Cesium Dihydrogen Phosphate Research Articles

The Structure and Proton Transport Mechanisms in the Superprotonic Phase of CsH2PO4: An Ab Initio Molecular Dynamics Study

Anhydrous proton conducting materials have attracted a burst of interest in recent years because of their potential application in fuel cells. However, a detailed microscopic picture of proton transport mechanisms has not been fully established. To gain a deeper understanding of such materials, the superprotonic phase of cesium dihydrogen phosphate (CDP), CsH2PO4, was studied using Car-Parrinello ab initio molecular dynamics (CPAIMD) simulations. Unlike previous classical molecular dynamics studies of related systems, such as CsHSO4, proton transfer is explicitly included in the present CPAIMD simulations. A detailed analysis of structural, dynamical and spectroscopic properties of CDP is reported. The cubic structure of the system is dynamically disordered as seen from the broad Cs−Cs and Cs−P radial distribution functions (RDF), but the fluctuations in the hydrogen bond distances (from the O−O RDF) are much smaller than those of the Cs−Cs and Cs−P RDFs. The orientations of PO4 groups are disordered as well, but a complete tumbling motion of the PO4 group was not observed within the time scale of the present CPAIMD simulations (∼20 ps). In fact, the reorientation rate of the PO4 group was found to be smaller than the proton hopping rate, contrary to previously held notions. Therefore, proton diffusion was observed to occur via the slow reorientation of PO4 followed by a fast proton hopping to a neighboring PO4 unit. A Grotthuss type structural diffusion mechanism, which does not require PO4 reorientation, was also observed with an associated order of magnitude faster time scale.

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

The dehydration behavior of caesium dihydrogen phosphate CsH2PO4 was investigated in the temperature range of 230 °C to 260 °C under high humidity, conditions of particular relevance to the operation of fuel cells based on this electrolyte. The onset temperature of dehydration was determined from changes in ionic conductivity on heating and confirmed by weight change measurements under isothermal conditions. The relationship between the onset temperature of dehydration (Tdehy) and water partial pressure (pH2O) was determined to be log(pH2O/atm = 6.11(±0.82) − 3.63(±0.42) × 1000/(Tdehy/K), from which the thermodynamic parameters of the dehydration reaction from CsH2PO4 to CsPO3 were evaluated. The dehydration pathway was then probed by X-ray powder diffraction analysis of the product phases and by thermogravimetric analysis under slow heating. It was found that, although the equilibrium dehydration product is solid caesium metaphosphate CsPO3, the reaction occurs via two overlapping steps: CsH2PO4 → Cs2H2P2O7 → CsPO3, with solid caesium hydrogen pyrophosphate, Cs2H2P2O7, appearing as a kinetically favored, transient phase.

Preparation of proton conductive composites with CsHSO 4/CsH 2PO 4 and phosphosilicate gel

New composite materials were prepared using cesium hydrogen sulfate (CsHSO 4) or cesium dihydrogen phosphate (CsH 2PO 4) and phosphosilicate gel (P 2O 5–SiO 2 gel). In X-ray diffraction patterns of these composites, diffraction peaks due to Cs 2H 5(SO 4) 2(PO 4) and CsH 5(PO 4) 2 were observed for CsHSO 4–(P 2O 5–SiO 2 gel) composites and CsH 2PO 4–(P 2O 5–SiO 2 gel) composites, respectively. These composites showed high conductivities in the order of 10 − 3 S cm − 1 at 150 °C due to melting of Cs 2H 5(SO 4) 2(PO 4) or CsH 5(PO 4) 2 in the composites. In the cooling process, the CsHSO 4–(P 2O 5–SiO 2 gel) composites kept relatively high conductivity to 110 °C where solidification of Cs 2H 5(SO 4) 2(PO 4) occurs, whereas CsH 2PO 4–(P 2O 5–SiO 2 gel) composites showed relatively high conductivity continuously to ambient temperature.

