Localized Proton Motions in Acceptor-Doped Barium Zirconates - The

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Localized Proton Motions in Acceptor-Doped Barium Zirconates Daria Noferini, Michael Marek Koza, and Maths Karlsson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00177 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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The Journal of Physical Chemistry

Localized Proton Motions in Acceptor-Doped Barium Zirconates Daria Noferini,†,‡ Michael Marek Koza,‡ and Maths Karlsson∗,† † Department ‡ Institut

of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France

ABSTRACT: Acceptor-doped barium zirconates are currently accumulating considerable interest because of their high proton conductivity, especially in the intermediate-temperature range targeted for next-generation solid oxide fuel cells, combined with their excellent chemical stability. However, fundamental questions surrounding the proton conduction mechanism in these materials remain, for instance regarding the nature of localized proton motions and how they depend on the local structural properties of the material. Here, we investigate the nature of localized proton motions in the two acceptor-doped proton conducting perovskites BaZr0.9 M0.1 O2.95 with M= Y and Sc, using quasielastic neutron scattering. We show the presence of pronounced localized proton dynamics, with mean residence periods on the time-scale of 1–30 ps and an activation energy of about 100 meV, for both materials. In view of first-principles calculations as reported elsewhere the experimentally established dynamics could comprise footprints from proton transfers as well as O-H rotational motions in several different types of proton sites, due to a range of various local proton sites present in both materials.

1

INTRODUCTION

Acceptor-doped barium zirconates have emerged as promising electrolytes for future intermediate-temperature, environmentally friendly, solid oxide fuel cells because of their high proton conductivity combined with high chemical stability. 1 The role of the acceptor-doping, such as Y substituted for Zr in BaZrO3 , is to create an oxygen deficient material (BaZr1−x Yx O3−x/2 ), which can be loaded with protons to give a hydrated, proton conducting, analogue, BaZr1−x Yx O3 Hx . The protons are bound to oxygens as hydroxyl groups (-OH) and migrate throughout the lattice via a sequence of (i) proton transfers between neighbouring oxygens and (ii) rotational motions of the O-H species between such transfers. Although the proton transfer has been mostly identified as the slower motion, 2–5 some computational studies have shown that the energy barrier for O-H rotation can be higher than that for proton transfer. 6,7 Furthermore, the effects of grain boundaries, strain, and dopant atoms on the proton dynamics are not fully understood. By changing the type and concentration of dopant atoms, proton conducting barium zirconates generally show considerably different proton conductivity. 8 A primary example is the case for 10% Y- and Sc-doped BaZrO3 (10Y:BZO and 10Sc:BZO), which, despite that the two materials may be considered as structurally nearly equivalent (both exhibit an average-cubic structure based on diffraction data), the proton mobility in 10Y:BZO (1.9·10−7 cm2 s−1 ) is at 500 K almost two orders of magnitude higher than in 10Sc:BZO (1.1·10−9 cm2 s−1 ). 8 Several properties of the dopant atoms have been proposed to play a significant role in causing these different proton conductivities: e.g., the ionic radius, 8–10 electronic structure, 9,11,12 and the absolute hard-

ness. 13 In particular, results from quasielastic neutron scattering (QENS), 5,14 nuclear magnetic resonance combined with conductivity measurements, 15 muon spectroscopy 16 and computer simulations 6,17–19 on selected compositions have shown that the introduction of dopants, with an effective negative charge relative to the species they replace, in combination with local strain effects, can trap protons and impede their long-range transport. However, a clear relationship between the macroscopic proton conductivity and atomic-scale structure and proton dynamics has not yet been established. 20 In this work we investigate the nature of localized proton motions in 10Y:BZO and 10Sc:BZO, using QENS. Previous QENS studies on these materials have revealed proton dynamics interpreted as both localized motions, on the time-scale of picoseconds, 21,22 and longer-range translational diffusion, on the time-scale of nanoseconds. 23 However, it has been difficult to determine the spatial geometry of the observed dynamics, i.e. whether it relates to proton transfers or O-H rotational motions, notably because previous studies were performed over a limited momentum transfer (Q)-range (≈ 0.2 − 2.0 Å−1 ). 21,22 Here we show, through measurements optimized to investigate an extended Q-range (up to ≈ 4.0 Å−1 ), that such a discrimination was not solely limited by the Q-range. The new results reveal proton dynamics with mean residence periods on the time-scale of 1–30 ps and an activation energy of about 100 meV, indeed compatible with both proton transfers and O-H rotational motions. Furthermore, we show that the experimentally established dynamics could comprise contributions from proton transfers as well as O-H rotational motions in several different types of proton sites in the materials. This new insight motivates efforts to determine the nature of local proton 1–8 | 1

