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Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates
SOSI-13547; No of Pages 7 Solid State Ionics xxx (2014) xxx–xxx
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Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates☆ S.N. Savvin a, A.V. Shlyakhtina b,⁎, A.B. Borunova b, L.G. Shcherbakova b, J.C. Ruiz-Morales a, P. Núñez a,⁎⁎ a b
Department of Inorganic Chemistry and Institute of Materials and Nanotechnology, University of La Laguna, 38200 La Laguna, Tenerife, Spain Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991 Russia
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 10 November 2014 Accepted 2 December 2014 Available online xxxx Keywords: Proton-conducting membranes Proton conductivity Electron conductivity
a b s t r a c t Zr-doped rare-earth molybdates (Nd5.4Zr0.6MoO12.3, Sm5.4Zr0.6MoO12.3, Dy5.4Zr0.6MoO12.3 and La5.8Zr0.2MoO12.1) demonstrate appreciable mixed electron–proton conductivity in the low and intermediate temperature range under wet oxidizing and mild reducing conditions. Proton contribution to their total conductivity decreases as the lanthanide cation radius decreases. Among the samples studied, La5.8Zr0.2MoO12.1 showed the highest total conductivity of about 2.5 × 10−5 S/cm at 500 °C in wet air. Its impedance spectra did not provide any evidence of the grain boundary contribution, which seems to be an inherent feature of lanthanum molybdates. Exposure of Nd5.4Zr0.6MoO12.3 to wet argon was found to increase its total conductivity by almost one order of magnitude relative to the values obtained in dry argon. Although all the rare-earth molybdates studied in this work became essentially electronic conductors in Ar–5% H2 atmosphere, La5.8Zr0.2MoO12.1 demonstrated much lower propensity to reduction than the rest of the samples. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Proton-conducting oxides attracted great attention over the last decades primarily due to the fact that quite a few of them at least potentially can be used as electrolytes in high-temperature fuel cells and steam electrolyzers or mixed electron–proton conductors in gas separation membranes [1]. However, apart from being good proton conductors state-of-the-art materials such as acceptor-doped BaCeO3 and BaZrO3 suffer from several major drawbacks, which impede their immediate implementation into practice [2]. The tendency for the alkaline-earth cerrates/zirconates to react with CO2 and H2O, large grain boundary resistance and complexity of their fabrication are among such drawbacks. On the other hand, lanthanide based oxides, such as fluorite-like rare-earth molybdates and tungstates or Ce/Zr pyrochlores generally possess much better chemical stability than the perovskites and as will be discussed later some of them seem to lack any grain boundary contribution to the total conductivity [3].
☆ This work was presented during the 11th International Symposium on Systems with Fast Ionic Transport, Gdansk, Poland 25–29.06.2014 as poster presentation. ⁎ Correspondence to: A.V. Shlyakhtina, Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991 Russia. Tel.:+7 495 9397950; fax: +7 499 2420253. ⁎⁎ Correspondence to: P. Núñez, Department of Inorganic Chemistry and Institute of Materials and Nanotechnology, University of La Laguna, 38200 La Laguna, Tenerife, Spain. Tel.:+34 922318501; fax: +34 922 318461. E-mail addresses: [email protected], [email protected] (A.V. Shlyakhtina), [email protected] (P. Núñez).
Both oxygen-ion and proton conductivity of the Ln6WO12 tungstates are known to decrease across the rare-earth series in the order La N Nd N Gd N Er [4–7]. This decrease is accompanied by drastic changes in their crystal structure as the rare-earth ionic radius gets smaller. For instance, while La6WO12 adopts a highly disordered fluorite-like cubic structure, medium-sized rare-earth elements (Nd–Gd) bring about tetragonal distortions of the Ln6WO12 crystal structure and, finally, the smallest rare-earth cations (Tb–Lu) give rise to rhombohedral tungstates [5]. It should be mentioned that unlike the tungstates, the crystal structure of the medium-sized rare-earth molybdates (Sm, Eu, Gd, Dy) remains cubic although La6MoO12, Nd6MoO12, and Ho6MoO12 are known to exist both as rhombohedral (R3) and fluorite-like ( Fm3m) cubic polymorphs. Available data on phase relations in the Ln2O3–WO3 [8–14] and Ln2O3–MoO3 [15] systems is scarce and somewhat contradictory. Among the tungstates the La2O3–WO3 quasi-binary system seems to have received most attention over the past years. In fact, hightemperature phase equilibria occurring in this system are quite complex [9] and structural similarities between phases found in the La2O3-rich portion of the phase diagram have recently resulted in some debate on whether the “La6WO12” proton conductor is actually stable at room temperature [11]. The phase diagram of the Sm2O3–WO3 system studied by L.L.Y. Chang et al. [13,14] in the 1960s at first glance does not seem to be as complicated as that of La2O3–WO3. The compound Sm6WO12 was described as a defect pyrochlore with the cubic cell parameter of
http://dx.doi.org/10.1016/j.ssi.2014.12.003 0167-2738/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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is available on phase relations in the Ln2O3-rich field of the Ln2O3– MoO3 systems. Recently we have demonstrated that Zr-doped samarium molybdates (Sm5ZrMoO12.5 and Sm5.4Zr0.6MoO12.3) possess mixed electron– proton conductivity at intermediate temperatures [16]. In this work we report on the synthesis, crystal structure and conductivity of the fluorite-like Ln5.4Zr0.6MoO12.3 (Ln = Nd, Sm, Dy) and rhombohedral (La5.8Zr0.2MoO12.1) molybdates — potential intermediatetemperature mixed electron–proton conductors.
