Hidden nature of the high-temperature phase transitions in crystals of KH2PO4-TYPE : Is it a physical change ?

Lee, Kwang-Sei

doi:10.1016/0022-3697(95)00233-2

Cited by 139 publications

(66 citation statements)

References 45 publications

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“…17 The controversy surrounding the properties of CsH 2 PO 4 began in earnest in 1996 with a publication by Lee suggesting that the observed conductivity effects were artifacts of thermal decomposition and partial polymerization at the surfaces of the CsH 2 PO 4 particles. 19 The hypothesis was based on a review of literature data, without the benefit of new experimental results. A later paper from this same author repeated these conclusions, but in this case experimental support was provided in the form of a limited set of optical micrographs showing the degradation of single crystal surfaces.…”

Section: Phase Transition Behaviormentioning

confidence: 99%

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: 99%

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

et al. 2007

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“…In any case, the phase behaviour above room temperature remains a controversial subject and still no clear picture has been presented of the nature of the high temperature phase transitions in KDP. It has even been suggested that the high protonic mobility observed above 180 °C is due to the onset of thermal dehydration [7,8] instead of a structural phase transition.…”

Section: Introductionmentioning

confidence: 97%

“…The high temperature phase behaviour has for example been studied by many researchers using electrical measurements [3][4][5][6]. The review of Lee [7] gives a good overview of all phases reported in KDP: below -151 °C a ferroelectric, ferroelastic and orthorhombic phase; between -151 and 180 °C a paraelectric, paraelastic and tetragonal phase; the phase between 180 and 233 °C has been suggested to be monoclinic while the crystal structure of the high temperature phase (233 °C < T < 259 °C, melting point) has not been identified. In any case, the phase behaviour above room temperature remains a controversial subject and still no clear picture has been presented of the nature of the high temperature phase transitions in KDP.…”

Section: Introductionmentioning

confidence: 99%

Dielectric relaxation of KH₂PO₄ above room temperature

Diosa¹,

Vargas

Albinsson

et al. 2004

Physica Status Solidi (b)

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The dispersion curves of the dielectric response of KH 2 PO 4 were obtained from 5 Hz to 13 MHz and over the temperature range 120 -206 °C for both single crystal and polycrystalline samples. Both types of samples reveal dielectric relaxation at low frequency, for example around 10 4 Hz at 140 °C, which shifts to higher frequencies (~1 MHz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E a = 0.82 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that causes distortion and change the local lattice polarizability inducing dipoles like 2 4 H PO − .

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“…Thermal events observed around Ttr =458 K, for example, have been attributed by some authors to a polymorphic phase transition at an intermediate-temperature KDP modification [10], while others have claimed that the behaviour at Ttr was in fact due to chemical changes, such as dehydration and onset of partial polymerization of the room temperature of tetragonal KDP phase [11,12].…”

Section: Introductionmentioning

confidence: 99%

“…The lattice parameters at 468 K were given as a=7.47 Å, b=7.33 Å, c=14.49 Å, α=β=90°and γ=92.2°. The spots in the Weissenberg photograph at 468 K were only consistently indexed by assuming a twin structure in the monoclinic phase appearing above 460 K. The complete assignment of the reflections was difficult to do, and until now detailed structural analyses including atomic coordinates have not been done [11]. Moreover, it has been reported that the monoclinic phase of KDP is metastable at temperatures below Ttr and that it reverts to the stable tetragonal phase after being kept for some days in air at room temperature [10,[12][13][14].…”

Section: Introductionmentioning

confidence: 99%

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

Ettoumi

Gao

Toumi

et al. 2013

Ionics

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Raman spectroscopy and differential thermal analysis (DTA) and thermogravimetric analysis have been carried out on Cu-doped KH 2 PO 4 (Cu-KDP). X-ray diffraction powder data reveal that the structure of the KDP crystal does not change with the additive Cu 2+ ion. DTA analysis and Raman study of Cu-KDP as a function of temperature reveal that this compound undergoes two phase transitions at about Ttr =453 and 473 K. The electrical conductivity measurements on polycrystalline pellet of Cu-KDP (5) are performed from room temperature (RT) up to 495 K. Only one phase transition is observed at 470 K. The activation energy in the migration is 0.42 eV in the temperature range from RT to 470 K. For temperature above 470 K, the activation energy of the superprotonic phase is 1.87 eV.

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Hidden nature of the high-temperature phase transitions in crystals of KH2PO4-TYPE : Is it a physical change ?

Cited by 139 publications

References 45 publications

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

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

Dielectric relaxation of KH₂PO₄ above room temperature

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

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Hidden nature of the high-temperature phase transitions in crystals of KH2PO4-TYPE : Is it a physical change ?

Cited by 139 publications

References 45 publications

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

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

Dielectric relaxation of KH2PO4 above room temperature

Thermal analysis, Raman spectroscopy and complex impedance analysis of Cu2+-doped KDP

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Product

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Dielectric relaxation of KH₂PO₄ above room temperature