ACS Paragon Plus Environment


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EXPERIMENTAL DETAILS

The QENS experiment was performed at the Institut LaueLangevin (ILL) on the time-of-flight spectrometer IN5. The spectrometer operated with 2.5 Å wavelength neutrons, yielding a Gaussian-shaped resolution function with a full width at half maximum of 620 µeV and an accessible Q-range of 0.02–4.6 Å−1 , which, for the purpose of distinguishing between proton transfer and O-H rotation, was deemed to be sufficient judging from previous studies. 21 The obtained quantity from the QENS measurements is proportional to the dynamic structure factor, S(Q, h¯ ω), which is featured by a (quasielastic) broadening of the elastic peak in the case of relaxational dynamics accessible to the instrument, as determined by its resolution. After masking out Q-ranges contaminated by small-Q scattering from the sample environment and Bragg peaks of the sample, QENS spectra were obtained at 1.70, 1.90, 2.30, 3.50 and 3.90 Å−1 , and for the temperatures 10, 200, 300, 350, 450 and 550 K. Further details of the experiment can be found in the Supporting Information (SI). A description of the sample preparation is given in ref. 22. While, generally, the intensity of scattered neutrons on the samples is comprised of both coherent and incoherent scattering, our previous analysis based on polarized neutron diffraction suggests that the incoherent scattering dominates offthe-Bragg-peaks; e.g., at Q = 1.70 and 1.90 Å−1 the incoherent scattering accounts for more than 80% of the total scattering. 22 The contribution of hydrogen to this incoherent scattering is approximately 96% for 10Sc:BZO and 99% for 10Y:BZO. 22 It follows that essentially incoherent dynamics is studied and hence that information about proton self-dynamics can be obtained from the experiment. The measured scattering intensity, Smeas (Q, h¯ ω), can be expressed as Smeas (Q, h¯ ω) = αS (Q, h¯ ω) exp −U iso Q2 × [1 + n (¯hω)] h¯ ωβ ⊗ R (Q, h¯ ω) .

(1)

Here α is a normalization factor, β is the reciprocal of the thermodynamic temperature, n(¯hω) is the Bose factor, R (Q, h¯ ω) is the resolution function, and S(Q, h¯ ω) is the dynamic structure factor that contains information about the elastic and quasielastic contributions and hence the physics in the materials. The exponential term exp −U iso Q2 is the Debye-Waller factor (DWF), and U iso the isotropic thermal displacement parameter of the atoms in the material. The DWF was estimated

1.00 0.95 0.90 0.85

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x10-2 10Sc:BZO 10Y:BZO

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Ue iso [Å2]

sites present, in combination with further QENS studies, as a route to elucidate the mechanism of proton conduction as well as which local structure that accommodates the highest proton conductivity. This is important for the further development of solid oxide fuel cells based on proton conducting oxides.

exp(-mTQ 2) [arb. units]

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0.70 0.65

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Q [Å-1]

Figure 1 DWF for 10Sc:BZO (dashed lines) and 10Y:BZO (solid lines) at 200 K (violet), 300 K (blue), 350 K (green), 450 K (red), 550 K (yellow). Inset: temperature dependence of the experimental isotropic total displacement parameter.

from the data in a separate procedure, thus reducing the number of parameters in the numerical fit routine. The experimental isotropic thermal displacement parameters, Ueiso , were derived from the data within the harmonic approximation, R Smeas (Q, h¯ ω)T iso 2 . exp −Ue Q = R Smeas (Q, h¯ ω)10K