2. Experimental
Fig. 1. (a) XRD patterns of (1) La5.8Zr0.2MoO12.1 (1600 °C, 3 h), (2) Sm5.4Zr0.6MoO12.3 (1600 °C, 3 h) and (3) Dy5.4Zr0.6MoO12.3 (1600 °C, 3 h), indices are given for the fluorite cubic cell, and refer to the patterns 2 and 3 only; (b) results of the Le-Bail refinement of the XRD data obtained for La5.8Zr0.2MoO12.1; the inset shows that the rhombohedral unit cell proposed does not account for some weak superstructure reflections; (c) results of the Le-Bail refinement of the XRD profile for the sample of nominal composition Nd5.4Zr0.6MoO12.3, the inset highlights the presence of a strong peak splitting.
10.8 Å. Authors [14] speculate that above 1800 °C there might exist complete miscibility between Sm6WO12 and Sm14W4O33, for their crystal structures are apparently quite similar at high temperatures. Both compounds have homogeneity ranges which become wider as the temperature increases reaching ~ 5 and ~ 3.3 mol% Sm 2O 3 at 1700 °C for Sm6WO12 and Sm14W4O33 respectively. Much less data
Nd5.4Zr0.6MoO12 + δ (06NZMO), Sm5.4Zr0.6MoO12 + δ (06SZMO), Dy5.4Zr0.6MoO12 + δ (06DZMO) and La5.8Zr0.2MoO12.1 (02LZMO) were prepared by the mechanical activation method. After preheating the starting Ln2O3 oxides at 1000 °C for 2 h, they were mixed with the rest of the components (ZrO2, MoO3) and co-milled in the SPEX8000 ball mill for 1 h. MoO3 was previously activated in the high energy Aronov ball mill for 4 min. The mechanically activated mixtures of the oxides were unaxially pressed at 914 MPa and sintered at 1400 °C and 1600 °C for 3 h. The geometric density of the asprepared ceramics ranged from 80.5 to 92.5% of the theoretical one. All samples were characterized both structurally and electrically. Powder X-ray diffraction patterns were obtained on a DRON-3M automatic diffractometer (filtered CuKα radiation, step scan mode) in the angular range 2θ = 10–65° and on a PANalytical X'Pert Pro diffractometer equipped with a Ge(111) incident beam monochromator (CuKα1 radiation) and X'Celerator silicon strip detector. In the latter case the patterns were collected in the 2θ range of 5–120°, with a step size of 0.017° and counting time of 300 s per step. Structure refinement was performed using the GSAS suite of Rietveld refinement software [17]. Electrical conductivity of the samples was studied by impedance spectroscopy. Prior to making measurements both faces of the ceramic pellets prepared as described above were covered by Pt paste (Metalor, # 6695) and fired at 950 °C for 1 h. Afterwards, each pellet was settled in a gas-tight sample stage outfitted with an outer ceramic housing, gas inlet and outlet tubes and a thermocouple located in the close vicinity of the sample. The temperature dependence of the total (electronic and ionic) grain interior conductivity in different dry and wet atmospheres (air, Ar, Ar–5% H2) was extracted from impedance spectra obtained using a Solartron 1260 frequency response analyzer. The spectra were recorded in the frequency range of 0.1 Hz to 1 MHz on cooling from 850 °C to 250 °C; the root mean square ac voltage amplitude was set to 150 mV. Depending on the atmosphere and temperature it took from 2 to 5 h for the samples to reach the equilibrium conductivity values. Relative humidity of the gases fed into the sample stage was controlled by passing them over a freshly dehydrated silica gel (designated “dry”) or through a water saturator held at 20 °C (designated “wet”), which ensured constant water content of about 2%. Under reducing conditions total ac conductivity was measured as a function of the oxygen partial pressure p(O2) in the atmosphere of CO/CO2, while under oxidizing conditions dry oxygen and Ar were mixed in different ratios to yield atmospheres with p(O2) ranging from ≈ 1 × 10− 5 atm to ≈ 1 atm at 800 °C. Most of the impedance spectra obtained under mild reducing (Ar) and oxidizing (air) conditions were fitted to an equivalent circuit consisting of two parallel (RQ) elements placed in series, which represented bulk and grain boundary contributions to conductivity. In some instances, e.g. at high temperatures or in the case of 06DZMO and 02LZMO compounds, only one semicircle appeared in the spectra. In these cases the equivalent circuit was modified accordingly in order to take into account the absence of the bulk or grain boundary semicircle.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Table 1 Characteristics of the as prepared samples. Sample no