Evidence for a Structural Transition to a Superprotonic CsH2PO4 Phase Under High Pressure

ABSTRACTWe have used synchrotron X-ray powder diffraction (SXRPD) to investigate the structural behavior of cesium dihydrogen phosphate upon heating. Temperature-resolved data collected at ambient-pressure demonstrate that a transition from the room-temperature monoclinic phase (P21/m; a=7.90Å, b=6.39Å, c=4.87Å, and β=107.64°) to a high-temperature cubic phase (Pm3m; a=4.96Å) occurs at T=237°C. The high-temperature phase is not stable under ambient-pressure conditions, even in the absence of further heating. On the other hand, SXRPD measurements carried out under high-pressure (∼1GPa) evidence a transition from monoclinic to a stable cubic phase (Pm3m, a=4.88Å) at a temperature within the 255°C-275°C range. A 1000-fold increase in the proton conductivity (indicating the transition to the superprotonic phase) was previously observed under the same non-ambient conditions. Therefore, our results represent strong evidence that the superprotonic behavior in cesium dihydrogen phosphate is associated with a monoclinic-to-cubic polymorphic structural transition and not with chemical modifications.

Effect of water vapor on proton conduction of cesium dihydrogen phosphateand application to intermediate temperature fuel cells

The proton conduction and superionic phase transition of cesium dihydrogen phosphate, CsH2PO4 (CDP), were investigated under various humid conditions to evaluate the applicability of a CsH2PO4 solid electrolyte to an intermediate temperature fuel cell operating between 230 °C and 300 °C. The phase stability, superionic phase transition, and reversibility of dehydration of CsH2PO4 were evaluated under different ambient water vapor concentrations, from 0 to 90 mol%, through the measurements of conductivity. The dependence of conductivity on the water vapor concentration and the demonstrated reversibility of dehydration clearly showed the range in which CsH2PO4 is applicable to the intermediate temperature fuel cell. Additionally, we evaluated the protonic transport number of CsH2PO4, which was almost unity, and demonstrated fuel cell operation at 250 °C using a single cell fabricated with the CsH2PO4 electrolyte.

Formation of Order of Cesium Dihydrogen Phosphate in a Depolarization Field

The antiparallel domains of cesium dihydrogen phosphate form a prefractal domain structure periodic along the wall normal in the presence of a depolarization field. We have observed the self-similarity of the domain structure and found that the domain walls are along the (801) plane. This indicates that the periodic lamellar domain structure is born of a transverse component of the polarization fluctuation. The component has a wave vector normal to the (801) plane. The periodic domain structure can acquire a fractal character while the electrostatic and the domain wall energies repeatedly compete with each other. The integration of the spontaneous polarization over the whole crystal conserves the value of zero in the formation of order.

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

Single crystal and powder diffraction investigations have revealed that the paraelectric phase of cesium dihydrogen phosphate, CsH2PO4 (CDP), from the crystallographic point of view is stable (space group P21/m) up to ca. 231 °C. At this temperature a structural phase transition from the paraelectric phase to the superionic phase (space group Pm–3m) has been observed. The superionic phase of CDP is unstable under normal air condition due to dehydration process. Under H2O-saturated atmosphere the superionic phase transition of CDP takes place without decomposition and is reversible with a temperature hysteresis on cooling.

A genetic algorithm for determination of the elastic constants of a monoclinic crystal

This paper describes a nonlinear inverse method which allows the determination of the second- and third-order elastic constants for a caesium dihydrogen phosphate lattice via ultrasonic velocity measurements. This analysis is based on a genetic algorithm. The efficiency and accuracy of the method and the influence of measurement errors are discussed.

A comparison of Ag- and proton-conducting ferroelectrics

A free-energy model of ionic conductors that are ferroelectric in their low-temperature phase is presented, together with data on systems that meet that description, including cesium dihydrogen phosphate, thallium antimony oxy-germanate, the ‘Nascion’ sodium scandium phosphates, and silver iodide tetratungstate.

On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m–C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.

Phase transition in caesium dihydrogen phosphate crystal with thallium impurities

A mixed crystal between two one-dimensional hydrogen bonding crystals of caesium dihydrogen phosphate (CDP) and thallium dihydrogen phosphate (TDP), TDP, was studied by dielectric constant measurements of phase transition characteristics. Dielectric constant dependence on temperature in the TDP crystal was best fitted by a power-law equation of the form with = 1.43 for CDP, and = 1.39 and = 1.34 for TDP mixed crystals of x = 0.003 and x = 0.008 respectively. A quasi-one-dimensional Ising model was employed to explain the experimental observations in terms of the decrease in the intrachain interaction constant, , and the increase in the interchain interaction constant, , with increasing TDP impurity concentration x.