(2)

The data were integrated in the energy range -2 6 h¯ ω 6 2 meV. Attempts with wider integration ranges did not change the DWF properties measurably. The temperature dependence of the so-obtained displacement parameters was approximated at higher temperatures as Ueiso = mT , so that the DWF can be expressed as exp[−(mT + U0iso )Q2 ], where U0iso is the isotropic thermal displacement parameter for the zeropoint vibrations. S(Q, h¯ ω) was in turn approximated with a function composed of one elastic component, aD δ (¯hω), together with a Lorentzian function, aL (Q)L(¯hω), that describes the quasielastic scattering, and a h¯ ω-independent background, bkg(Q), i.e. S (Q, h¯ ω) = aD (Q)δ (¯hω) + aL (Q)L(Q, h¯ ω) + bkg(Q). (3) The quasielastic width, Γ(Q), did not show any significant Qdependence and was therefore fitted as a global parameter for each temperature. Since the Stokes part of the spectra was affected by elastic multiple scattering artefacts from the environment, the fit range was limited to -5 6 h¯ ω 6 0.8 meV.

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T = 550 K T = 450 K T = 350 K T = 300 K

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Data background Elastic Quasielastic Fit

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Figure 2 Smeas (Q, h¯ ω) of (a) 10Sc:BZO and (b) 10Y:BZO (b), and fits according to Eq. (1) and (3). Data are presented at Q= 2.3 Å−1 in a T -range of 300–550 K.

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RESULTS AND DISCUSSION

Figure 1 shows the Q- and T -dependence of exp −mT Q2 . The inset shows the temperature dependence of Ueiso . Notice that the best conducting material, 10Y:BZO, presents larger displacements, and consequently a larger DWF in respect to 10Sc:BZO. Figure 2 displays Smeas (Q, h¯ ω) together with their fits, for both materials. Data are shown for Q = 2.3 Å−1 and T = 300– 550 K, for which a clear quasielastic signal can be observed. At the two lowest temperatures, 10 and 200 K, no quasielastic signal was observed at any of the investigated Q-values and these spectra are therefore not shown. A similar behavior, i.e. the “onset” of dynamics “accessible” to the instrument at a temperature of around 300 K, was reported for the same materials in an earlier QENS experiment that was featured by a higher resolution (≈100 μeV) as compared to the present one. 21 We notice that 300 K corresponds to a thermal energy of approximately 26 meV, which coincides roughly to the lowfrequency optical lattice vibrations in BaZrO3 , which are in the range ≈15–80 meV. 24 The width, Γ, of the quasielastic signal is overall similar for the two materials and broadens with increasing temperature. The background is associated with quasielastic scattering too fast to be adequately analyzed and/or with inelastic scattering from phonons that overlap with the quasielastic region; the latter is in agreement with an observed Q2 -dependence of the background. In Figure 3 we compare the Q-dependences of the amplitudes of the elastic and quasielastic components, aD (Q) and aL (Q), normalised for their sum, with those expected within the model for a jump diffusion over two or four equivalent

