Proton-Containing Yttrium-Doped Barium Cerate: A Simultaneous

Apr 7, 2007 - Uptake of hydrogen occurs from room temperature up to 500−600 °C, beyond which the system dehydrates. Onset of dehydration is observe...
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J. Phys. Chem. C 2007, 111, 6574-6580

Proton-Containing Yttrium-Doped Barium Cerate: A Simultaneous Structural and Dynamic Study by Neutron Scattering N. Malikova,*,‡,§ C.-K. Loong,‡ J.-M. Zanotti,§ and F. Fernandez-Alonso¶,† Intense Pulsed Neutron Source DiVision, Argonne National Laboratory, 9700 S Cass AVenue, Argonne, Illinois 60439, Laboratoire Le´ on Brillouin (CEA-CNRS), CEA Saclay, 91191 Gif-sur-YVette cedex, France, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom ReceiVed: January 17, 2007; In Final Form: March 3, 2007

The structural response to proton incorporation in a yttrium-doped barium cerate (BCY), a perovskite exhibiting high protonic and low oxide ion conductivity, and the proton motion therein are investigated in situ as a function of temperature and surrounding atmosphere (flow of moist or dry air). Undoped barium cerate (BC) is taken as a nonproton-conducting reference. Using simultaneous neutron diffraction and quasielastic neutron scattering measurements, we determine and correlate the phase transitions in the BCY lattice, the hydrogen content, and the relative mean square displacement (MSD) of hydrogen atoms over the temperature range of 150-900 °C. Presence of OH groups in the BCY system affects the temperature of the high-temperature rhombohedral-cubic phase transition. Uptake of hydrogen occurs from room temperature up to 500-600 °C, beyond which the system dehydrates. Onset of dehydration is observed before the rhombohedral-cubic phase transition at 700 °C. The relative MSD follows the trend of hydrogen content, peaking also at 500600 °C. Decrease in MSD over a large range of temperatures (500-800 °C) suggests a corresponding significant variation of the bonding strengths between the hydrogen atoms and the lattice.

Introduction Yttrium-doped barium cerate (BCY) is a perovskite-type oxide with a number of properties suitable for applications as a solid electrolyte in fuel cells, hydrogen separation membranes, and hydrogen sensors. Attention has been drawn to operating devices such as fuel cells at elevated temperatures due to the accelerated electrode reactions without the use of expensive noble-metal electrocatalysts, thereby gaining an overall more efficient energy conversion. The desire for electrolytes that are chemically stable with high protonic conductivity at elevated temperatures has promoted numerous investigations focusing on a variety of perovskite-type oxides.1,2 Among these, BCY exhibits one of the most favorable combinations of conductivities for fuel cell operation, high protonic and low oxide conductivity over the 600-1000 °C temperature range, depending on the operational environment such as the partial pressure of oxidizing/reducing gases in the atmosphere.2,3 Proton mobility in BCY seems to be closely linked to the crystal phases of the perovskite lattice host. We begin here with a brief overview of the phases seen for both doped and undoped barium cerate, which belong to the ABO3-type perovskite (A and B stand for two different metal atoms; O stands for oxygen atoms). For these perovskites, the ideal, highest symmetry phase is the cubic Pm3hm phase. Its stability can be assessed using the Goldschmidt tolerance factor (t), which is a simple geometrical factor composed of the radii of the three atoms in the structure, * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. ‡ Argonne National Laboratory. § CEA Saclay. ¶ Rutherford Appleton Laboratory. † Also at the Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom.

t ) (rA + rO)/[x2(rB + rO)]. Any deviation from t ) 1 signals a mismatch of the A-O and B-O distances necessary for the ideal cubic arrangement. In real systems, this mismatch results in a cooperative rotation/tilting of the BO6 octahedra, which lowers the overall symmetry of the structure, usually toward the orthorhombic or rhombohedral phases.4,5 Detailed neutron diffraction studies have shown that, at room temperature, undoped barium cerate (BC) is of the orthorhombic Pmcn phase and undergoes a series of phase transitions between room temperature and 900 °C: Pmcn f Incn (at 290 °C) f F3h2/n (at 400 °C) f Pm3hm (at 900 °C). The highest symmetry phase is, therefore, recovered at high temperature.6 In BCY, substitutions of Ce4+ by Y3+ at the B metal position cause the formation of oxygen vacancies (VO¨ ) in order to maintain the overall electro-neutrality of the lattice. Knight et al. reported that doping of barium cerate with up to 10% of Y (or up to 20% of Nd) does not modify the sequence of phase transitions seen in BC as a function of temperature; however, it changes the widths of the phase fields.6 At this stage, it is important to note that BCY is a reactive substance. It is hygroscopic, whereby initially absorbed water molecules dissociate, resulting in the formation of hydroxyl groups (OHO˙ ) on the sites of the original oxygen vacancies. This can be summarized in the following defect reaction written in the Kro¨ger-Vink notation

