Ambient Pressure Behavior

Typical thermal analysis results for CsH2PO4 samples of different particle sizes and fixed

heating rate (5 ◦C min-1_{) are presented in Figure 3.4. The thermal gravimetry (TG)}

and differential thermal gravimetry (DTG) results are shown in the top panel, differen- tial scanning calorimetry (DSC) in the middle panel, and mass spectroscopy measurements of H2O (m18.00) in the evolved gases in the bottom panel. For the complete dehydration

of CsH2PO4 by the following reaction:

CsH2PO4(s)→ CsPO3(s) + H2O(g) ,

a weight loss of 7.83% is expected, as noted in the top panel. It is evident from these thermal measurements that surface area plays a significant role in the decomposition/dehydration of CsH2PO4 and may well explain the literature discrepancies noted above. For example, a

weight loss of 7% was observed at 278 ◦C for the fine powder, while for single crystals the same weight-loss was not observed until 352 ◦C. Quite significantly, regardless of sample surface area, in all cases a polymorphic transition, independent of decomposition, is clearly evident at ∼ 230 ◦C. Although the impact of this transition on conductivity cannot be assessed from thermal analysis alone, we identify this transformation as the superprotonic transition described earlier by Baranovet al.8 At temperatures just beyond this structural transformation, dehydration occurs via multiple steps. The superprotonic phase transition (SP) and what are defined here as the first (I), second (II), third (III), and fourth (IV) dehydration processes are indicated in Figure 3.4. For fine powders only dehydration pro- cesses I and II were observed, while the course powder and single crystals exhibited only slight or no dehydration at I and II, with the majority of dehydration taking place at high temperature processes III and IV.

The coincidence of the superprotonic phase transition with dehydration presents a chal- lenge for the accurate measurements of the enthalpy ∆HSP of the superprotonic phase

transition. To address this challenge, the enthalpy of the transition was evaluated only from single crystals, in which dehydration processes I and II are minimized. A value of 49.0

±2.5 J g-1was obtained for a heating rate of 5 ◦C min-1.

An inherent thermal lag in the thermal analysis can lead to systematic errors between true and measured transition and reaction temperatures as a function of heating rate.

Figure 3.4: Thermal analysis of fine powder, course powder, and single crystals of CsH2PO4heated at a rate of 5 ◦C min-1 under flowing (40 cm3min-1) dry Ar. Analysis consisted of simultaneous thermal gravimetric (TG) and differential thermal gravimetric (DTG) analysis—top panel, differ- ential scanning calorimetry (DSC)—middle panel, and mass spectroscopy of evolved H2O vapor (m18.00)—bottom panel.

The temperature difference ∆T between the measured peak temperature Tpeak and the

equilibrium onset temperature Tonset of a transition or reaction of a pure compound is

proportional to the heating rateυ, sample massm, transition enthalpy ∆H, and the thermal resistance %23:

∆T =p2mυ%∆H. (3.3)

Assuming a constant%and utilizing samples of equal massm, a linear relationship between ∆T and √υ can be obtained. As the heating rate υ → 0, the difference in temperature between the equilibrium onset and measured peak temperatures ∆T →0, which implies that

Tpeak→Tonset. Therefore, by carrying out thermal analysis at several different heating rates

and extrapolating to a heating rate of zero, one can estimate the “true” onset temperature for any thermal event.

As an example of the heating-rate dependent thermal behavior of CsH2PO4, a plot of

the TG and DSC traces obtained at several different heating rates from a fine powder of CsH2PO4 is shown in Figure 3.5. Analogous results were obtained for coarse powders and

single crystal samples. The Tpeak values for each process, as measured by DSC, DTG, and

results obtained from these three thermal analysis techniques are in good agreement. More- over, a linear relationship betweenTpeak and

√

υis indeed evident. The true or equilibrium onset temperatures, Tonset, determined by extrapolation to zero heating rate for each pro-

cess and for each of the three sample types are summarized in Table 3.1. As is apparent from these data, whereasTSP is relatively independent of surface area, the temperature of

the dehydration processes, I–IV, are not. Thus, for crystals of CsH2PO4 heated rapidly,

the superprotonic phase transition will likely be observed before dehydration, however, if sufficiently slow heating rates (<0.3◦C min-1) and large surface area powders are utilized, dehydration can precede the superprotonic transition.

