Proton Dynamics in Hydrated BaZr0.9M0.1O2.95 (M = Y and Sc

Article pubs.acs.org/JPCC

Proton Dynamics in Hydrated BaZr0.9M0.1O2.95 (M = Y and Sc) Investigated with Neutron Spin−Echo Daria Noferini,†,‡ Michael M. Koza,‡ Peter Fouquet,‡ Gøran J. Nilsen,‡ Moureen C. Kemei,§ Seikh M. H. Rahman,∥ Marco Maccarini,‡ Sten Eriksson,∥ and Maths Karlsson*,† †

Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France § Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States ∥ Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ‡

S Supporting Information *

ABSTRACT: Hydrated samples of the two proton conducting perovskites BaZr0.9M0.1O2.95 (M = Y and Sc) were investigated using neutron spin−echo spectroscopy together with thermal gravimetric measurements, polarized neutron diffraction, and infrared spectroscopy, with the aim to determine how the atomicscale proton dynamics depend on temperature, and type of dopant atom, M. The results show the presence of pronounced localized proton motions for temperatures above ca. 300 K, characterized by relaxation times on the order of picoseconds to nanoseconds and governed by a wide distribution of activation energies due to a heterogeneous distribution of proton sites present, with no strong dependence on the type of dopant atom.

1. INTRODUCTION Perovskites, sometimes referred to as inorganic chameleons, can display a wide range of useful properties due to their ability to accommodate varying cations, substitution (doping), nonstoichiometry, and structural defects. Of specific concern for this work are acceptor doped, hydrated perovskites, which show proton conducting properties. The primary interest in these materials comes from their huge potential to contribute to a sustainable future through their usage as efficient electrolytes in various electrochemical devices, such as hydrogen sensors, electrochromic displays, and, perhaps most importantly, in next-generation intermediate-temperature (≈200−500 °C) solid oxide fuel cell (SOFC) technology.1,2 The role of the acceptor doping, such as Y substituted for Zr in BaZrO 3 , is to create an oxygen deficient structure (BaZr1−xYxO3−x/2), which can be hydrated by heat treatment in a humid atmosphere.2 During this process, water molecules from the gaseous phase dissociate into hydroxyl groups (OH−), which fill the oxygen vacancies, and protons (H+), which bind to lattice oxygens, thus forming ideally compounds of the form BaZr1−xYxO3Hx. The protons are generally not stuck to any particular oxygens but can, at elevated temperatures, move from one oxygen to another one. This leads to these materials’ high proton conductivities, reaching a typical value of the order 10−4−10−2 S cm−1 in the upper part of the intermediate temperature range.2 An atomistic view of the proton conduction mechanism started to emerge in the mid 1990s and is based mainly on © 2016 American Chemical Society

results obtained from computer simulations and quasielastic neutron scattering (QENS) on bulk powder samples of acceptor doped BaZrO3 and BaCeO3, as summarized recently.3 The conduction mechanism is divided into two main processes: (i) proton transfers (jumps) between neighboring oxygens and (ii) reorientational motions of the hydroxyl group between such jumps.3 However, local structural defects, related to the presence of, e.g., dopant atoms and oxygen vacancies, complicate the description of the proton diffusivity on an atomic length scale. It remains still not fully understood for even the simplest systems. For example, the relationship between proton conduction and the type of dopant atom, M, or the degree of protonation, has not yet been established. In this work, we investigate the proton dynamics in hydrated samples of the two proton conducting perovskites BaZr0.9M0.1O2.95 (M = Y and Sc). The aim is to determine how the dynamics depend on the temperature and type of dopant atom, M. Our research is based on a multitechnique approach, through the usage of QENS by the technique called neutron spin−echo (NSE), thermogravimetric analysis (TGA), polarized neutron diffraction, and infrared (IR) spectroscopy. In comparison to other QENS techniques, such as backscattering and time-of-flight (TOF) methods, NSE gives access to a larger dynamical range, from picoseconds to nanoseconds, Received: April 12, 2016 Revised: June 9, 2016 Published: June 10, 2016 13963