H2O + VO¨ + OxO f 2OHO˙

(1)

The phase of a BCY system at a particular temperature is likely to be influenced by at least two factors, (a) the degree of yttrium doping and (b) the surrounding atmosphere determining the degree of occupation of oxygen vacancies by hydroxyl groups. This has been demonstrated in a previous study, in which BCY

10.1021/jp070395+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007

Proton-Containing BCY samples (0-30% Y doping) were annealed under oxidizing, reducing, and water-vapor-containing atmospheres and thereafter analyzed by neutron diffraction at ambient temperature.7 Under an oxidizing atmosphere (unfavorable for hydroxyl group formation), the phase remained orthorhombic up to 15% Y doping (in agreement with the observation of Knight et al.6); thereafter however, it became rhombohedral F3h2/n (a nonstandard setting of the R3hc quoted in reference 7). Under a hydrogenor water-containing atmosphere, an additional monoclinic I2/m phase was identified for samples with Y doping greater than 10%. It was suggested that the monoclinic I2/m phase is important for the dissolution of water in the lattice. (As the current data shows, the absence of the monoclinic I2/m phase in 20% doped BCY does not prevent water dissolution in the lattice. However, water dissolution does seem to occur to a lesser extent.) In the current study, we concentrate on a barium cerate (BC, a control sample) and a yttrium-doped barium cerate sample (BCY). As a function of temperature and surrounding atmosphere (moist or dry), we simultaneously analyze the phase transitions that take place and assess the content and the mean square displacement of hydrogen atoms in the sample. On the basis of previous results which show that, under conditions favoring the formation of hydroxyl groups, the total conductivity increases with increasing yttrium concentration from 5 to 20%,8 we use here a 20% yttrium-doped barium cerate. Experimental Techniques Barium cerate samples, both undoped and yttrium-doped, were prepared by a solid-state reaction; the details particular to this procedure can be found in Takeuchi et al.7 Neutron diffraction measurements were performed on the General Purpose Powder Diffractometer (GPPD) at the Intense Pulsed Neutron Source (IPNS) of Argonne National Laboratory at ambient temperature and pressure. Samples, in the form of a powder (approximately 10 g), were held inside a cylindrical vanadium can (diameter: 1.1 cm). For quasielastic neutron scattering measurements, we used a flow cell setup (Figure 1), in which the sample (a coarse powder, approximately 25 g) was held inside a quartz tube (inner diameter: 1.0 cm) on a porous quartz frit and could be subjected to a flow of moist or dry gas (or held under vacuum). Quartz was chosen for its zero incoherent scattering contribution as well as ease and low cost to manufacture. Generation of moist gas was achieved simply by passing the gas through distilled water (a “bubbler”) prior to its passage into the quartz tube. The central part of the quartz tube, containing the sample, was surrounded by a heating element and could be heated up to approximately 1100 °C, the temperature being recorded by two thermocouples placed inside the tube both beneath the frit and above the sample itself. At first, simultaneous quasielastic and diffraction experiments were carried out on the QENS spectrometer at IPNS. QENS is an inverse-geometry time-of-flight spectrometer capable of quasielastic measurements over 22 scattered flight paths.9,10 Overall, the range of accessible wave vectors is 0.3-2.5 Å-1, and the energy resolution (fwhm) is about 80 µeV. Two sets of diffraction detectors are also available on QENS; they cover a wave vector (Q) range of 0.1-24 Å-1 (0.25-62 Å in d-spacing) with a resolution (∆d/d) of 1%. In our QENS experiments, samples were heated/cooled between room temperature and approximately 800-900 °C at a rate of 3 °C/min, while being held for 30 min at 30 °C intervals for quasielastic scattering measurements to be taken. For each sample, the entire heat

