Atomic-Scale Picture of the Ion Conduction Mechanism in a

Article pubs.acs.org/cm

Atomic-Scale Picture of the Ion Conduction Mechanism in a Tetrahedral Network of Lanthanum Barium Gallate Niina Jalarvo,*,† Olivier Gourdon,*,‡,§ Zhonghe Bi,∥ Delphine Gout,‡,§ Michael Ohl,† and M. Parans Paranthaman∥ †

Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science (JCNS), Outstation at Spallation Neutron Source (SNS), Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6473, United States ‡ Chemical and Engineering Materials Division, Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37861-6475, United States § International Centre for Diffraction Data, Newtown Square, Pennsylvania 19073, United States ∥ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6100, United States S Supporting Information *

ABSTRACT: Combined experimental study of impedance spectroscopy, neutron powder diffraction, and quasielastic neutron scattering was performed to shed light onto the atomic-scale ion migration processes of protons and oxide ions in La0.8Ba1.2GaO3.9. This material consists of tetrahedral GaO4units, which are rather flexible, and rocking motion of these units promotes the ionic migration process. The oxide ion (vacancy) conduction takes place on channels along the c axis, involving a single elementary step, which occurs between adjacent tetrahedra (intertetrahedra jump). The proton conduction mechanism consists of intratetrahedron and intertetrahedra elementary processes. The intratetrahedron proton transport along the c axis is the rate-limiting process, with activation energy of 0.44 eV. The intertetrahedra proton transport has the activation energy of 0.068 eV. KEYWORDS: ion conduction mechanism, proton conductor, oxide ion conductor, impedance spectroscopy, neutron powder diffraction (NPD), quasielastic neutron scattering (QENS)

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INTRODUCTION One of the current objectives of fuel cell research is to develop a stable and efficient ion-conducting electrolyte for intermediate temperature applications (400−700 °C).1,2 Solid oxides are in the spotlight, with many advantages compared to the other candidates. In these materials oxide ion conductivity usually occurs via oxygen vacancies, which are introduced for charge compensation by acceptor doping. Protonic defects, responsible for proton conduction, can be incorporated from water into the oxygen vacancies as given in the following equation: H 2O (g) + V ··o + Oo = 2OH·o

Here, we present a detailed study of the ionic conduction mechanism in La1−xBa1+xGaO4−x/2 (x = 0.2), where the oxygens are in tetrahedral coordination around the gallium, forming GaO4 units.13 Increasing the barium content at the expense of lanthanum content will introduce more oxygen vacancies in the system.11 Protons from a humid atmosphere are incorporated into the oxygen vacancies. The material exhibits substantial ionic conductivities, especially noteworthy proton conduction in an intermediate temperature range.11,12,14 The structure is orthorhombic with a space group P212121, with remarkably distorted GaO4 tetrahedra forming Ga2O7 units.15 More insight into the structure and defect concentration has been obtained using synchrotron X-ray diffraction and extended X-ray absorption fine structure analyses,16 and by combining density functional theory (DFT) calculations with neutron powder diffraction (NPD).15,17 Also, activation energies for proton migration between different sites were calculated using DFT.15 Below, we present a detailed experimental study based on impedance spectroscopy and neutron scattering (quasi-elastic

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Conventional categories of solid oxide ionic conductors include perovskite and fluorite structures.3−6 Lately, oxides with structures containing tetrahedral units and improved properties have been attracting attention, for example, rare-earth orthoniobates and ortho-tantalates, 7 apatites, 8 − 1 0 and La1−xBa1+xGaO4−x/2.11,12 These alternate structures exhibit defect association and migration that may vary greatly from the conventional types. A detailed understanding of the atomicscale ionic conduction process is needed for optimization and development of new materials. © 2013 American Chemical Society