DOI: 10.1021/acs.jpcc.6b03683 J. Phys. Chem. C 2016, 120, 13963−13969

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The Journal of Physical Chemistry C ⎛ dσ ⎞ H ⎛ dσ ⎞ ⎜ ⎟ ⎟ =⎜ − ⎝ dΩ ⎠inc ⎝ dΩ ⎠inc

which is of importance in the likely presence of nontrivial dynamics. Previous QENS studies on the same materials have revealed the presence of localized proton motions, characterized by a short relaxation time (≈2−4 ps) and low activation energy (≈10−30 meV), with only a small dependency on the type of dopant atom,4 as well as proton diffusion on an intermediate length scale characterized by a relaxation time of some nanoseconds.5 In comparison, the present study demonstrates the presence of pronounced localized proton motions above a temperature of ca. 300 K, characterized by relaxation times on the order of picoseconds to nanoseconds and governed by a wide distribution of activation energies due to a heterogeneous distribution of proton sites present, with no strong dependence on the type of dopant atom.

∑ i≠H

i i σinc n 4π

(1)

where (dσ/dΩ)Hinc is the incoherent scattering cross section of hydrogen and (dσ/dΩ)inc that over the sum of all atoms in the material. σiinc refers to the incoherent scattering cross section of the ith element in the compound (i = Ba, Zr, Y, Sc, O, H), weighted for its stoichiometric factor ni. Dividing (dσ/dΩ)Hinc by σHinc/4π yields the hydrogen molar content per formula unit. It should be noted that the spin-incoherent cross section of all elements except H is small. The experiment was performed on the diffuse scattering spectrometer D7 at the Institut Laue-Langevin (ILL) in Grenoble, France. An incident wavelength of 4.8 Å was used to access a Q-range of 0.25−2.47 Å−1. The measurements were carried out at room temperature (≈300 K), with the samples held in cylindrical Al cans with a wall thickness of approximately 0.5 mm and an inner diameter of 10 mm. The containers were vacuum-sealed using a stainless-steal head that was pinched to the Al head of the cells. The transmission through the samples was measured to be around 80%. Measurements of cadmium and quartz standards were used for the correction for beam attenuation and flipping-ratio calibration, respectively. A measurement of an empty container was used for sample-holder subtraction. The correction of the detector efficiency and the calculation of the signal in absolute units was carried out by means of a vanadium standard of known mass. 2.3. NSE Experiment. The NSE measurements were performed on the IN11C (hereafter referred to as IN11) spectrometer at the ILL. The spectrometer was set to an effective average neutron wavelength of 5.5 Å with the 30° wide-angle detector centered at a scattering angle of 50°. With this setup, we accessed simultaneously an NSE time window of ca. 5 ps to 1.3 ns and a Q-range of 0.7−1.2 Å−1. The same sample specimens were used as on D7. Spectra were measured at 2, 249, 421, 454, 500, 528, and 546 K for 10Y:BZO and at 2, 299, 373, 453, 491, and 546 K for 10Sc:BZO. The acquisition periods were 8 h (249−528 K) and 16 h (2 and 546 K), respectively. The 2 K spectra were used as resolution functions in the data analysis. To increase the statistics, the spectra measured at different temperatures were summed over the entire Q-range, thus giving an average Q-value of 0.96 Å−1. For a Q-dependent analysis, data accumulated for 16 h were grouped into three spectra with average Q-values of 0.77, 0.99, and 1.17 Å−1. Furthermore, the coherent to incoherent intensity ratio was monitored during each measurement. At any probed temperature, it was reproduced within the experimental precision, giving evidence that no unwanted dehydration of the samples took place during the measurements.