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Figure 1. Flow cell sample environment in conjunction with the socalled Howe furnace used for simultaneous diffraction and quasielastic measurements on doped and undoped barium cerate samples. Moist or dry gas flows through the sample held inside a quartz tube, while temperature can be varied between 25 and approximately 1100 °C. Figure after K. Volin.

treatment cycle consisted of heating under moist air, annealing under a flow of pure oxygen for 1 h at the highest temperature, and then cooling under dry air. Additional high-resolution quasielastic experiments were carried out on the IRIS spectrometer at the ISIS facility of the Rutherford Appleton Laboratory, another inverse-geometry timeof-flight spectrometer.11 Detectors in close to a back-scattering geometry achieve a higher energy resolution, here 17.5 µeV (fwhm) at the elastic line, for a Q range of 0.4-1.8 Å-1. A slightly different heating scheme for the sample under flowing moist air was adopted in this case, a heating rate of 2 °C/min from room temperature to 800 °C, while being held for 20 min at 20 °C intervals for measurements to be taken. Under the setting used, diffraction data over a smaller range of d-spacing was also collected on the IRIS spectrometer (3.0-4.0 Å). Results and Discussion On the basis of initial structural characterization of the undoped barium cerate (BC) and 20% yttrium-doped barium cerate (BCY) by neutron diffraction (GPPD spectrometer), we conclude that, at room temperature, these samples were of the orthorhombic Pmcn and rhombohedral F3h2/n phase, respectively (Figure 2). Using the combination of the diffraction and quasielastic data from the QENS spectrometer, the main pieces of information obtained, as a function of temperature and type of gas passing across the sample, were (1) the crystal structure of the perovskite lattice from the diffraction pattern, (2) the approximate hydrogen content as indicated by the background in the diffraction pattern between 2.80 and 3.00 Å in d-spacing, and (3) the mean square displacement (MSD) determined from the Debye-Waller factor. In the elastic region, the effect of fast vibrational motion comes in the form of the Debye-Waller factor, an overall multiplication factor of the total elastically scattered intensity

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Malikova et al.

Figure 3. Series of diffractograms as a function of temperature for BC (a) and BCY (b) under a flow of moist air. Disappearance of the peak at 2.7 Å indicates the rhombohedral F3h2/n-cubic Pm3hm phase transition. The furnace background is primarily responsible for the observed intense peak at 2.37 Å.

Figure 2. High-resolution diffractograms of the starting materials, BC (a) and BCY (b), showing the orthorhombic Pmcn and rhombohedral F3h2/n (R3hc) phases, respectively. The bottom row of tick marks indicates the positions of Bragg reflections.

at a given Q. For an isotropic or powder averaged system, we can write SEL(Q,ω) ∝ e-〈u2〉Q2/3, with 〈u2〉 being the mean square displacement.12,13 MSD can therefore be extracted from the linear slope of a plot of the logarithm of the scattered intensity integrated over the elastic region, that is, ln(∫EL S(Q,ω)dω) versus Q2 (from now on, MSD shall refer to this slope and thus rigorously to 〈u2〉/3). In practice, the integration is carried out across the width of the resolution function. In our case, we used the integration limit of (0.04 meV for the medium-resolution QENS data and (0.01 meV for the high-resolution IRIS data. For a crystalline or glassy solid at low temperatures, where an atom is bound by a harmonic-like potential at its equilibrium position, MSD increases linearly with temperature according to the quasiharmonic approximation. For systems where stochastic, longer-range motion in time and space is possible, such as rotational/translational motion of H atoms in BCY, a part of the scattered intensity appears outside the elastic region (quasielastic broadening). The overall result is a faster decrease of elastically scattered intensity as a function of Q2 at a given temperature. In the presence of quasielastic broadening, the MSD determined using the above method is then significantly larger; it no longer reflects only atomic vibrational amplitudes but also the longer-range types of motion. Figure 3 features a series of diffractograms for BC and BCY upon heating under flowing moist air. For the BC system, we observe a peak at 2.81 Å of the orthorhombic Pmcn phase, which disappears upon heating at around 200 °C, in agreement with Knight et al.6 Note that the behavior of another peak