Received: February 5, 2013 Revised: June 21, 2013 Published: June 26, 2013 2741

dx.doi.org/10.1021/cm400452n | Chem. Mater. 2013, 25, 2741−2748

Chemistry of Materials

Article

shows up as a high-frequency arc corresponding to a specific capacitance of ∼10−11 F cm−2. To investigate the crystal structures, and especially to get more detailed information about the localization of D and O atoms and oxygen vacancies, samples of dried and hydrated (D2O) phases were preloaded in 6 mm diameter vanadium cans sealed under Ar. The time-of-flight (TOF) neutron data were collected at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory, on the new high-resolution neutron powder diffractometer POWGEN.20 Data were collected at 300 K for both samples, dry and hydrated (using D2O for hydration instead of H2O to decrease incoherent background and for better contrast). For each sample two data collections using two different center wavelengths (CWLs) have been used. On the basis of the actual POWGEN detector configuration, CWLs of 1.066 and 3.731 Å were chosen. The first CWL, which covers d-spacing from 0.29 to 3.09 Å, was used to have accurate information on the nuclear structure as well as the atomic displacement parameters (ADPs) whereas the second CWL, which covers higher d-spacing from 1.65 to 8.24 Å, can give access to additional nuclear reflections if further ordering occurs. The crystallographic structures were refined using JANA2006 software21 based on the La1−xBa1+xGaO4−x/2 crystal structure, known from the literature,13,15,17 as a starting model. The refinements were performed using simultaneously the two banks (each of them being associated with a different CWL data set) to obtain unit cell parameters, atomic positions, and ADPs. The QENS experiments were performed on the BASIS timeof-flight backscattering spectrometer at Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.22 The fullwidth at half-maximum of the instrumental resolution of the elastic peak (fwhm) for Q-averaged spectra is 3.4 μeV and the dynamic range was selected from −100 to +100 μeV, capable of probing time scales from picoseconds to nanoseconds range. About 1 mm thick layer of the sample was sealed under argon into a cylindrical annular sample container made of aluminum. The outer diameter of the sample container was 29 mm and the total mass of the sample was ∼9 g. Measurements were performed at temperatures from 30 to 500 K. The data collected at 30 K was used as the instrument resolution function. The QENS spectra were recorded at Q values ranging from 0.2 to 2.0 Å−1; however, the spectra above 1.3 Å−1 contain strong coherent Bragg scattering from structural reflections and was therefore not used for the QENS data analysis. The TOF spectra were converted to energy-transfer presentation and averaged to selected Q values. The QENS data analysis was performed using Dave software package.23

neutron scattering [QENS] and NPD), providing the migration paths for oxide ions and protons, and also the associated activation energies for the proton jumps.

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EXPERIMENTAL SECTION La1−xBa1+xGaO4−x/2, for x = 0.2 and x = 0, were prepared from high-purity La2O3, BaCO3, and Ga2O3 powders. These powders were mixed according to the expected molar ratios and heated to 1400 °C in air for a day. Phase purity was examined by powder X-ray diffraction (PANalytical powder diffractometer). Diffraction patterns showed no evidence of impurities and full peaks indexation is possible using the orthorhombic cell parameters.13 To prepare the samples for neutron scattering experiments, La0.8Ba1.2GaO3.9 powders were placed in a sealed quartz tube, which was then connected to a gas supply. Dehydrated material was obtained by heat-treating at 1100 °C for 4 h with flowing dry O2, and subsequently, the samples were cooled to room temperature with a cooling rate of 5 °C/min. Hydrated materials for QENS and NPD were obtained with flowing O2 through H2O and D2O bubbling systems, respectively. It is worth pointing out that for QENS measurements a protonated sample is preferred to probe the incoherent scattering from hydrogen diffusion. For NPD a deuterated sample is desired to minimize the incoherent background from hydrogen, and for better coherent scattering contrast of deuterium. To hydrate the materials, the samples were first dried at 1100 °C in dry O2 for 1 h, and then slowly cooled down to 150 °C under wet O2 atmospheres with a cooling rate of 0.5 °C/min. To avoid condensation and formation of surface water, cooling below 150 °C was done in a dry atmosphere. La0.8Ba1.2GaO3.9 pellets for a.c. conductivity measurements were sintered at 1450 °C for 4 h in air, and then the surfaces were ground and polished to a thickness of 1.0 mm using SiC paper. Two symmetrical porous Pt electrodes of diameter 7.0 mm were attached directly to both sides of the pellets by painting with two layers of Pt paint (Heans 901) on each side, then dried, and fired at 900 °C for 2 h in air. Pt mesh attached to a Pt wire was placed on the electrode surface to complete the electrical connections. Water uptake of La1−xBa1+xGaO4−x/2 (x = 0 and x = 0.2) was determined using differential thermal analysis-thermogravimetric analysis (DTA-TGA) equipment (NETZSCH STA 409 PC). The samples were first heated up to 1100 °C and dried for 1.0 h in high-purity Ar (99.99%) gas with a flow rate of 50 mL/ min. Subsequently, the high-purity Ar was saturated with water vapor at 22 °C at the same flow rate. After 0.5 h at 1100 °C in wet Ar, the samples were slowly cooled down to room temperature with a cooling rate of 1.0 °C/min in humidified Ar. Similar background measurement was performed to subtract the buoyancy effects. The a.c. conductivity measurements were performed in an assembled cell placed in a quartz reactor which was supported in a tubular furnace. Impedance spectra were recorded at the temperature range from 300 to 1000 °C under dry (dried by silica gel dryer) and wet air (p(H2O) = 22 mbar), respectively. At each temperature point, impedance spectra were recorded at 1 h intervals until concurrent results were obtained. Impedance spectra were recorded in the frequency range 106 to 0.01 Hz with signal amplitude of 100 mV using VersaSTAT 4 (Princeton Applied Research) with an internal frequency response analyzer. ZSimpWin software was used to fit the acquired impedance data using different equivalent circuits.18,19 Below 450 °C, in Cole−Cole representations the bulk response