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. Powder samples of BaZr0.9Y0.1O2.95 and BaZr0.9Sc0.1O2.95 were prepared by solid state sintering after mixing stoichiometric amounts of BaCO3, ZrO2, and Y2O3 or Sc2O3, respectively, with the sintering process divided into three heat treatments: 1100 °C for 12 h, 1250 °C for 24 h, and 1450 °C for 24 h, with intermediate cooling, grinding, and compacting of pellets between each heat treatment. Hydrated samples, BaZr0.9Y0.1O2.95(H2O)x and BaZr 0.9 Sc 0.1 O 2.95 (H 2 O) x , referred to as 10Y:BZO and 10Sc:BZO, respectively, were prepared by annealing the assintered powders at approximately 300 °C under a flow of N2 saturated with water vapor for a period of a few days. The hydrated samples were characterized by TGA, using a TG 209 F1 Iris instrument from Netzsch. For each measurement, about 100 mg of sample was placed in an open platinum pan, which was heated under a flow of N2 gas (25 mL/min) from room temperature up to 1300 K, with a heating rate of 10 K/min. Additionally, IR diffuse reflectance spectroscopy measurements were performed on the hydrated samples, using an Alpha FTIR spectrometer from Bruker. Absorbance-like spectra of the two samples were derived by taking the logarithm of the ratio between the spectrum of a rough Au mirrora “perfectly” diffuse scatterer and used as a referenceand the sample spectra. X-ray diffraction (XRD) patterns of the samples confirmed a cubic structure (space group Pm3̅m) with no detectable amounts of impurities, in agreement with the literature.6−8 2.2. Neutron Diffraction with Polarization Analysis. Complementary to the TGA, the proton, or more accurately hydrogen (H), concentrations in the two hydrated samples were also extracted from neutron diffraction measurements using xyz-polarization analysis. The technique, described in detail in refs 9 and 10, is based on measuring the spin-flip and non-spin-flip cross sections for three orthogonal orientations of the neutron beam polarization and then extracting from linear combinations of these the nuclear coherent + isotope incoherent, spin incoherent, and magnetic contributions. Note that, for the compounds studied here, 10Y:BZO and 10Sc:BZO, the nuclear isotope incoherent scattering cross section is insignificant.11 We therefore refer to the spin incoherent scattering cross section as the incoherent scattering cross section hereafter. The hydrogen concentration is obtained from the incoherent part after subtraction of the contribution of all other atoms in the material according to

3. NSE ON PROTON CONDUCTING PEROVSKITES The measured quantity in NSE experiments is the intermediate scattering function, I(Q, t), which comprises a coherent and an incoherent component, Icoh(Q, t) and Iinc(Q, t), related to collective and single-particle (self) dynamics, respectively.3 Here, the incoherent component is dominated by the spin incoherent scattering from hydrogen, which changes the beam polarization to −1/3 of its nominal value. The normalized intermediate scattering function measured by NSE can therefore be expressed as12 13964


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The Journal of Physical Chemistry C Icoh(Q , t ) − I (Q , t ) = I(Q , t = 0) Scoh(Q ) −

1 3 1 3

i ∑i Iinc (Q , t ) i ∑i Sinc (Q )

between 300 and 1300 K, and assuming a gradual dehydration of the material as the only effect upon increased temperature, we estimate the initial hydrogen content to x = 0.40 in 10Y:BZO and to x = 0.13 in 10Sc:BZO. The coherent, incoherent, and magnetic scattering responses obtained by neutron diffraction with polarization analysis are shown in Figure 2a. Note that the magnetic scattering component is essentially zero within the experimental uncertainty. The coherent signal, and thus the coherent to incoherent signal ratio, is constant only at 0.7 Å−1 ≲ Q ≲ 1.4 Å−1. This momentum range is embraced at higher Q-values by Bragg peaks of the BaZrO3 crystalline structure, and toward lower Q-values by an ascending small-angle signal related to the powder nature of the samples. Thus, we estimate the proton concentration from this Q-range with a precision of ca. 10% to be x = 0.34 in 10Y:BZO and x = 0.18 in 10Sc:BZO. These numbers are in good agreement with the mass loss calculated from the TGA data (Figure 1) and show that this loss is predominantly related to hydrogen. In Figure 2b is highlighted the good agreement of the coherent and incoherent relative intensities as measured at IN11 and D7. The D7 data proved to be, however, of higher reliability and were thus exploited for the calculation of the hypothetical m values according to eq 3. We derived values of m = −0.46 ± 0.01 in 10Y:BZO and m = −0.79 ± 0.02 in 10Sc:BZO. Figure 2b also confirms that at these Q-values the incoherent signal is largely predominant. Using the values of the proton concentration as obtained from the D7 data in combination with literature data of the neutron scattering cross sections of the constituent atoms,11 we find that the contribution of hydrogen to the incoherent scattering signal is approximately 96% for 10Sc:BZO and 99% for 10Y:BZO. 4.2. Proton Dynamics. Figure 3 shows the T-dependence of the intermediate scattering function I(Q, t), which has been averaged over the Q-range 0.7−1.2 Å−1. At the lowest temperatures, the I(Q, t) shows almost no decrease for 10Y:BZO (249 K) and a weak decay for 10Sc:BZO (299 K). Upon heating, the I(Q, t) follows a quite monotonic, featureless, decrease, with a slope becoming gradually more pronounced. At the highest temperature (546 K), the I(Q, t) has decayed to similar values, of about 0.55 for 10Y:BZO and 0.6 for 10Sc:BZO in the long-time limit. Hence, it remains at very high levels in comparison to the hypothetical m values of −0.46 ± 0.01 for 10Y:BZO and −0.79 ± 0.02 for 10Sc:BZO, as expected for a complete decay of correlations related to the protons in the two samples. The quite monotonic decrease of I(Q, t) suggests that the recorded dynamics do not follow a basic microscopic process characterized by a unique activation energy. Such a process would be observable as a single exponential function with a unique relaxation time, τ. Formally, the best approximation to the experimental data is offered by a stretched exponential function, exp[−(t/τ)β], which points to an ensemble of microscopic processes, each governed by a characteristic activation energy. However, the parameters β and τ, derived from the entirety of the experimental data, do not establish systematic Q- and T-dependences with sufficient reliability. This may be appreciated from the similarity between the single and stretched exponential fits to the 546 K data, which are included in Figure 3, despite the large difference in the values of the fitting parameters (see figure caption). We conclude that the time window probed in the NSE experiment is too narrow to discriminate between a single and stretched exponential function, or other relaxation functions, widely used