Figure 4. Diffraction background averaged between 2.80 and 3.00 Å for the BC ()) and BCY (4) samples upon heating under moist conditions.

corresponding to the orthorhombic Pmcn phase, at 2.37 Å, is obscured by the background furnace peak at the same d-spacing, which remains throughout the entire temperature range studied. In addition, the disappearance of the peak at 2.7 Å at approximately 850 °C, which corresponds to the rhombohedral F3h2/n-cubic Pm3hm phase transition, is clearly seen in Figure 3a. This transition temperature is the same for both the heating and cooling part of the cycle (data on cooling not shown). The series in Figure 3b for the BCY sample shows a similar change, corresponding to the analogous rhombohedral-cubic phase transition; however, in this case, a “hysteresis” was observed. Upon heating (under moist conditions), the phase transition occurs at approximately 700 °C, whereas upon cooling (under dry conditions), it decreases to 540 °C (data on cooling not shown). Figure 4 compares the diffraction background averaged between 2.80 and 3.00 Å (region free of Bragg peaks) for the BC and BCY samples upon heating under moist conditions. The steady increase for the BC sample should be treated as a reference, reflecting combined signals from the perovskite

Proton-Containing BCY

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lattice, quartz tube, and furnace shielding as the entire system heats up (note that no background subtraction was performed to produce data for Figure 4). Ascribing the difference between the BC and BCY data sets to the signal from the H atoms (strongest incoherent scatterers present) absorbed in the form of OH groups, we observe an increase in water absorption by BCY up to 600 °C, beyond which the system dehydrates. It has been discussed by Kruth et al.14 that, for BCY with 10% yttrium doping, it might be the orthorhombic Incn-rhombohedral F3h2/n phase transition that drives the dehydration of the system. For our system, richer in yttrium, no such transition exists; the phase is rhombohedral already at room temperature, and dehydration occurs before the phase transforms into the high-temperature cubic phase. This suggests that dehydration is not necessarily driven by a phase transition of the underlying lattice. At the same time, the presence or absence of hydroxyl groups in the structure almost certainly affects the phase transition temperatures, as is seen in the above-mentioned hysteresis for the rhombohedral F3h2/n-cubic Pm3hm phase transition in 20% doped BCY. We return now to the quasielastic measurements. In the system under investigation, H atoms of the hydroxyl groups are the strongest incoherent scatterers. As such, they can be treated as the primary contributors to the scattered intensity for wave vectors far from any coherent scattering (Bragg) peaks. This reasoning has been exploited in the previous section in the assessment of the hydrogen content from the diffraction background between Bragg peaks. For the quasielastic measurements, the separation of the coherent and incoherent contributions is not straightforward. In addition to the incoherent scattering of interest, the integrated elastic intensities in the QENS accessible Q range (0.3-2.5 Å-1) contain both a broad peak at 1.4 Å-1 arising from the coherent scattering of the quartz tube (amorphous silica) and a number of Bragg peaks from the perovskite lattice. Extraction of a Debye-Waller factor (and the corresponding MSD) directly from the observed data is impossible. Instead, for each series of measurements as a function of temperature (BC-moist, BC-dry, BCY-moist, BCYdry), we normalized the integrated elastic intensities by a lowtemperature run (reference run) under the same conditions. We refer to the MSD extracted from these normalized elastic intensities as a relative MSD. The value of the relative MSD corresponds to the difference of MSD between the temperature of interest and the reference temperature, as can be seen from uref2 2 〉Q /3

-〈 the following. For IEL ref ) Arefe

EL ITi,Norm )

IEL Ti IEL ref

EL ln(ITi,Norm ) ) ln

)

-〈 and IEL Ti ) ATie

uTi2 2 〉Q /3

ATi -[〈uTi2〉-〈uref2〉]Q2/3 e Aref

( )

ATi - Q2[〈u2Ti〉 - 〈u2ref〉]/3 Aref

(2)

Figure 5 gives examples of the normalized elastic intensities versus Q2, together with the best linear fits, for both the medium(QENS) and high-resolution (IRIS) data. Relative MSD was EL ) versus Q2 in the obtained from a linear fitting of ln(ITi,Norm -1 region of Q < 2.4 Å for QENS data and Q < 1.35 Å-1 for IRIS data. In the IRIS data set, the contribution to the elastic intensity from the background (quartz) and the perovskite network dominated over the signal from the absorbed H atoms beyond Q ) 1.35 Å-1. The normalized elastic intensities showed almost no Q2 dependence in that region and were excluded from the fitting. This problem was less serious in the case of the QENS data, and fitting could be carried out up to Q ) 2.4 Å-1.