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RESULTS AND DISCUSSION Hydration. Thermogravimetry was used to determine the hydration characteristics of La1−xBa1+xGaO4−x/2 (x = 0 and x = 0.2). Figure 1 illustrates the results, showing water uptake for La0.8Ba1.2GaO3.9 upon cooling in a wet argon atmosphere. The hydration starts smoothly below 800 °C, and saturation is reached at around 400 °C. The hydration ratio can be determined, n = 0.08, and the hydrated form of the sample can then be written as La0.8Ba1.2GaO3.9·0.08H2O. LaBaGaO4 does not exhibit any water uptake, as expected. Ionic Conductivity. The total ac conductivity from the impedance spectra of La0.8Ba1.2GaO3.9 versus inverse temperature in dry and humidified air (pH2O = 22 mbar) is shown in Figure 2. The bulk conductivity in wet air was extracted from 2742



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ones obtained by X-ray diffraction. The La/Ba ratio has also been refined to a value of 0.82(5)/1.18(5), in good agreement with the loaded composition. At this stage of the refinement, an unrealistic large value of the O3 atomic displacement parameters (ADP) suggests possible vacancies on that specific site. Relaxing the O3 occupation conducts a drastic improvement in the refinement for only one additional parameter. In the final refinement, anisotropic ADP for three of the four oxygen sites (O2, O3, and O4) were necessary to fully explain the residues observed by a Fourier difference map. Table 1 summarizes the refined atomic parameters and the equivalent ADPs. As can be seen in Figure 4a, the strong elongation of the O3 density along the axis is consistent with the presence of local Ga2O7 units as proposed by Kendrick et al.15 The existence of such entities requires a strong flexibility of the GaO4 tetrahedral sites, especially a rocking of clusters around the a direction consistent with the shape of the O2 and O4 oxygen density sites. Such atomic organization emphasizes a possible conduction process through oxygen vacancies along the c direction. For the hydrated sample, the atomic positions obtained from the dry sample have been used as a starting model. After a few refinement cycles, an increase of the cell volume of ∼0.5% was obtained, which is in good agreement with incorporation of water into the structure. Nevertheless, the increase of the unit cell occurs anisotropically with a major expansion along the a direction, providing the unit cell parameters a = 10.1319(11) Å, b = 7.3681(7) Å, and c = 5.9507(3) Å. The hydration of the material is also clearly visible in the neutron diffraction pattern by an increase of intensities of some major diffraction peaks as illustrated in Figure 3b(arrows). Final refinement, using the same constraints, converges to a satisfying (gof) value of 1.9 (Rp = 4.01%) for the same number of refined parameters. As illustrated in Table 2 and Figure 4b, the results show that (1) only a few percentage points of oxygen vacancies remain on the O3 sites, (2) a decrease of the ADPs for most of the oxygen sites is observed, and (3) a residue “path” is observed by a Fourier difference map that could be attributed to the average localization of the D atoms over all the unit cells. The decrease of the oxygen vacancies in the hydrated sample is consistent with the accommodation of extra oxygen atoms provided through the hydration water molecules. It also explains the decrease of the elongation of the oxygen density map since the structure does not need to accommodate as many [Ga2O7] entities as for the dry sample. A decrease of the conduction through oxygen vacancies is therefore expected. However, Fourier map studies clearly show densities “flowing” from the O2 to the O4 atoms, which could be attributed to the D atoms based on the O−D distances (∼0.97 Å). However, the strong delocalization of the D atoms does not allow the refinement of a specific atomic position as illustrated in Figure 4b. Such observation strongly demonstrates the proton conduction mechanism in this material. The rocking motion of the O2 and O4 atoms should facilitate the D intertetrahedra jump and therefore a small amount of vacancies (on O3 site) seems suitable to facilitate the process. Proton Diffusion. The QENS spectra comprise two major fragments: the elastic peak and the quasielastic broadening of the elastic peak. With respect to the instrumental time scale, the quasielastic (QE) broadening represents the species that undergo diffusive motion, whereas the immobile species contribute to the elastic peak. The scattering function for QENS spectra can be written as

