X-ray study of the high-temperature phase transition of CsH2PO4 crystals

Bronowska, W.; Pietraszko, A.

doi:10.1016/0038-1098(90)90840-8

Cited by 36 publications

(20 citation statements)

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“…Shortly after Baranov's study, Bronowska and Pietraszko reported the structure of superprotonic CsH 2 PO 4 and provided the first clear demonstration of the significance of water partial pressure in suppressing dehydration. 15 All of the peaks in the high temperature X-ray powder diffraction pattern, along with their relative intensities, could be explained on the basis of the proposed high temperature structure. The subsequent Raman study of Romain and Novak 16 supported Bronowska's conclusions regarding the structural features of the superprotonic state, while Vargas and Torijano in 1993 also agreed (initially) with the existence of a reversible, but hysteretic, phase transition at 227 1C on the basis of differential scanning calorimetry.…”

Section: Phase Transition Behaviormentioning

confidence: 86%

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

et al. 2007

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The compound CsH 2 PO 4 has emerged as a viable electrolyte for intermediate temperature (200-300 1C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H 2 O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 1C, with the conductivity rising to a value of 2.2 Â 10 À2 S cm À1 at 240 1C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH 2 PO 4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH 2 PO 4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm À2. Thus, for fuel cells in which the supported electrolyte membrane was only 25 mm in thickness and in which a peak power density of 415 mW cm À2 was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH 2 PO 4 , a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

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Section: Phase Transition Behaviormentioning

confidence: 86%

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

et al. 2007

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“…1 This crystal has a transition sequence similar to that in KDP and RDP though the crystal structures are different. The CDP crystal undergoes the following three phase transitions: [25][26][27][28][29][30][31][32][33] Ferroelectric…”

Section: Introductionmentioning

confidence: 99%

Possible origin of the proton conduction mechanism ofCsH2PO4crystals at high temperatures

Park

2004

Phys. Rev. B

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The high-temperature transformation of CsH 2 PO 4 was studied by means of impedance spectroscopy, differential thermal analysis, and thermogravimetric methods. The dielectric constant showed a high-temperature anomaly around T p ϭ230°C. The thermal transformation which appears around T p ϭ230°C is endothermic in addition to showing weight loss. Our results show evidence that the high-temperature transformation of CsH 2 PO 4 near T p is not a structural phase transition, but an onset of thermal decomposition of CsH 2 PO 4 into Cs n H 2 P n O 3nϩ1 ͓probably nӷ1, (CsPO 3 ) n ] at reaction sites at the surface of the crystal. From complex ac impedance spectra, we observed impedance consisting of two regions for measuring frequency: the lowfrequency region may be due to the formation and migration of H 2 O molecules at the surface while the conduction mechanism of the high-frequency region seems to be Cole-Cole type and may be due to the proton migration in the bulk. Our results support that the phase of CsH 2 PO 4 above T p is not a superionic conductor phase caused by a structural transition (monoclinic→cubic) but rather a polymeric state caused by partial thermal decomposition. The possible origin of proton conduction for CsH 2 PO 4 is considered to be the partial thermal decomposition of the crystal surface because the polymeric transition tends to increase breaking and reforming of the hydrogen bond at the surface.

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“…2͒. That strategy of the experiment is very good for presenting the thermal decomposition process of CDP as a function of annealing temperature; however, it is insufficient to conclude that the reported transition [3][4][5][6][7][8][9][10][11][12] associated with endothermic anomaly at ca. 231°C does not take place and, moreover, that the high conductivity above 231°C is only a consequence of the dehydration of the crystal surface.…”

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“…9 Lattice parameters of CDP were calculated on the basis of the measurements of the Bragg angles of the reflections with high angles using the least-square method. The accuracy of the measurements of 2, which was better than 10Љ of arc, allowed the determination of the lattice parameters to an accuracy better than 3ϫ10…”

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confidence: 99%

“…The powder x-ray study performed in a normal air atmosphere ͑from 127°C to 327°C on heating with a rate 0.1°C per minute͒ using the high-temperature Guinier-camera 9 allowed study of structural changes and thermal decomposition of CDP. Slightly above 227°C the powder diffraction pattern shows the transformation of the monoclinic cesium dihydrogen phosphate CsH 2 PO 4 to the orthorhombic cesium hydrogen pyrophosphate Cs 2 H 2 P 2 O 7 with the lattice parameters a ϭ4.571 (7) The application of the Guinier camera was insufficient to accurately study the structural changes related to the reported superionic phase transition.…”

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Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]

Bronowska

2001

The Journal of Chemical Physics

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Superionic materials have intrigued researchers because they exhibit structural transitions from the paraelectric to the superionic phase which are accompanied by an increase of the electrical conductivity as much as five orders of magnitude. Notwithstanding the number of studies and variety of experiments that have been carried out on superprotonic crystals of the KH 2 PO 4 type, no generally agreed-upon model describing the structural and chemical features that induce superionic transitions exists. Discrepancies between the high-temperature results reported by different groups of scientists have been well summarized in recent papers. 1,2 Lee 1 has discussed the similarities of the phase transitions in the KH 2 PO 4 -type compounds and concluded that the high-temperature phenomena of these compounds are not related to physical changes like structural phase transitions, but related to chemical ones and has suggested that the term ''high-temperature phase transition'' should be replaced by ''onset of partial polymerization at reaction sites distributed on the surface of solids. '' In order to check if the reported high-temperature phase transitions of CsH 2 PO 4 ͑CDP͒ are only related to thermal dehydration, power x-ray measurements were performed by Ortiz et al.2 A fresh powder sample was subsequently heat treated 1 min at 130, 165, 200, 238, and 250°C, and their respective x-ray diffraction patterns taken under dry conditions after cooling at room temperature ͑25°C͒ were plotted ͑see Fig. 3 of Ref. 2͒. That strategy of the experiment is very good for presenting the thermal decomposition process of CDP as a function of annealing temperature; however, it is insufficient to conclude that the reported transition 3-12 associated with endothermic anomaly at ca. 231°C does not take place and, moreover, that the high conductivity above 231°C is only a consequence of the dehydration of the crystal surface.Under normal air conditions the high-temperature transition occurs very close to the region where CDP decomposes by dehydration and these two simultaneous effects considerably complicate the interpretation of the phase relation.Ten years ago the Bond method of precise lattice parameter determination was applied as a sensitive monitor of the structural changes occurring in the CDP crystal.9 Lattice parameters of CDP were calculated on the basis of the measurements of the Bragg angles of the reflections with high angles using the least-square method. The accuracy of the measurements of 2, which was better than 10Љ of arc, allowed the determination of the lattice parameters to an accuracy better than 3ϫ10Ϫ5 . Investigations of the lattice parameters of CDP above room temperature have revealed that up to ca. 137°C all parameters are almost linear. In the vicinity of 149°C a discrepancy of the linearity of the lattice parameters has been noticed, but the space group ( P2 1 /m) of the crystal above room temperature up to ca. 231°C does not change. The investigation of the single-crystal samples of CDP with the Bond diff...

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X-ray study of the high-temperature phase transition of CsH2PO4 crystals

Cited by 36 publications

References 10 publications

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

Possible origin of the proton conduction mechanism ofCsH2PO4crystals at high temperatures

Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]

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