(2)

Here, Icoh(Q, t = 0) = Scoh(Q) and Iinc(Q, t = 0) = Sinc(Q) are the relative coherent and incoherent intensities, which can be determined by measuring the intensities of spin-flip and noni i spin-flip scattering, respectively, and Sinc (Q) = niσ inc / j ∑j njσincSinc(Q), i = Ba, Zr, O, H, and Y/Sc. In the present case, we note that oxygen diffusion in 10Y:BZO and 10Sc:BZO is a process characterized by a relaxation time of the order of 10−6 s at the highest temperatures applied here.13 Consequently, oxygen diffusion is not expected to be detected in the NSE time window accessible at IN11. This justifies the assumption that Icoh(Q, t) = Scoh(Q) and Iiinc(Q, t) = Sinc(Q) for all other elements except for hydrogen. Considering also the very large difference between the coherent and incoherent scattering cross sections of protons, it is clear that predominantly self-dynamics of the protons is measured with NSE. Therefore, under the assumption that only protons and all protons contribute to the diffusion process(es), the complete decay of the intermediate scattering function to a final value m can be approximated as 1

i Scoh(Q ) − 3 ∑i ≠ H Sinc (Q ) I(Q , t = tmax ) = =m 1 I(Q , t = 0) Scoh(Q ) − 3 Sinc(Q )

(3)

The estimation of m is essential for the comparison of results from compounds with different compositions and recorded with different techniques such as NSE (applied here) and TOF (applied previously4) spectroscopy. This will be shown hereafter.

4. RESULTS AND DISCUSSION 4.1. Proton Concentration. The TGA heating curves are shown in Figure 1. Both samples gradually lose mass upon heating, starting almost immediately above room temperature. After an initial loss, which is particularly strong for 10Y:BZO, the signal levels off for both samples between 700 and 900 K. Upon further heating, the loss rate again increases in 10Y:BZO, whereas it flattens out for 10Sc:BZO. Considering the mass loss

Figure 1. TGA curves for 10Y:BZO (red) and 10Sc:BZO (blue), respectively, as measured with a heating rate of 10 K/min. 13965


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Figure 2. (a) Coherent (green), incoherent (red), and magnetic (blue) scattering signals of 10Y:BZO (left) and 10Sc:BZO (right) measured on D7. (b) Polarization, coherent and incoherent relative weights for 10Y:BZO and 10Sc:BZO measured on IN11 at a temperature of 200 K, and extracted from the D7 data at 300 K.

Figure 3. Temperature dependence of the normalized intermediate scattering function, I(Q, t)/S(Q), averaged between 0.7 and 1.2 Å−1 for (a) 10Y:BZO and (b) 10Sc:BZO. Solid lines are free fits to the 546 K data using single exponential functions, I(Q, t)/S(Q) = 0.62 + 0.23 exp(−t/0.07) for 10Y:BZO and I(Q, t)/S(Q) = 0.61 + 0.27 exp(−t/0.09) for 10Sc:BZO. Dashed lines are free fits to the 546 K data using stretched exponential functions, I(Q, t)/S(Q) = 0.4 + 1 exp[−(t/0.03)0.2] for 10Y:BZO and I(Q, t)/S(Q) = 0.58 + 0.34 exp[−(t/0.09)0.6] for 10Sc:BZO.

