Background

Cesium dihydrogen phosphate, CsH2PO4, has received attention because of both its high

temperature proton transport properties and its low temperature ferroelectric properties. While there is wide agreement on the low-temperature behavior of this material—that it undergoes a ferroelectric phase transition at 154 K (P21/m→P21)6,7—there is significant

that, upon heating, the conductivity of CsH2PO4undergoes a sharp increase at 230◦C8,9,10.

Some, including Baranov et al.8, Romain and Novak11, Preisinger et al.12, and Luspin et al.13 attribute this behavior to a structural transition to a stable, high-temperature phase (a so-called superprotonic transition), whereas others, including Lee14 and Ortiz et al.9, attribute the increase in conductivity to an artifact of water loss due to thermal decomposition.

As aforementioned, whether or not CsH2PO4 undergoes a superprotonic transition is of

particular relevance to understanding the role of hydrogen bonding in these phase transfor- mations. Under ambient temperature and pressure conditions, this compound has a unique hydrogen bond network in which PO3₄− oxy-anions are linked via O–H· · ·O bonds at all corners of the phosphate tetrahedra, forming [H2(PO4)−]∞ layers. Cesium cations reside

at sites between these layers. Plakida has treated the superprotonic phase transition in the related solid acid compound, CsHSO4, in terms of a microscopic model that assumes

an order-disorder relationship between the low and high temperature phases15. Although not stated explicitly, such a model rules out the possibility of a superprotonic transition in a compound such as CsH2PO4, in which all oxygen atoms participate in hydrogen bond

formation.

As an example of the controversy surrounding the high temperature behavior of CsH2PO4,

the thermal gravimetric and differential thermal analysis (TGA and DTA) data reported by Rashkovich et al.16, by Ortizet al.9, and by Nirsha et al.17 are presented in Figure 3.2. It is immediately evident that there are significant discrepancies between these three sets of results. Rashkovich et al. have reported two structural (polymorphic) phase transitions, the onset of which occurs at 230 and 256 ◦C, prior to decomposition, which begins at ∼

300◦C. Data were collected only for temperatures of 180 ◦C and higher. In a later paper, Rashkovich and Meteva reported high temperature diffraction data and proposed that the formation of an intermediate phase of “symmetry no higher than monoclinic” was responsi- ble for the transition at 230 ◦C18_{. In addition, they showed that the main peaks of cesium}

phosphite, CsPO3, appeared at temperatures as low as 100 ◦C, and that this phase was

well-formed by 250 ◦C, although weight loss was not observed until higher temperatures were reached. Differential scanning calorimetry (DSC) measurements of Metcalfe and Clark (not shown in Figure 3.2) indicate the presence of two polymorphic transitions, one at 149 ◦_{C (quasi-irreversible) and the latter at 230} ◦_{C (reversible), with decomposition occurring}

at temperatures above∼250◦C19, which is in relatively good agreement with Rashkovich et al. for the temperature regime of mutual examination.

(a) Thermal gravimetric analysis (b) Differential scanning calorimetry and differential thermal analysis Figure 3.2: Prior thermal analysis results reported for CsH2PO4 by Ortizet al.9, Nirsh et al.17, and Rashkovich et al.18_{; (a) thermal gravimetric analysis (TGA), and (b) differential scanning} calorimetry (DSC) and differential thermal analysis (DTA) data.

In contrast, Nirsha et al. reported that thermal events occur at ∼220 and 255◦C and are both due to thermal decomposition, with the compound dicesium dihydrogen pyrophos- phate, Cs2H2P2O7, forming over the temperature range 175–225 ◦C and CsPO3 over the

range 235–285 ◦C17. This interpretation was supported by high temperature diffraction data and IR spectra. Thermal events evident in the DTA data at higher temperatures were ascribed to the crystallization of the initially amorphous CsPO3. The conclusions that

Ortiz et al. draw from their results are in general agreement with the findings of Nirsha et al. although they differ in detail. Quite surprisingly, the DSC data of Ortiz et al. match the DTA data of Rashkovich et al., but their TGA results differ significantly. Consistent with the results of Rashkovich et al., Ortiz and coworkers observe thermal events at ∼

149, 231, and 273◦C, all of which coincide with peaks in their differential thermal analysis data. Accordingly, they ascribe all three, along with discontinuities in the conductivity of CsH2PO4 (see Figure 3.3, below) at 149 and 231 ◦C, to thermal dehydration and partial

polymerization (i.e. polycondensation of phosphate groups).