Figure 3.5: Thermal analysis of fine powders of CsH2PO4 heated at rates of 0.5, 1, 5, 10, 15, and 25◦C min-1under flowing (40 cm3min-1) dry Ar. Results of simultaneous thermal gravimetric (TG) analysis—top panel, and differential scanning calorimetry (DSC)—bottom panel.

Figure 3.6: The peak temperatureTpeakof thermal events in fine powders of CsH2PO4as measured by differential scanning calorimetry (DSC), differential thermal gravimetric (DTG) analysis, and mass spectroscopy (Mass Spec) of evolved H2O vapor (m18.00) plotted as function of

√

υ, the square root of the heating rate employed for each measurement. Lines, calculated from a least squares fit of the data, identify the thermal events resulting from the superprotonic phase transition (SP), first dehydration process (I), and second dehydration process (II). The equilibrium onset temperature Tonset is indicated for each thermal event at

√ υ= 0.

Table 3.1: Summary of measured onset temperatures for the superprotonic phase transition (TSP), and the first (TI), second (TII), third (TIII), and fourth (TIV) decomposition/dehydration processes in CsH2PO4 samples of various surface areas. Numbers in parentheses indicate the uncertainty in the final digit(s).

Sample Type TSP TI TII TIII TIV /◦C fine powders 228(2) 224(3) 258(5) — —

course powders 228(2) 230(4) 261(4) 292(5) 311(10) single crystals 228(2) 230(5) 261(6) 298(8) 325(12)

Polarized Light Microscopy

Optical polarized light microscopy studies support the conclusion that single crystal CsH2PO4

undergoes a structural transition prior to decomposition. Typical images obtained under cross-polarizers are shown in Figure 3.7. At room temperature (prior to heating), the opti- cally anisotropic, monoclinic form of CsH2PO4 was observed, Figure 3.7(a). Upon heating

to ∼ 245 ◦C, a rainbow-colored phase-front traveled from left to right across the crystal, leaving behind a new phase, which is optically isotropic (black) and presumably cubic, Fig- ure 3.7(b) to (e). Upon cooling to room temperature, the crystal transformed back to an optically anisotropic phase, Figure 3.7(e) to (f), with some evidence of slight damage to the crystal quality. The reversible nature of this observed transformation is strong evidence for a polymorphic phase transition as opposed to decomposition.

(a) 25◦C (d) 240 ◦C

(b) 230◦C (e) 245◦C

(c) 235◦C (f) 25 ◦C

Figure 3.7: Polarized light microscopy images of a single crystal of CsH2PO4taken in the sequence of temperatures: (a) 25◦C→(b) 230 ◦C→(c) 235◦C→(d) 240◦C→(e) 245◦C→(f) 25◦C.

Impedance Spectroscopy

The results of AC impedance measurements for both single crystal and polycrystalline sam- ples under ambient pressure conditions are presented in Figure 3.8 in an Arrhenius plot. Representative Nyquist plots obtained at temperatures above the superprotonic phase tran- sition temperatureTSP are presented in Figure 3.9. The conductivity of both sample types

increased sharply at the transition temperature, however, in neither case to the high value reported by Baranovet al.8. Moreover the jump in conductivity was significantly lower for the polycrystalline pellet than for the single crystal sample. AboveTSP, neither sample type

exhibited a linear, Arrhenius region, as is typical for other superprotonic conductors. The polycrystalline pellet exhibited a monotonic decrease in conductivity, also evident in Fig- ure 3.9(b), whereas the single crystal sample exhibited more erratic behavior, although the conductivity generally increased from 245 to 260◦C, Figure 3.9(a). As might be expected, the high-temperature behavior of CsH2PO4 was highly sample dependent, with polycrys-

talline samples and small single crystals typically exhibiting lower conductivity and lower stability aboveTSP than large single crystals. These results are consistent with previous in-

vestigations, in which the electrical behavior of the superprotonic phase could be observed, such as by Baranov et al.,8 for sufficiently large crystals, whereas for smaller crystals, de- hydration prevented such observations.10 In almost every case, samples measured upon cooling, either single crystals or polycrystalline pellets, did not exhibit conductivities as high as that obtained upon heating (not shown), indicating that the sample had undergone some degree of decomposition at elevated temperatures.