Figure 4. Q-dependence of the normalized intermediate scattering function, I(Q, t)/S(Q), as measured at 546 K, for (a) 10Y:BZO and (b) 10Sc:BZO.

in the literature. The slight downshift of the whole I(Q, t) curves upon increasing Q (Figure 4) is most likely due to vibrational dynamics of the systems that become more pronounced at larger Q-values (cf. the Debye−Waller factor, exp(−Q2⟨u2⟩), where ⟨u2⟩ is the mean-square displacement of the vibrations).14

The stretched profile of the I(Q, t)s and thus the anticipated distribution of microscopic relaxation steps is in agreement with scenarios forged from computer models. First-principles calculations identify different activation energies for proton transfers and rotational processes depending on the distance from the protons to the dopant atoms and on structural 13966


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The Journal of Physical Chemistry C variations in the vicinity of the dopants.15 Surface and interface effects at grain boundaries have been identified as another important factor imposing a distribution of activation energies for the microscopic proton transport mechanism.16,17 The prospect of a distribution of different proton sites can be examined on purely experimental grounds by, e.g., IR spectroscopy. In Figure 5, we show the IR absorbance spectra

Figure 6. I(Q, t) of 10Sc:BZO and 10Y:BZO computed from the present data and approximated from data measured with the TOF spectrometer IN6 on a shorter time scale. The dashed line indicates the extension of the TOF calculated I(Q, t) beyond the resolution limit of IN6. Figure 5. IR absorbance spectra of the O−H stretch region for 10Y:BZO and 10Sc:BZO. The spectra have been vertically shifted to merge at 4000 cm−1 for easier comparison. The tiny peaks at ∼1750 and ∼2450 cm−1 are related to a small amount of an unidentified impurity. The band at around 4200 cm−1 relates to a combination of O−H stretch and O−H wag modes.18

which under the explained assumptions can take values between 1 and m in the time window 0−1.3 ns, have been rescaled in the following way I resc(Q , t )IN11 =

I(Q , t )NSE − m 1 + | m|

(4)

in order to take on values between 1 and 0. The intermediate scattering functions associated with the TOF experiment, Icalc(Q, t)IN6, have been calculated by exploiting the temperature dependence of the harmonic and total mean-square displacement, ⟨uharm2(T)⟩ and ⟨utot2(T)⟩, established in the TOF study.4 Thereby, the short-time limit, IIN6(Q, t < 0.1 ps)IN6, was assumed to be determined by harmonic vibrations and was approximated from the harmonic contribution to ⟨uharm2(T)⟩. The long-time limit, Icalc(Q, t > 0.1 ns)IN6, was calculated from the total ⟨utot2(T)⟩. Both values were reduced by the zero point value ⟨u2(T → 0)⟩, for equivalence with Iresc(Q, t)IN11. Overall, the TOF-recorded I(Q, t) complements the dynamics observed in the NSE experiment toward shorter time scales. Both responses overlap in the time range 5−10 ps, albeit an offset in the 10Sc:BZO data is obvious. The resolution of the TOF spectrometer limits the validity of the response and reduces the significance of its line shape to a curve adapting the stretched exponential characteristics followed by the IN11 response. It is thus in line with the conclusion upon the presence of a distribution of different proton sites and activation energies drawn from the NSE and IR spectroscopy experiments. The comparison between TOF and NSE data is made under the assumption of equal nominal compositions of the systems investigated in the two experiments. Small differences in the materials’ chemical composition or in the hydration level may easily occur, which may relate to the relative qualitative behavior of the different samples, such as the offset between the TOF and NSE data. As discussed above, the present data do not allow a reliable quantification of the width of the distribution. Note that because of the very stretched decay of I(Q, t), the footprint of proton dynamics should, at time-scales longer than those accessed on IN11, enter into the time domain of oxygen