A completely different interpretation of the high-temperature behavior of CsH2PO4 has

a phase of cubic symmetry with superprotonic conductivity. The conductivity of CsH2PO4,

as reported by several authors1,9,10, is reproduced in Figure 3.3. It is evident that all authors are in agreement that the conductivity increases sharply at ∼ 230 ◦C, while only Baranov et al. have attributed the increase to a transformation to a thermally stable, superprotonic phase1. They report the activation energy for proton transport in this phase to be 0.32 eV, and the magnitude of the conductivity at 230 ◦C to be 2.3 × 10-2 Ω-1cm-1, somewhat higher than others have observed. In contrast, Ortiz et al. flatly ruled out the possibility of a polymorphic phase transformation, as described above9. Haile et al. observed that the conductivity of CsH2PO4 immediately decreased after the transition at

230 ◦C, and concluded that if a superprotonic phase did indeed exist, it was a highly transient state, immediately followed by decomposition10. Prolonged examinations of the electrical properties of CsH2PO4by Ortizet al. showed a similar degradation in conductivity

with time9.

Figure 3.3: Prior conductivity data reported for CsH2PO4 by Baranov et al.8, Ortizet al.9, and Haileet al.10, plotted in Arrhenius form.

Very convincing data for the presence of a cubic, high-temperature phase of CsH2PO4

has been presented by Preisinger et al. (see also reference20), who performed diffraction experiments at elevated temperatures. Under dry atmospheres, dehydration and the for- mation of Cs2H2P2O7 was observed, but under humidified conditions, a stable cubic phase

(P m3m,a= 4.961 ˚A) appeared at temperatures above 230◦C12. Baranov et al. had ear- lier proposed that their superprotonic phase was cubic on the basis of optical microscopy

investigations, which showed the phase to be optically isotropic8. Further evidence for a polymorphic transition at 230 ◦C comes from the work of Romain and Novak, who showed by high temperature Raman spectroscopy that there is no evidence of Cs2H2P2O7 forma-

tion up to temperatures as high as 246 ◦C11. In addition, in a comprehensive study of the high-temperature, high-pressure behavior of CsH2PO4, Rapoport et al. observed a stable,

high-temperature phase, that transformed reversibly to and from the lower temperature monoclinic form at elevated pressures21.

In addition to the competing views as to whether the transition at 230◦C in CsH2PO4

corresponds to decomposition or a polymorphic transition, there is significant discrepancy regarding the behavior in the temperature regime 100–150◦C. As noted above, Ortiz et al. suggest that CsH2PO4undergoes a thermal decomposition event at a temperature of 149◦C

which, much like the transition at 230 ◦C, gives rise to an increase in conductivity9. Oth- ers have ascribed the transition at this temperature to a subtle, polymorphic, monoclinic

→ monoclinic transformation. Evidence for this interpretation is provided by Bronowska and Pietraszko, who observed a slight change in the thermal expansion coefficients at this temperature20, and by Luspinet al., who noted a small change in elastic constants13. Bara- nowski and coworkers22 observed a subtle transition, not at 149 ◦C, as most other groups have reported, but rather at 107◦C. What is quite clear from their work is that the thermal behavior of CsH2PO4 in this lower temperature regime is highly dependent on sample state

and environment, that is, whether the material is subjected to mechanical grinding or has been freshly prepared, and whether the atmosphere is humid or dry. Moreover, in no case have these transitions at 107 and 149◦C been found to be reversible, nor are they associated with superprotonic conductivity. Given that the presence or absence of these transitions is not relevant to the question of whether or not a superprotonic phase can exist in a com- pound such as CsH2PO4, with its particular hydrogen bond network. Therefore, no effort

was made in this work to obtain a detailed understanding of the behavior of CsH2PO4 in

3.1.1.2 Ambient Pressure Behavior