Figure 1. Water uptake of La1−xBa1+xGaO4−x/2 (x = 0 and x = 0.2) while cooling in wet argon.

Figure 2. Total and bulk conductivities of La0.8Ba1.2GaO3.9.

the impedance spectra for low temperatures, also shown in Figure 2. Above 800 °C the conductivities in wet and dry air become equivalent as a result of dehydration of the sample, and the conductivity is dominated by oxygen vacancies, probably including n-type electronic transport as well. Below 800 °C the total conductivity in wet and dry air are remarkably different, the gain at wet atmosphere arising from proton conduction.11 Between 800 and 450 °C, however, the conductivity is only partially dependent on the [OH·o], whereas below 450 °C the conductivity is predominantly protonic in wet air.14 Therefore, the extracted bulk conductivity values, as shown in Figure 2, present the protonic conduction in this material. The activation energy from the bulk conductivity was determined as 0.56 ± 0.05 eV. Structure. Observed and calculated patterns of La0.8Ba1.2GaO3.9 at room temperature for the short wavelength bank are shown in Figure 3a. Refinement of the neutron data, which included background coefficients, scale factors, profile functions, and absorption coefficients, atomic position parameters, and atomic displacement parameters, smoothly converged to a reasonable solution with a goodness of fit (gof) value of 2.1 (Rp= 3.94%) for 72 parameters (30 atomic parameters + 42 profile parameters). La0.8Ba1.2GaO3.9 exhibits an orthorhombic structure with a space group of P212121 and a lattice parameters of a = 10.0665(11) Å, b = 7.3417(6) Å, and c = 5.9433(4) Å. Cell parameter values are consistent with the 2743



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Figure 3. NPD pattern of La0.8Ba1.2GaO3.9 at room temperature for (a) the dried sample, (b) the hydrated sample, and (c) the superimposed patterns of both in the region where major differences between the dry and the hydrated samples are observed. Dots indicate normalized profile, solid line is the calculated profile, tick marks below profile indicate the positions of all allowed reflections, and the difference curve is shown above the tick marks on the same scale (d-spacing range 0.40−3.1 Å). Arrows indicate the major intensity changes upon hydration.

data analysis in related systems can be found in the literature, for example, see refs 24−27. The measured QENS spectra were fitted using eq 2. The data and the fits are shown in Figure 5 for temperatures ranging from 400 to 500 K. In this temperature range two Lorentzian components were required to describe the QE broadening. Each one corresponds to a dynamical process observable on the measured time scale. Below, we describe these two processes separately in greater detail. Slow Process. The half-width at half-maximum (Γ) of the narrower Lorentzian component has a clear Q-dependent behavior, as can been seen in Figure 6. The Γ(Q) indicates a continuous diffusion in the presented Q range, which can be described with Fick’s law, given as Γ(Q ) = DQ 2