sites located on a circle with radius r. These models are used to describe the proton transfer between neighboring oxygens and the O-H rotational diffusion, respectively. 25 Note that for the two-site model the comparison with the quasielastic signal derived from the experimental data is straightforward, since the two-site model contains only one quasielastic component, with width Γ1 . However, for the four-site model the assumption is that aL (Q) L(¯hω) approximates the sum of the two Lorentzians expected from the model, with widths Γ1 and Γ2 = 2Γ1 . Mathematical expressions for the two models as well as a derivation of relevant quantities, such as the mean residence time, are given in SI. As can be seen from Figure 3, both models equally well describe the Q-dependence of the amplitudes. The radius of the circle, r, as obtained from the fits in Figure 3, represents, in the case of jump diffusion over two sites (proton transfers), half of the jump length and, in the case of jump diffusion over four sites (O-H rotation), the OH distance. The obtained jump lengths, 1.4–1.6 Å for proton transfers, and 0.85–1.0 Å for O-H rotational motions (Table S1 in SI), are in good agreement with both crystallographic data and calculations of O-O distances (≈2.6–2.9 Å) and calculated O-H bond lengths (0.97-1.00 Å), 4,11,12,26,27 as well as with jump distances obtained in other QENS studies of the same 21 and similar materials such as SrCe0.95 Yb0.05 H0.02 O2.985 and Ba[Ca(1+x)/3 Nb(2−x)/3 ]O3−x/2 . 5,28 Previous work, 21 carried out at higher resolution (≈100 μeV) but over a smaller Qrange (≈ 0.2 − 2.0 Å−1 ), suggested the possibility to distinguish between transfer and O-H rotational diffusion in the Qrange probed by the present study. It is shown here, however, that a set of physically reasonable parameters allows a good description of the experimental data according to both models. 1–8 | 3



1.0

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300 K 350 K 450 K 550 K

Figure 4 Arrhenius plot of the widths of the quasi-elastic component Γ1 (see text and SI for their derivation). Lines are linear fits that correspond to a mean activation energy of 93±51 meV (10Y:BZO) and 96±24 meV (10Sc:BZO) for proton transfer, and 84±45 meV (10Y:BZO) and 103±22 meV (10Sc:BZO) for O-H rotation.

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Figure 3 Q-dependence of the relative weights of the elastic and quasielastic components for 10Y:BZO and 10Sc:BZO. Lines are fits according to a jump diffusion model over two sites (solid lines) and four sites (dashed lines), respectively.

The determination of the geometry of the observed dynamics is therefore not unambiguous even in this extended Q-range, at the resolution and dynamical range of the present experiment. The mean residence time between successive jumps, τ, takes values in the range of 4–18 ps (10Sc:BZO) and 4–28 ps (10Y:BZO) for proton transfers, and in the range 2–11 ps (10Sc:BZO) and 3–16 ps (10Y:BZO) for O-H rotational diffusion, at 300–550 K. The overall similar values, together with the fact that at some of the probed temperatures the longest residence times are found in the material for which the highest proton conductivities are reported (10Y:BZO), 8 suggest that the observed dynamics is not rate-limiting for proton conduction. The widths, Γ1 , as derived from the two-site and four-site model, respectively, follow an Arrhenius dependence (Figure 4) and have been used to derive activation energies of ca. 100 meV for both processes and materials. These values are comparable with values reported from calculations, 2,6,7,17,18,29 especially once taken into account quantum effects, 6,18 as well as with other QENS studies of proton conducting oxides 5,28 The values are, however, higher than the 10–30 meV that was extracted from QENS spectra of the same materials in a previous study. 21 Slight differences in the chemical composition of the samples and/or in the dynamical range covered by the respective spectrometers may be at the origin of this difference. Importantly, the activation energies are considerably lower than the activation energy for proton conductivity of 0.43 eV (10:BZO) and 0.50 eV (10Sc:BZO), which shows in accordance with our results that the observed dynamics is not

rate-limiting for long-range proton transport. We note that a proton experiences a different local structural surrounding, and hence potential energy surface (PES), depending on to which oxygen it is bound in the perovskite structure. The inset in Figure 5 shows the relevant migration paths for the proton around and between oxygens in the first (O1) and second (O2) oxygen coordination shell of a dopant atom M, where R1 and R2 denote the rotational processes at O1 and O2, respectively, and T11, T12 and T22 represent proton transfers between adjacent O1 atoms, between adjacent O1 and O2 atoms, and between adjacent O2 atoms, respectively. Further away from the dopant atom the PES is less distorted and there is essentially only one type of rotational (Rdf) and one type of transfer (Tdf) process, since the structure is cubic. Björketun et al. 17 modelled these local motions by density functional theory (DFT) calculations on a system corresponding to a dopant concentration of 3.7%. Using as input in transition state theory, τ −1 = ν0 exp (-Ea / kB T ), the classical migration barriers (Ea ) and O-H stretch and bend mode frequencies (used as pre-factor ν0 ), as reported elsewhere, 17 we have calculated for each of these processes a mean residence time τ. The τ-values were used to derive the widths of the Lorentzian functions associated with each of these processes modelled according to a jump diffusion over two sites (transfers) or four sites (rotations), respectively; see SI for details. Figure 5 compares the widths, Γ, of the quasielastic signal derived from experimental data (bullets) with the results concluded from these computer simulations (lines). 17 A key result is the fact that several components are present in the dynamical range probed here. At each sampled temperature, faster processes would give non-specific lineshapes undistinguishable from the background, whereas slower motions would be hidden within the instrumental resolution. The remaining components, whose widths are in some cases similar to each other, contribute to the signal according to their Q-