Figure 5. Plots of the logarithm of the normalized elastic intensity 2 (IEL Norm) versus Q , together with best linear fits, for the BCY sample under moist conditions at 270-280 (black), 360 (purple), and 540 °C (green). The two higher temperature sets are offset along the y-axis for clarity. (a) Data from the medium-resolution QENS spectrometer (30 min data collection time per temperature; integration limits, (0.04 meV). (b) Data from the high-resolution IRIS spectrometer (20 min data collection time per temperature; integration limits, (0.01 meV).

However, we note that some (but tolerable) distortion to the linear trend over the Q range considered in the QENS data sets is seen (Figure 5a). In the following analysis, we concentrate on the trends of relative MSD as a function of temperature rather than on its absolute value. The comparison of the relative MSD data sets for the BC and BCY systems under the same conditions is the most pertinent. While the BC data set gives an indication of the signal arising from atoms other than those in OH groups (perovskite lattice, quartz tube, mainly coherent scatterers), the difference in trends between the BC and BCY data sets can be almost exclusively ascribed to H atoms in the BCY system. Figure 6 summarizes the relative MSD for the four series of temperature measurements made on the QENS spectrometer. The series of BC-moist upon heating shows a moderate, steady increase of the relative MSD with increasing temperature, and that of BC-dry upon cooling traces the relative MSD values of BC-moist. A similar behavior is observed for the cooling BCdry system. The heating of BCY-moist contrasts with the others, featuring a significantly greater increase in the relative MSD

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Figure 6. Relative mean square displacements for the BC (a) and BCY (b) samples upon heating under moist conditions (] for BC, 4 for BCY) and cooling under dry conditions (× in both cases). Data were obtained from the medium-resolution QENS spectrometer.

Figure 7. Relative mean square displacements for the BCY sample upon heating under moist conditions. Data were obtained from the highresolution IRIS spectrometer.

up to a maximum at 500-600 °C and, thereafter, a rapid decrease. This turnover temperature coincides with the onset temperature of dehydration of the BCY system, as evident from the decreasing incoherent scattering background in the simultaneously measured diffractograms. The heat treatment of BCY under flowing moist air was further repeated on the IRIS spectrometer. The results are summarized in Figure 7. In very good agreement with the QENS data set for BCY under a flow of moist air, we observe a rapid increase in the relative MSD from 100 to 500-600 °C followed by a rapid decrease. The background of the simultaneously measured diffractogram also reaches its maximum at around 600 °C (not shown). Note that the relative MSD values extracted from the IRIS data set are, overall, higher than those from the QENS data set. This is an immediate consequence of the