Polarization Reversal and Ferroelectric Domain Structure Observed in Electroded Cesium Dihydrogen Phosphate Crystals Using an X-Ray Anomalous Dispersion Effect

We have carried out an X-ray intensity measurement and X-ray topography on electroded b plates of ferroelectric cesium dihydrogen phosphate, CsH 2 PO 4 (CDP), using a synchrotron radiation with a wavelength of 2.482 A above the Cs L 3 -absorption edge. We have found that integrated intensities I (150) and I (\bar1\bar50) show an anomalously large breakdown of Friedel's law, I (150)/ I (\bar1\bar50)=10.4 at 125 K, and display a ferroelectric hysteresis loop. The hysteresis loop determines that spontaneous polarization is antiparallel to the b axes set in both ferroelectric crystal structures related by inversions. The (150) diffraction topography shows that a single domain turns into a lamellar domain structure without fractal aspects after short-circuiting the b plate. The atomic displacement associated with polarization reversal is shown in a crystal structure model of 180° domains observed in the X-ray topograph.

133Cs NMR study of the ferroelectric and antiferroelectric transitions in CsH2PO4

Abstract Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric (FE) and antiferroelectric (AFE) phase transitions in cesium dihydrogen phosphate were studied using 133Cs nuclear magnetic resonance at 6.5 MHz. The spin-latticerelaxation time T1 was measured at temperatures down to the FE Curie point TC at pressures of 1 bar and at 1.5 and 3.0 kbar, down to the triple point Tt = 124.6 K at 3.3 kbar, and down to the AFE Neel point TN at 3.6 kbar. Far from the transition T1 decreases exponentially with decreasing temperature due to 1D fluctuations associated with the Jb interactions in disordered hydrogen-bonded chains running along the b axis. As temperature is decreased further, T1 decreases linearly as the JC interaction between hydrogen-bonded chains in b-c planes becomes important. From these results the pressure dependences of Jb, JC and the interplanar interaction Ja were calculated. At 3.3 kbar Ja changes sign, so the stacking of ordered planes becomes AFE rather than FE. At pressures above about 9 kbar, where the interaction JC extrapolates to zero, a new AFE phase is predicted in which each b-axis chain is oriented AFE with respect to nearest neighbors in both the a and c directions.

133Cs nuclear magnetic resonance study of one-dimensional fluctuations in CsH2PO4 and its ferroelectric and antiferroelectric transitions at high pressure.

Pressure and temperature effects on the one dimensional (1D) and higher-dimensionality correlations associated with the ferroelectric and antiferroelectric phase transitions in cesium dihydrogen phosphate were studied by means of the $^{133}\mathrm{Cs}$ nuclear magnetic resonance (NMR) spin-lattice relaxation time ${T}_{1}$. We measured ${T}_{1}$ at 6.5 MHz at temperatures down to the ferroelectric (FE) Curie point ${T}_{C}$ at 1 bar and at 1.5 and 3.0 kbar, down to the triple point ${T}_{t}$=124.6 K at 3.3 kbar, and down to the antiferroelectric (AFE) N\'eel point ${T}_{N}$ at 3.6 kbar. With decreasing temperature, ${T}_{1}$ first decreases exponentially due to 1D fluctuations associated with the ${J}_{b}$ interactions in disordered hydrogen-bonded chains running along b. As the temperature falls further, ${T}_{1}$ then decreases linearly as the ${J}_{c}$ interaction between these chains in hydrogen-bonded planes comes into play. From these results and the known pressure derivatives of ${T}_{C}$ and ${T}_{N}$, we calculated pressure dependences for ${J}_{b}$, ${J}_{c}$, and for the interplanar interaction ${J}_{a}$. At 3.3 kbar ${J}_{a}$ changes sign, so the plane stacking becomes AFE instead of FE. Above 8.9 kbar, where ${J}_{c}$ extrapolates to zero, a new AFE phase with a checkerboard arrangement of FE b chains is predicted.

