature (OC) Heat/Cool rate = 0.5OC/min
under ambient atmosphere heating cooling Temperature (OC)
L o g [σ T ] Ω-1 cm-1
1000 / T (K-1)
Figure 3.1 Measurements on the α, β, γ-CsHSO4-III compounds. DSC curves upon heating, a), and cooling, b) for all three modified forms of CsHSO4-III. Also, b-axis conductivity and low temperature DSC measurements for the Na compound, c) and d), respectively. Figure a) shows an increase in the phase III-II transition temperature with K to Na to Li substitution. The difference of the Na compound from pure CsHSO4 is shown in its two and three reverse transitions visible in the DSC, b), and conductivity data, c), as well as the presence of a low temperature (apparently second order) transition, d).
Experimental parameters given on graphs.
For the Cs/K system, this insolubility phenomenon is particularly easy to see in that only α-CsHSO4-III and K3H(SO4)2 crystals grew from the solutions. The K3H(SO4)2
compound belongs to another class of superprotonic conductors with general formula M3H(XO4)2 (M = Cs, Rb, NH4, K, Na and X = S, Se). This compound had been
previously synthesized and its structure determined, but its high temperature properties had not been sufficiently investigated23. Our studies revealed K3H(SO4)2 to have two high temperature transitions before decomposition, both of which are superprotonic in nature
and neither of which are analogous to the superprotonic transitions found in the other M3H(XO4)2 compounds110. Typically, these transitions involve very small structural changes from pseudo-trigonal to trigonal unit cells, with superprotonic conduction primarily in the basal planes100. The tetrahedra in the superprotonic phases do not undergo true rotations, but simply librate around a site with C3 symmetry113. These librations primarily effect the positions of basal plane oxygen atoms, hence the
anisotropic proton conduction of the phases. It is therefore not appropriate to compare the superprotonic transitions of the M3H(XO4)2 compounds to those of the MHXO4
compounds, and so the results for K3H(SO4)2 will not be included in this work.
The Cs/Li system resulted in a new mixed compound, Cs2Li3H(SO4)3·H2O. DSC, TGA and conductivity measurements show no evidence for a superprotonic transition before the start of decomposition above 105°C (see appendix A). The lack of a
superprotonic transition is not surprising as the average radius for the four- and tenfold oxygen coordinated lithium and cesium ions, respectively, is 1.078 Å83. Also, as this compound is hydrated and has a cation to tetrahedra ratio of 5:3 (instead of the desired 1:1 ratio), any correlations between its structure and properties are not particularly pertinent to the present discussion.
Fortunately, the Cs/Na system did produce two new mixed solid acids in the MHXO4 family with chemical formulas of Cs2Na(HSO4)3 and CsNa2(HSO4)325. The unit cell of Cs2Na(HSO4)3 is hexagonal while that of CsNa2(HSO4)3 is cubic, both novel symmetries for the room temperature structures of the MHXO4 compounds. Moreover, the single asymmetric hydrogen bond in both compounds links the SO4 groups into unique three-membered (HSO4)3 rings. These rings are most likely due to the Na atom’s
preference for a 6-fold oxygen coordination, with the resulting NaO6 octahedra serving as a template for the (HSO4)3 units22. The Cs atoms in both compounds reside in irregular polyhedra with a coordination of 9 to 12 oxygens, depending on the upper limit one sets for the Cs−O bonds. The rings in Cs2Na(HSO4)3 are linked together by NaO6 octahedra to form infinite Na(HSO4)3 chains that extend along [001], Figure 3.2 a and b, while in CsNa2(HSO4)3 the rings form a distorted cubic close-packed array. In this array, the Cs atoms are located within the “octahedral” sites and the Na atoms within the “tetrahedral”
sites, Figure 3.2 c and d.
Figure 3.2 Crystal structures of the mixed Cs/Na compounds. The hexagonal structure of Cs2Na(HSO4)3 is projected down [001]: a) unit cell contents from z = 0 to ½ and b) from z = ½ to 1. Sodium atoms have elevations of z = 0 and ½, while those of the
cesiums are as indicated. Cubic structure of CsNa2(HSO4)3 projected along [100]: c) unit cell contents from x = -¼ to +¼ and d) from x = +¼ to ¾. Elevation of cations as
indicated. Some oxygen atoms have been omitted for clarity25.
Neither of these compounds undergoes a superprotonic phase transition before melting at 139 and 125°C for Cs2Na(HSO4)3 and CsNa2(HSO4)3, respectively, as
established by thermal analysis and visual inspection. The DSC curves for the
compounds are shown in Figure 3.3, along with conductivity measurements which show the compounds to be fairly poor protonic conductors despite their high crystalline symmetry.
-170 0 1 2 3
CsNa2(HSO4)3 Cs2Na(HSO4)3 - heat Cs2Na(HSO4)3 - cool
endoexo
heat cool
90 110 130 150 170 190 Temperature ( C)O
Heat Flow (W/g)
a) b)
2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 -7
-6 -5 -4 -3 -2 -1 0
1 160 140 120T, OC100 80
∆Eσ = 0.26 eV
∆E σ = 0.66 eV
∆E σ ~ 1.0 eV log(σT) [Ω-1 cm-1 K]
1000/T (K-1)
Cs2Na a-axis Cs2Na c-axis CsNa2 pellet CsHSO4 pellet
Figure 3.3 a) DSC and b) conductivity measurements on Cs2Na(HSO4)3 and
CsNa2(HSO4)3. Figure a) shows the melting transitions of the compounds beginning at 139 and 125°C, respectively. The cooling curve for CsNa2(HSO4)3 does not reveal a solidification peak, which is in agreement with visual observations that the compound solidifies as a glass upon cooling from the melt. Conductivity measurements revealed the compounds’ protonic conductivity to be lower and activation energy higher than that of CsHSO4’s room temperature phase. The observed curvature in the conductivity of Cs2Na(HSO4)3’s a-axis is likely due to the onset of melting. DSC and conductivity measurements taken under flowing N2 and dry argon atmospheres, respectively, with heating/cooling rates of 10°C/min and 0.5°C/min, respectively.