Malikova et al. narrower Q region considered for the linear fitting in the case of the IRIS data, as mentioned earlier. Fitting of the QENS data sets only up to Q ) 1.35 Å-1, as in the case of IRIS data, produces very similar results for the two data sets. Most importantly, even if the exact Q region over which linear fitting is carried out has some effect on the value of the relative MSD (also seen in ref 15), it does not change the overall trend of MSD seen as a function of temperature. The latter is the crucial piece of information for our purposes. The trends in the two data sets of MSD for BCY under moist conditions have to be interpreted in conjunction with the total H content in the system as determined from the simultaneous diffractograms. The value of MSD at any given temperature is an average of the mean square displacement of all of the H atoms in the system. Even at room temperature, there is nonzero H content in the BCY sample; the MSD, however, does not show significantly higher values in comparison to those of the other atoms in the system (compare to the dry-BCY sample set). In the temperature range between 200 and 400 °C, the system takes up H atoms, and the average MSD rapidly increases. This indicates that the newly absorbed H atoms are much more mobile. The decrease in MSD in the range of 500800 °C is a result of a number of phenomena. It is unusual for the MSD of a given H atom to decrease with increasing temperature (though we can imagine a temperature-induced phase transition leading to proton ordering); the observed decreasing MSD is a consequence of depopulation of the H atoms in the perovskite lattice governed by the bonding strengths. It seems probable that H atoms with the highest mobility (largest MSD) are bonded weakly to the lattice; they leave the system first, causing the average MSD to drop. Qualitatively, the wider the temperature range over which the MSD decreases in this dehydration stage, the wider the bonding energy distribution over the H sites. To properly assess the range of H-bonding strengths, it would be desirable to carry out longer measurements at a given temperature to avoid any kinetic effects. Furthermore, OH stretching frequencies from inelastic neutron scattering, Raman, or infrared spectroscopy are of value here and would give more quantitative information. Care should be, however, taken to make such measurements under comparable H loading of the system. In the present study, the self-consistency in the analysis of the simultaneously measured diffraction and quasielastic scattering data, obtained from independent measurements by two different spectrometers, lends credence to our qualitative assessment of the temperature dependence of the H content and mobility. Furthermore, our own and other thermal gravimetric measurements on BCY show the onset of dehydration around 600 °C for yttrium-rich samples, in agreement with the present data.14,16,17 For lower yttrium doping, less than 10%, dehydration starts at temperatures as low as 400 °C.14 Combining the current results with those of a previous ex situ diffraction study which highlights the importance of a monoclinic I2/m phase for water dissolution in BCY,7 we conclude here that the absence of this monoclinic phase does not preclude water absorption. However, comparing roomtemperature high-resolution diffractograms of two BCY samples (20% yttrium doping), one with and one without the monoclinic phase, we observe a higher background in the former, which suggests that the presence of the monoclinic phase induces a higher water content. Simultaneous diffraction and quasielastic measurements on a BCY sample containing a monoclinic phase are envisaged. Previous attempts of measurements using such a sample suggested a possible link between the disappearance

Proton-Containing BCY

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6579 air at 510 °C. This quasielastic region of interest is bounded by the elastic peak at ω < 0.1 meV and the increasing quartz background beyond 2 meV. (The quartz background contains a dispersive mode, which becomes more prominent as Q increases, at around 5 meV toward the high Q end). No clear peaks are observed in the quasielastic region. Modeling of the quasielastic broadening with two components (narrow translational and wider transrotational contribution18) shows the peaks of the two components in the susceptibility curve as too wide to be seen distinctly between 0.1 and 2 meV. The wider, transrotational contribution is also masked by the increasing background beyond 2 meV. More detailed analysis of the quasielastic broadening is intended for a future communication. Conclusion

Figure 8. Susceptibility data for BCY-moist (green), BCY-dry (red), and the background (empty quartz tube, black) at 510 °C. The three curves at each Q value coincide except over the region of 0.1-2 meV where the high-to-low-intensity curves correspond to BCY-wet, BCYdry, and the empty quartz. Data were obtained from the QENS spectrometer.

of the monoclinic phase at around 340 °C and an increase in the observed relative MSD. Determination of MSD is only an initial step in studying the proton motion in the system. More information concerning the longer-range motion can be obtained, in principle, from the analysis of the quasielastic broadening in terms of relaxation times.18-20 The current MSD analysis serves to identify the temperature range most appropriate for such an analysis. Here, we present, only briefly, a set of susceptibility curves from the quasielastic measurements on the QENS spectrometer. These help to clearly distinguish the multiple contributions in the observed quasielastic signal. The imaginary part of the susceptibility, χ′′(Q,ω), is defined as the scattering function, S(Q,ω), divided by a Bose factor, n(ω,T) or [n(ω,T) + 1] for neutron energy gain (NE gain) and neutron energy loss (NE loss), respectively.21-23 Mathematically

χ′′(Q,ω)NEgain )

χ′′(Q,ω)NEloss )

S(Q,ω)NEgain n(ω,T)

) S(Q,ω)NEgain(eω/kBT - 1) (3)

S(Q,ω)NEloss

) n(ω,T) + 1 S(Q,ω)NElosse-ω/kBT(eω/kBT - 1) (4)

where ω is the energy transfer, kB the Boltzmann constant, and T the temperature. In the susceptibility representation, a mode appears as a peak at an energy transfer equal to its characteristic broadening, HWHM () half width at half-maximum).21,24 From Figure 8, an increased susceptibility signal in the region of 0.1-2 meV is observed for BCY samples under a flow of moist