At temperatures below TSP, a large difference between the conductivities of the single

crystal (b-axis oriented) and polycrystalline samples is evident, Figure 3.8. Furthermore, both appear more conductive than the CsH2PO4 reported by Baranov et al.8 Fitting the

conductivityσof the CsH2PO4single crystal to an Arrhenius law (σT =A0exp ∆Ha/kBT),

yields a pre-exponential factor A0 of 4.0(2)×106 Ω-1cm-1K and activation enthalpy ∆Ha

of 0.91(4) eV. Values ofA0 and ∆Ha for the polycrystalline samples are not reported here

as they were highly dependent on sample history. Specifically, upon drying for long periods the conductivity of polycrystalline pellets gradually decreased.

The apparent discrepancy between the conductivities of the single crystal and polycrys- talline samples of CsH2PO4 was further investigated by measuring the conductivity of a

Figure 3.8: Arrhenius plot of the conductivity upon heating at 0.5◦C min-1 _{undering flowing dry} N2 a CsH2PO4 single crystal and polycrystalline pellet, as compared to Baranov et al. results8. The temperature of the superprotonic transition is indicated byTSP.

(a) Single crystal (b) Polycrystalline pellet

Figure 3.9: Nyquist plots at various temperatures of CsH2PO4 upon heating above the superpro- tonic transition temperatureTSP for (a) a single crystal, and (b) a polycrystalline pellet.

polycrystalline pellet as a function of time at 200 ◦C under flowing dry N2. The pellet

was prepared from powders obtained by methanol-induced precipitation. The impedance data initially showed a single arc in the Nyquist representation, but after approximately 24 hours a second low-frequency arc became evident, Figure 3.10(a). This low-frequency arc was attributed to grain boundary conductivity (σgb), and the high-frequency arc to bulk

conductivity (σbulk). With increasing time, both σgb and σbulk decreased, and σbulk ap-

proached that of the conductivity of a CsH2PO4 single crystalσxtal, Figure 3.10(b). These

results suggest that the apparently high conductivity measured in polycrystalline samples is due to chemi-sorbed H2O, which is very gradually desorbed at elevated temperatures.

somewhat similar humidity dependence in their transport behavior due to adsorbed water between the layers of the highly layered crystal structure24. However, retention of water to such high temperatures and for such long periods, as observed here, is unusual for a crys- talline acid phosphate. Finally, we note that in this analysis,σgb has not been normalized

to account for the grain boundary density, as would be necessary to compare samples with different grain sizes25.

(a) Nyquist plots with time (b) Conductivity versus time

Figure 3.10: Impedance spectroscropy measurements on a polycrystalline pellet of CsH2PO4carried out at 200◦C under flowing dry N2gas over 10 days. (a) Nyquist plots at various times, in which two arcs are fit to the impedance data, the first, high frequency arc, attributed to the bulk conductivity (σbulk) , and the second, low frequency arc to the grain boundary conductivity (σgb), as compared

to the conductivity of a single crystal (σxtal). (b) Log plot of these conductivities as a function of

time.

NMR Spectroscopy

Presented in Figure 3.11 are 1H- NMR spectra of two CsH2PO4 powders: (1) produced

by methanol-induced precipitation, and (2) by grinding a single crystal. The methanol- precipitated powder was dried at 200 ◦C for 24 hours prior to measurement, whereas the ground single crystal was examined immediately after grinding. For both powders, peaks were observed at 14.5 and 10.9 ppm, which correspond to the two crystallographically dis- tinct hydrogen atoms observed in the room temperature structure of CsH2PO426. However,

for the methanol-precipitated powder an additional peak was observed at 6.6 ppm. On the basis of extensive studies of chemically similar calcium phosphates, in which chemical shifts of 5.5–6.2 ppm were observed for surface-adsorbed water,27 it has been concluded that this third peak corresponds to chemically surface-adsorbed water. It is thus evident that chemi-sorbed surface water is particularly difficult to remove from CsH2PO4, and that this

arc normally associated with bulk conductivity behavior. It is for this reason that well- dried single crystals were used (in crushed form) in the following high pressure conductivity experiments.

Figure 3.11: Solid-state1H-NMR spectra of CsH2PO4powders obtained by (1) methanol induced precipitation and then dried for 24-hours at 200 ◦C prior to measurement, and (2) from ground single crystals.

3.1.1.3 High Pressure Behavior