Proton dynamics in the hydrogen bond. Inelastic neutron scattering by single crystals of CsH2PO4 at 20 K

Inelastic neutron scattering spectra (INS) of the powder and of oriented single crystals of cesium dihydrogen phosphate (CsH2PO4, or CDP) at 20 K have been investigated. For single crystals the incident neutron beam was perpendicular to either the [100] or [001] crystal planes in order to distinguish between the short and long hydrogen bonds. The proposed assignments are based on previous infrared and Raman data and on the INS band intensities and polarisation. The optical and INS OH stretching band profiles are compared. Their shapes are described in terms of mechanical and electrical coupling of the two stretching modes of a OH…O hydrogen bond. For the longer bond a Fermi resonance of the OH stretching mode with an overtone of the bending mode is observed. Finally, a broad central mode observed in the INS at very low frequency has been tentatively assigned to the relaxation of the proton transfer along the hydrogen bond.

Proton dynamics in the hydrogen bond. Inelastic neutron scattering by single crystals of CsH 2PO 4 at 20 K

Inelastic neutron scattering spectra (INS) of the powder and of oriented single crystals of cesium dihydrogen phosphate (CsH 2PO 4, or CDP) at 20 K have been investigated, in order to distinguish between the short and long hydrogen bonds. The optical and INS OH stretching band profiles are compared.

The Room Temperature Elastic Behaviour of CsH2PO4

The components of the elastic constant matrix of monoclinic caesium dihydrogen phosphate (CDP) have been determined using ultrasonic velocity measurements to be Cl1 = 28� 83 � 0 '43, C22 = 26�67 � O� 37, C33 = 65 �45 � 0'48, C44 = 8 .1O� 0,15, Css = 5 �20� 0,24, C66 = 9�17 � 0,22, C12 = 1l'4�3'6, C13 = 42�87�1�58, CIS = 5'13�0'67, C23 = 14�5�4�4, C 2S = 8'4�4'3, C3S = 7�50�0�81 and C46 = -2�25�0�31 GPa. Calculations of the velocity surfaces, ray directions, Young's modulus surfaces and linear compressibility show marked elastic anisotropy, which has been correlated with the chain and layer-like structure of CDP.

EPR study of the symmetry breaking effect in ferroelectric cesium dihydrogen phosphate doped with Cr 5+ ions

The (PO 4) 3− units in a CsH 2PO 4 (CDP) crystal were replaced in a small fraction of sites by (CrO 4) 3− groups and the EPR of the Cr 5+ center was investigated. Splitting of the EPR line appears at T ∗ c=245 K, 91 K higher that the ferroelectric transition temperature T c=154 K. The electronic wave function of Cr 5+ (3d 1) is identified as d x2 −y2. The d x2 −y2 function couples with the near protons and the reorientation of this unit in the two possible configurations occurs in the paraelectric phase and breaks the symmetry far above T c. The observed correlation time 10 −9 sec and associated activation energy ΔU=0.215 eV are discussed.

Raman OH stretching band shape of and proton dynamics in CsH2PO4

Abstract Cesium dihydrogen phosphate is a pseudo-one-dimensional ferroelectric with two different hydrogen bonds which can be distinguished by Raman spectroscopy. The shorter bond ( R o...o = 2.48 A) shows a typical ABC spectrum while the longer one ( R o...o = 2.54 A) gives rise to a strong band near 2700 cm −1 and to weaker narrow features at 2400, 2340 and 1680 cm −1 . These bands can be calculated assuming a time expansion of the autocorrelation function of the transition moment according to a recent theory. The fast decay of this function generates broad features while long-time contributions give narrow bands which are compared with the “zero-phonon” transition proposed by Fischer, Hofacker and Ratner. Adiabatic potential functions for the slow ν o...o mode in the fundamental and in the excited state of the OH stretching vibrations are computed in order to fit the observed spectra and the main terms of the electrical coupling between the slow and fast stretching coordinates are determined. Fermi and Evans resonances do not appear to be important band-shaping factors. Potential functions governing the O...O stretching motions are completely different for the two hydrogen bonds.

Neutron scattering studies of quasi one- and two-dimensional hydrogen-bonded ferroelectrics

Abstract This work reviews neutron scattering studies of cooperative ordering in the hydrogen bond networks of stannous chloride dihydrate (SCD) copper formate tetrahydrate (CPT) and cesium dihydrogen phosphate (CDP). In each case, there is an interesting nontrivial comparison to be made with an exactly solvable lattice model.