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Γ Rdf

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upon the heating, the suppression of the relative weights of the quasielastic signal indicates that with increasing temperature the dynamic response of the fastest relaxation processes becomes too wide to be discerned from the flat background signal within the analyzable QENS window. This hypothesis is supported by the increase of the background and the decrease of the total integrated signal with increasing temperature. The situation of several different localized proton motions contributing to the overall QENS signal is indeed in agreement with the results obtained from recent neutron spinecho measurements, which explored the dynamics in the timerange of 5 ps to 1.3 ns in the very same samples. 22 The faster dynamics probed by the present experiment thus complements this picture with compatible results.

Γ R1 2

Γ 2 R2

200

300

400

500

600

T [K]

Figure 5 Comparison of the widths, Γ, of the quasielastic signal derived from experimental data (bullets) with results concluded from computer simulation studies of acceptor-doped BaZrO3 17 (lines). The calculated widths, Γ1 Tdf, Γ1 T11,..., Γ2 R2, where Γ1 = Γ2 /2, are shown for the migration steps shown in the inset.

dependent amplitudes and their probabilities. Processes of the same type have similar geometrical constraints, and therefore similar amplitudes. This implies that the observed quasielastic broadening is likely to have contributions from several different proton transfer and O-H rotational motion processes, due to the variety of different types of local proton sites present in both materials. In this scenario, the quasielastic widths and related relaxation times are mean values, with a temperaturespecific average over those components that become evident in the analyzable QENS window at the respective T . Thus, the extracted energy values do not reflect the activation energies for a specific proton motion, but are rather “apparent” values reflecting an average over the probed components. Support to the hypothesis of a variety of contributions also comes from the decrease in the relative weight of the quasielastic part with increasing temperature (Figure 3). Since the sample composition (proton concentration) did not change significantly

In conclusions, we have investigated the nature of proton dynamics in the two acceptor-doped proton conducting perovskites BaZr0.9 M0.1 O2.95 with M= Y and Sc, using QENS. We show the presence of pronounced localized proton dynamics, with mean residence periods on the time-scale of 1–30 ps and an activation energy of about 100 meV, for both materials. In view of recent first-principles calculations by Björketun et al. 17 the experimentally established dynamics could comprise footprints from proton transfers as well as O-H rotational motions in several different types of proton sites, due to a range of different types of local protons sites present in both materials. This insight motivates efforts to determine the nature of local proton sites present, in combination with further QENS studies, as a route to elucidate the mechanism of proton conduction as well as which local structure that accommodates the highest level of proton conductivity.

ASSOCIATED CONTENT Supporting Information Details of the QENS experiment, the jump-diffusion models, and the fitting equations and parameters, and a full list of author names in ref. 23 (PDF)

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. 1–8 | 5



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ACKNOWLEDGMENTS We are grateful for financial support from the Swedish Research Council (grant No. 2010-3519 and 2011-4887) and the ILL (grant No. ILL1279.1). The ILL is thanked for access to neutron beam facilities (experiment 7-03-126). The group of R. Seshadri at the UCSB are thanked for assistance in the preparation of the samples. J. Halbwachs is thanked for technical assistance at the IN5 spectrometer, and R. Ammer for the design and manufacturing of the sample cells.

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Localized Proton Motions in Acceptor-Doped Barium Zirconates - The

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