of the two materials in the energy range of the O−H stretch band. The O−H stretch band extends over a remarkably wide range of energies from about 2500 to 3500 cm−1. This feature is similarly observed in other proton conducting oxide systems as elaborated in refs 18−21. It is related to a distribution of different proton sites, being, thus, in full accordance with the scenario outlined above. The notable extent of the distribution of O−H stretch frequencies, related to different proton sites, can be illustrated by comparing the here measured O−H stretch bands with the IR spectrum of a hydrated sample of the perovskite KTaO3. Hydrated KTaO3 contains virtually only one, well-defined, proton site for which the O−H stretch mode is manifested as a sharp peak at 3487 cm−1 contrasting the spectra shown in Figure 5.22 The presence of a much more well-defined proton site in hydrated KTaO3 than for the 10Sc:BZO and 10Y:BZO materials is related to its more ordered local structure. In 10Sc:BZO and 10Y:BZO, the acceptor doping allows for the possibility of different local proton arrangements in the vicinity of the dopant atoms, e.g., Zr−OH−Zr, Zr−OH−In, and In−OH−In, and even more in the likely presence of some (unfilled) oxygen vacancies, thus leading to several different proton sites present. Further information about the proton dynamics present can be obtained from the offset of the I(Q, t) functions at the shortest Fourier times investigated here (∼5 ps). This offset evidences the presence of proton dynamics at a shorter time scale than observable within the NSE time window accessible on IN11. A previous TOF spectroscopy study carried out by Karlsson et al.4 on the same perovskite compounds examined this shorter time scale. We compare the results from both experiments, NSE and TOF, as intermediate scattering functions in Figure 6 for T = 546 K and for the Q-range of the present NSE experiment. To do so, the NSE I(Q, t) data, 13967


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Structural and Mechanistic Features. Chem. Soc. Rev. 2010, 39, 4370− 4387. (3) Karlsson, M. Proton Dynamics in Oxides: Insight into the Mechanics of Proton Conduction from Quasielastic Neutron Scattering. Phys. Chem. Chem. Phys. 2015, 17, 26−38. (4) Karlsson, M.; Matic, A.; Engberg, D.; Björketun, M. E.; Koza, M. M.; Ahmed, A.; Wahnström, G.; Berastegui, P.; Börjesson, L.; Eriksson, S. G. Quasielastic Neutron Scattering of Hydrated BaZr0.90A0.10O2.95 (A = Y and Sc). Solid State Ionics 2009, 180, 22−28. (5) Karlsson, M.; Engberg, D.; Björketun, M. E.; Matic, A.; Wahnström, G.; Sundell, P. G.; Berastegui, P.; Ahmed, I.; Falus, P.; Farago, B.; et al. Using Neutron Spin-Echo To Investigate Proton Dynamics in Proton-Conducting Perovskites. Chem. Mater. 2010, 22, 740−742. (6) Kreuer, K. D.; Adams, S.; Münch, W.; Fuchs, A.; Klock, U.; Maier, J. Proton Conducting Alkaline Earth Zirconates and Titanates for High Drain Electrochemical Applications. Solid State Ionics 2001, 145, 295−306. (7) Schober, T.; Bohn, H. G. Water Vapor Solubility and Electrochemical Characterization of the High Temperature Proton Conductor BaZr0.9Y0.1O2.95. Solid State Ionics 2000, 127, 351−360. (8) Karlsson, M.; Ahmed, I.; Matic, A.; Eriksson, S. Short-range Structure of Proton-conducting BaM0.10Zr0.90O2.95 (M = Y, In, Sc and Ga) Investigated with Vibrational Spectroscopy. Solid State Ionics 2010, 181, 126−129. (9) Stewart, J. R.; Deen, P. P.; Andersen, K. H.; Schober, H.; Barhélémy, J.-F.; Hillier, J. M.; Murani, A. P.; Hayes, T.; Lindenau, B. Disordered Materials Studied Using Neutron Polarization Analysis on the Multi-detector Spectrometer, D7. J. Appl. Crystallogr. 2009, 42, 69−84. (10) Moze, O.; Hicks, T. J.; McLaren, A. C. Determination of the Hydrogen Content of Synthetic Quartz by Neutron Polarization Analysis. Phys. Chem. Miner. 1980, 5, 309−314. (11) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26−37. (12) Farago, B. The Basics of Neutron Spin Echo. In Neutron Data Booklet; Dianoux, A.-J., Lander, G., Eds.; OCP Science: Philadelphia, PA, 2003; pp 2.8-1−2.8-16. (13) Kreuer, K.-D.; Adams, S.; Münch, W.; Fuchs, A.; Klock, U.; Maier, J. Proton Conducting Alkaline Earth Zirconates and Titanates for High Drain electrochemical applications. Solid State Ionics 2001, 145, 295−306. (14) Squires, G. L. Introduction to the Theory of Thermal Neutron Scattering; Cambridge University Press: Cambridge, U.K., 1978. (15) Björketun, M. E.; Sundell, P. G.; Wahnström, G. Structure and Thermodynamic Stability of Hydrogen Interstitials in BaZrO 3 Perovskite Oxide from Density Functional Calculations. Faraday Discuss. 2007, 134, 247−265. (16) Kim, D.-H.; Kim, B.-K.; Kim, Y.-C. Effect of Dopant Nature on Structures and Lattice Dynamics of Proton-conducting BaZrO3. Solid State Ionics 2012, 213, 18−21. (17) Van Duin, A. C. T.; Merinov, B. V.; Han, S. S.; Dorso, C. O.; Goddard, W. A., III ReaxFF Reactive Force Field for the Y-doped BaZrO3 Proton Conductor with Applications to Diffusion Rates for Multigranular Systems. J. Phys. Chem. A 2008, 112, 11414−11422. (18) Karlsson, M.; Björketun, M. E.; Sundell, P. G.; Matic, A.; Wahnström, G.; Engberg, D.; Börjesson, L.; Ahmed, I.; Eriksson, S. G.; Berastegui, P. Vibrational Properties of Protons in Hydrated BaInxZr1−xO3−x/2. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 094303:1−7. (19) Ahmed, I.; Knee, C. S.; Karlsson, M.; Eriksson, S. G.; Henry, P. F.; Matic, A.; Engberg, D.; Börjesson, L. Location of Deuteron Sites in the Proton Conducting Perovskite BaZr0.50In0.50O3−y. J. Alloys Compd. 2008, 450, 103−110. (20) Bielecki, J.; Parker, S. F.; Ekanayake, D.; Rahman, S. M. H.; Börjesson, L.; Karlsson, M. Short-Range Structure of the Brownmillerite-type Oxide Ba2In2O5 and its Hydrated Proton-Conducting Form BaInO3H. J. Mater. Chem. A 2014, 2, 16915−16924.