We have investigated the structure and local proton dynamics, as characterized by the mean square displacement, in 20% yttrium-doped barium cerate under conditions of flowing moist and dry air at temperatures between 150 and 900 °C. Taking the undoped barium cerate as a reference sample, we have analyzed the simultaneously measured diffraction, elastic incoherent scattering, and quasielastic scattering data to correlate the structural phase transitions in the perovskite lattice, the hydrogen content, and the relative MSD of the H atoms in a self-consistent manner. The major observations and conclusions are as follows. (a) While barium cerate undergoes a rhombohedral F3h2/ncubic Pm3hm phase transition at 850 °C irrespective of the surrounding atmosphere (moist or dry air), a hysteresis is observed for this transition in the case of 20% yttrium-doped barium cerate. The transition occurs at approximately 700 °C upon heating under moist air and at approximately 540 °C upon cooling under dry air. Overall, the surrounding atmosphere, and thus the presence or absence of hydroxyl groups in BCY, has an effect on the phase transition temperature. (b) Even in the absence of the monoclinic I2/m phase, which was thought to be crucial for OH incorporation in BCY,7 we observe an uptake of hydrogen (presumably as OH groups from dissociated absorbed water). However, BCY samples containing the monoclinic phase may have higher water-absorbing capacity, a topic of interest for a future neutron scattering study. (c) Uptake of hydrogen by the BCY system with the temperature increasing from 25 to approximately 500-600 °C is accompanied by an increase in the relative mean square displacement of the H atoms. This is consistently seen in independent medium-resolution and high-resolution measurements. (d) Dehydration of the system, even under flowing moist air, occurs around 600 °C, accompanied by a decrease in the relative MSD. The fact that the relative MSD declines over a temperature range of almost 300 °C (500-800 °C) provides evidence for the presence of H sites with a variable degree of bonding strengths to the perovskite lattice. If there existed only one H site of uniform bonding energy, the drop in relative MSD would be very abrupt. After ruling out a kinetic effect, more direct information on the bonding strength of H sites in BCY is to be sought from inelastic neutron scattering, Raman, or infrared spectroscopy. No evidence is seen for the dehydration being driven by a phase transition, as it begins before the rhombohedral F3h2/n-cubic Pm3hm phase transition. Overall, in continuation of the work in ref 7, we identify a shift of the rhombohedral F3h2/n-cubic Pm3hm transition temperature in a BCY sample with respect to the surrounding atmosphere (moist or dry). The measurements carried out here

6580 J. Phys. Chem. C, Vol. 111, No. 17, 2007 and in ref 7 are still under relatively low water vapor pressure (0.02 atm). While some water absorption occurs for BCY already under ambient humidity at room temperature, more efficient “loading” of the system with hydroxyl groups occurs at elevated temperatures under significantly higher water vapor pressures (up to the order of 1 atm).17 (Even in these latter cases, at most, 80% of the oxygen vacancies tend to be occupied.14,17) To fully understand the dependence of the structural phase on the concentration of hydroxyl groups in the material, a systematic neutron diffraction study with BCY being preconditioned up to significantly higher water vapor pressures might be instructive. Acknowledgment. N.M. gratefully acknowledges financial support of the European Community in the form of the Marie Curie Outgoing International Fellowship (Contract MOIF-CT2005-021792). F.F.A. gratefully acknowledges financial support from the U.K. Science and Technology Facilities Council. This work is further supported by the Argonne National Laboratory (U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357) and by the Laboratoire Le´on Brillouin (Commissariat a l’Energie Atomique/Centre National de la Recherche Scientifique, France). The authors are indebted to Dr. K. Takeuchi for the provision of BC and BCY samples, further to K. Volin, R. Ziegler, and Dr. N.R. de Souza at IPNS and A.J. Church, C.M. Goodway, and R. Haynes (ISIS User Support Group) for technical support prior to and during experiments. They also thank Professor B. Dabrowski, Dr. R. Kiyanagi, and Dr. K.S. Knight for fruitful discussions. References and Notes (1) Kreuer, K. D. Solid State Ionics 1997, 97, 1. (2) Kreuer, K. D. Annu. ReV. Mater. Res. 2003, 33, 333.

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