where D is the diffusion coefficient. Equation 3 was fitted to the observed Γ(Q) values; the fits are shown as continuous lines in Figure 6. The Q range presented here corresponds to d values ranging from 5.2 to 31.4 Å (d = 2π/Q). The Fick’s law is fulfilled, indicating that, on this length scale, the proton diffusion is rather free. Therefore, all elementary steps necessarily take place at distances smaller than 5.2 Å. The obtained D values are shown in Table 3. Figure 7 shows the Arrhenius plot of the diffusion coefficients obtained from QENS spectroscopy. The corresponding activation energy is 0.44 ± 0.01 eV, which reflects the proton diffusion from about 3 unit cell lengths to sub unit cell scale. The activation energy obtained from the impedance spectroscopy 0.56 ± 0.05 eV is in relatively good agreement, although impedance spectroscopy probes rather macroscopic length scales compared to the

Figure 4. View along the a direction of the (a) dry La0.8Ba1.2GaO3.9 unit cell and (b) hydrated La0.8Ba1.2GaO3.9 unit cell. Atomic probability density maps have been used to represent the oxygen atoms in yellow. The surface of the probability represents 5% of the probability density maximum (see text for further details).

S(Q , ω) = F[x(Q )δ(ω) + (1 − x)(Q )Lj(Q , ω)] ⊗ R(Q , ω) + B(Q , ω)

(3)

(2)

where F is the scaling factor. The elastic peak δ with amplitude x and the quasielastic (QE) contribution, which is described as a number of Lorentzian functions, Lj, are convoluted with the instrumental resolution function R(Q,ω). The last term, B, stands for the background. More detailed description of QENS 2744



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Table 1. La0.8Ba1.2GaO3.9: (a) Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters (in Å2); (b) Anisotropic Displacement Parameters for the O2, O3, and O4 (in Å2) occupation

x

1 0.82(5)/ 0.18 1 1 1 0.88(3) 1

0.6730(9) 0.0475(5) 0.3336(5) 0.1754(6) n/a n/a n/a

(a) Ba1 La/Ba2 Ga O1 O2 O3 O4 (b) O2 O3 O4

y 0.3333(10) 0.4944(7) 0.2829(6) 0.1562(8) n/a n/a n/a U12

U11

U22

U33

0.04090(15) 0.02170(16) 0.01806(15)

0.08748(15) 0.02109(16) 0.09877(17)

0.01372(15) 0.0860(16) 0.01292(15)

0.01029(7) 0.01123(8) 0.01568(8)

z

Ueq

0.255(2) 0.2466(15) 0.2625(12) 0.2284(14) n/a n/a n/a U13

0.0094(14) 0.0092(13) 0.0066(9) 0.0051(13) n/a n/a n/a U23

−0.00384(8) 0.001245(7) 0.00011(9)

0.01975(7) 0.01888(9) −0.02708(7)

Table 2. Hydrated La0.8Ba1.2GaO3.9: (a) Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters (in Å2); (b) Anisotropic Displacement Parameters for the O2, O3, and O4 (in Å2) (a) Ba1 La/Ba2 Ga O1 O2 O3 O4 (b) O2 O3 O4

occupation

x

y

1 0.84(7)/0.16 1 1 1 0.96(3) 1

0.6736(11) 0.0484(6) 0.3345(7) 0.1757(9) n/a n/a n/a

0.3341(13) 0.4966(10) 0.2779(8) 0.1592(11) n/a n/a n/a U12

U11

U22

U33

0.03402(15) 0.01221(16) 0.01749(16)

0.07485(16) 0.01650(16) 0.12105(16)

0.00943(16) 0.07161(17) 0.01642(16)

0.01149(8) 0.00881(9) −0.00105(7)

z

Ueq

0.254(3) 0.252(2) 0.250(2) 0.2259(16) n/a n/a n/a U13

0.0126(18) 0.0118(16) 0.0074(11) 0.0066(18) n/a n/a n/a U23

−0.01046(7) 0.01183(7) 0.00674(9)