diffusion. However, we cannot exclude that a prominent portion of the hydrogens remain immobile on longer time scales, forming a plateau in I(Q, t). This remains to be verified by future experiments. Another point to be tested experimentally is the effect of hydroxide secondary phases, which might be present in the compounds as conjectured by Jalarvo et al.23 from studies of similar proton conducting materials.

5. CONCLUSIONS To conclude, we have investigated the proton dynamics in the two well-known proton conducting perovskites BaZr0.9M0.1O2.95 (M = Y and Sc), using NSE spectroscopy together with polarized neutron diffraction, TGA, and IR spectroscopy. The results elaborate the presence of pronounced localized proton dynamics at temperatures higher than 300 K. The observed neutron scattering data cannot be characterized by a unique relaxation time featuring a microscopic relaxation process with a well-defined activation energy. They rather suggest the contribution of varying relaxation processes governed by a wide distribution of activation energies. The IR spectra give evidence, by the extent of the O−H stretch band, of a notable distribution of different proton sites in the materials. Consequently, the IR spectra support the conjecture of a distribution of activation energies driving the microscopic relaxation processes of protons due to a heterogeneous distribution of proton sites. The reported experimental results are also in line with data reported in the literature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03683. A full list of author names in ref 5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +46 31 772 2090. Phone: +46 31 772 8038. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported primarily by the Swedish Research Council (Grant No. 2010-3519 and 2011-4887), the ILL (Grant No. ILL1279.1), and the National Science Foundation (Grant No. DMR 1105301 and 08-43934). We also acknowledge the use of shared experimental facilities of the Materials Research Laboratory (MRL): an NSF MRSEC, supported by NSF Grant No. DMR 1121053. ILL is thanked for access to neutron beam facilities (experiments 7-03-124 and 7-03-130). E. Thaveron, D. Renzy, and W. Clancy are thanked for technical assistance with the instruments. R. Ammer is gratefully acknowledged for the design and manufacturing of the sample cells. M.C.K. was supported by the Schlumberger Foundation Faculty for the Future Fellowship. M.K. thanks Prof. R. Seshadri at the MRL for support as a visiting scientist.

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REFERENCES

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Proton Dynamics in Hydrated BaZr0.9M0.1O2.95 (M = Y and Sc

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