0.00341(10) 0.02663(9) −0.00119(7)

neighboring tetrahedra are very similar makes this type of analysis impractical for trying to understand the preferred local proton jump. However, we argue that the observed localized process is a proton jump between neighboring tetrahedra, in agreement with the work of Kendrick et al.15 In Figure 9 we illustrate our interpretation of the proton migration path in this material. Protons are found in the vicinity of particular corners of the GaO4 tetrahedra, denoted as O2 and O4 sites. The intratetrahedron proton transfer takes place between these two sites along the c axis, parallel to the observed oxide ion migration path. This is the only intratetrahedron proton transfer process observed experimentally. Intertetrahedra proton transfer ought to take place between these two sites, either (i) directly along the c axis or (ii) intermediated via the O3 site of a third tetrahedron. The first alternative is supported by our experimental observation of only one intertetrahedra proton transfer process. The second option is possible in light of previous studies;17 since a proton was located at a low temperature near the O3 site and an O4−H---O3 interaction was found. However, our experimental observation at much higher temperatures does not reveal this route nor does it rule it out. The intertetrahedra proton transfer from the O4 site to the O3 site and then from the O3 site to the O2 site may take place on a similar time scale and have similar activation energies, and therefore these two processes would not be distinguishable at the observed QENS spectra. In both cases (i) and (ii), the long-range proton transport takes place along the c axis. The overall proton transport in La0.8Ba1.2GaO3.9 can be described as a well-known Grotthus-type mechanism, where protons diffuse stepwise through material along the hydrogen

QENS measurements. These values are very similar to the values obtained by DFT15 for the intratetrahedron proton migration. Actually, the value of 0.44 ± 0.01 eV corresponds perfectly to the calculated value for the intratetrahedron proton jump between O2 and O4 sites, which is also the path very clearly visible in our NPD results. These results give strong evidence that this is the preferred intratetrahedron proton path, which at the same time is the rate-limiting step for the longrange proton diffusion in this material. The marginally higher activation energy obtained from the impedance measurements contains macroscopic features, which have their impact on the proton diffusion as well. Fast Process. The wider Lorentzian component was fitted to QENS spectra for temperatures ranging from 350 to 500 K. At temperatures below 400 K, the narrow component was not resolvable within the instrumental time scale. The spectra at 350 K were well-described using only the wider Lorentzian component. The width of the wider Lorentzian component has Q-independent character, whereas the intensity of this component increases with increasing Q values. These characteristics of the wider QE component strongly indicate a localized process. An average Γ for each temperature was taken, as shown in Figure 8 as an Arrhenius plot. Activation energy of 0.068 ± 0.006 eV for this process was obtained, which is similar to the calculated15 intertetrahedra proton transfer values. To obtain more insight into the localized diffusion process, modeling of the EISF (elastic incoherent structure factor) was performed. However, in the available Q range the EISF models do not give an explicit answer about the localized proton jump distance. Furthermore, the fact that possible proton site distances within a tetrahedron and between 2745



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Figure 5. S(Q,ω) at 400, 433, 466, and 500 K. The open circles represent the experimental data, and the solid black line is the fit of eq 2. Also, the elastic peak (blue line) and the two QE components (red lines) are shown.

Table 3. Diffusion Coefficients for Proton Diffusion in La0.8Ba1.2GaO3.9·0.08H2O temperature (K) 500 466 433 400

D (10−8 cm2 s−1) QENS 3.91 2.07 1.14 0.47

± ± ± ±

0.19 0.08 0.09 0.04

almost an order of magnitude higher than that for the intertetrahedra transfer. The former one occurs between the gallium and barium cations, whereas the latter takes place between or around two lanthanum cations. The nature and location of these cations should not explain such a huge difference in the potential barrier, and therefore a curved path beside the adjacent O3 site might be preferred to evade the barrier created by the lanthanum cations.

Figure 6. Γ(Q) of the narrower Lorentzian component at 500 (red triangles), 466 (green squares), 433 (lila circles), and 400 K (blue open circles). The continuous lines are the fits of eq 3.

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CONCLUSIONS These results show for the first time how anharmonic ADPs and Fourier difference maps delivered from the neutron powder diffraction results in combination with quasielastic incoherent neutron spectroscopy can be used to identify ionic conduction paths in atomic scale. In this study we have shown that, in the dry La0.8Ba1.2GaO3.9 sample, most of the oxygen vacancies are located on particular corners of the GaO4

bond network. Between each step a proton is located in the vicinity of oxygen, and it can perform localized motions like vibrations or librations around the equilibrium position and reorientations. Also, the GaO4 tetrahedra are rocking, which elevates the protons to a higher exited state. The energy barrier experienced by protons for the intratetrahedron transfer is 2746



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counterpart the elementary steps of proton conduction consist of two processes, which both take place at distances less than 5.2 Å. The proton diffusion appears along the c axis as well, rate-limiting step being the intratetrahedron proton migration with activation energy of 0.44 eV. A small number of oxygen vacancies still exist in the hydrated materials, which preserves to some degree the rocking of GaO4 units. This may support the intertetrahedra proton transport, which has activation energy of 0.068 eV. These results indicate that the flexibility of the tetrahedral network in an ionic conductor is an advantage, facilitating the localized ion transport.

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ASSOCIATED CONTENT

S Supporting Information *

Selected impedance spectra and details on how the bulk conductivity was separated. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. Proton diffusion coefficients from QENS, the solid line presents the fit of Arrhenius law.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Neutron research at Spallation Neutron Science was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. N.J. is supported by the Jülich Centre for Neutron Science. Materials characterization work (M.P.P. and Z.B.) was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Development of the program Jana2006 is supported by Praemium Academiae of Czech Academy of Sciences. N.J. thanks Eugene Mamontov for the discussion about the QENS data analysis.

Figure 8. Γ of the fast process at temperatures ranging from 350 to 500 K.

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REFERENCES

(1) Norby, T. Solid State Ionics 1999, 125, 1−11. (2) Goodenough, J. B.; Manthiram, A.; Paranthaman, M.; Zhen, Y. S. Mater. Sci. Eng., B 1992, 12, 357−364. (3) Kreuer, K. D. Ann. Rev. Mater. Res. 2003, 33, 333−359. (4) Norby, T. Perovskite Oxides for Solid Oxide Fuel Cells; Ishihara, T., Ed.; Springer US: Boston, MA, 2009; Vols. 217−241. (5) Iwahara, H. Solid State Ionics 1996, 86−88, 9−15. (6) Ishihara, T.; Matsuda, H.; Takita, Y. J. Am. Chem. Soc. 1994, 116, 3801−3803. (7) Haugsrud, R.; Norby, T. Nat. Mater. 2006, 5, 193−196. (8) Nakayama, S.; Aono, H.; Sadaoka, Y. Chem. Lett. 1995, 24, 431− 432. (9) Slater, P. R.; Sansom, J. R.; Tolchard, J. R. Chem. Rec. 2004, 4, 373−384. (10) Kendrick, E.; Islam, M. S.; Slater, P. R. J. Mater. Chem. 2007, 17, 3104−3111. (11) Li, S.; Schönberger, F.; Slater, P. Chem. Commun. 2003, 21, 2694−2695. (12) Schönberger, F.; Kendrick, E.; Islam, M. S.; Slater, P. Solid State Ionics 2005, 176, 2951−2953. (13) Rüter, I.; Müller-Buschbaum, H. K. Z. Anorg. Allg. Chem. 1990, 584, 119−124. (14) Lee, K. H.; Kim, J. H.; Kim, H. L.; Kim, S.; Lee, H. L. Jpn. J. Appl. Phys. 2005, 44, 254−257. (15) Kendrick, E.; Kendrick, J.; Knight, K. S.; Islam, M. S.; Slater, P. R. Nat. Mater. 2007, 6, 871−875. (16) Giannici, F.; Messana, D.; Longo, A.; Martorana, A. J. Phys. Chem. C 2011, 115, 298.

Figure 9. Protons are represented as small gray-colored circles bound to particular corners of the GaO4 tetrahedra. The proton migration takes place along the c axis, shown as a gray dashed arrow. The insert shows a slice parallel to the ac plane with the elementary steps: the intratetrahedron path is represented with gray arrows and two possibilities for intertetrahedra paths are represented with (i) blue and (ii) purple arrows.

tetrahedra, that is, the O3 sites. Anisotropic ADPs of O2, O3, and O4 sites give evidence of the rocking motion of the tetrahedra leading to temporary formation of Ga2O7 units. The rocking motion clearly promotes the ionic conduction in this material. The oxide ion (vacancy) conduction occurs via O3 sites in channels along the c axis. Besides, in the hydrated 2747



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Atomic-Scale Picture of the Ion Conduction Mechanism in a

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