Composition
Heat treatment
Color
Relative density, %
Structure type
Cell parameters, Å
1 2 3 4
La5.8Zr0.2MoO12.1 Nd5.4Zr0.6MoO12.3 Sm5.4Zr0.6MoO12.3 Dy5.4Zr0.6MoO12.3
Air, 1600 °С, 3 h Air, 1400 °С, 3 h Air, 1600 °С, 3 h Air, 1600 °С, 3 h
Yellow Vinous Bright yellow Orange-yellow
92.5 92 80.5 91.2
Fluorite-like; rhombohedral Fluorite; 2 phases Fluorite Fluorite
a = 3.9785(1); c = 9.8633(2) a1 = 5.4572(1); a2 = 5.4269(1); 5.3991(1) 5.3036(2)
3. Results and discussion 3.1. Crystal structure of Ln5.4Zr0.6MoO12 + La5.8Zr0.2MoO12.1
δ
(Ln = Nd, Sm, Dy) and
According to the JCPDS PDF database and in contrast to La6WO12 (# 16-0391), whose crystal structure, to a first approximation, can be considered cubic defect fluorite, La6MoO12 (record # 34-1220) crystallizes in the rhombohedral structure of space group R3. Nd6MoO12 reportedly can adopt either cubic (# 24-1111) or rhombohedral (# 35-0246) crystal structure, while the other two molybdates, namely Sm6MoO12 (# 24-1121) and Dy6MoO12 (# 25-0331), were found to have only one stable polymorph, with a defect-fluorite cubic structure. The XRD patterns of the as-prepared Ln5.4Zr0.6MoO12.3 (Ln = Nd, Sm, Dy) and La5.8Zr0.2MoO12.1 as well as their cell parameters, color and relative density are given in Fig. 1 and Table 1, respectively. All reflections observed in the patterns of 06SZMO and 06DZMO were indexed on a face-centered cubic lattice (space group Fm3m), which resulted in quite similar cell parameters for both phases. However, closer inspection revealed that the XRD data obtained for the larger rare-earth molybdates, i.e. 02LZMO and 06NZMO, were more complex. Apart from a number of low-intensity and presumably superstructure reflections, the XRD pattern of 02LZMO contained intense peaks which were split in a way consistent with the trigonal distortion of an fcc lattice. It should be noted that our attempts to prepare single-phase La5.4Zr0.6MoO12.3 material did not succeed. Since its XRD patterns, regardless of the way of synthesis and conditions of thermal treatment, persistently showed traces of the La2Zr2O7 pyrochlore impurity we opted to reduce the amount of Zr and prepared less heavily doped La5.8Zr0.2MoO12.1 instead. Interestingly, M. Amsif et al. [18] recently observed a diffraction pattern quite similar to that given in Fig. 1b. They suggested that additional weak reflections present in the XRD pattern of La6Mo0.8W0.2O12 − δ could have resulted from a 7 × 7 superstructure formed in the ab plane of the rhombohedral subcell. Ordering of oxygen vacancies, typically observed in fluorite-based systems, was considered the prime reason for the superstructure to appear.
Indexing of the main reflections in the XRD pattern of 02LZMO allowed us to determine parameters of the subcell (Table 1). The values obtained agree quite well with those reported in [18] if allowance for the smaller ionic radius of Zr4+ is made. The XRD profile of 06NZMO showed clearly that the sample was not phase-pure and contained small amount of impurity whose cell parameter was close to that of the principal fluorite phase. Indeed, the experimental profile exhibited a significant splitting of all but the lowest angle diffraction peaks, which could be satisfactorily accounted for by including a second cubic phase in the Le-Bail refinement (Fig. 1c). Cell parameters of both co-existing fluorite phases are given in Table 1. The XRD pattern of 06DZMO was analyzed by the Rietveld method (Fig. 2). As mentioned above quite close values of the cubic cell parameter were obtained after indexing the diffraction patterns of 06SZMO and 06DZMO, which implied that both phases have similar crystal structures. Therefore, in the course of Rietveld refinement atomic positions in the unit cell of 06DZMO as well as initial values for isotropic displacement parameters were set equal to those reported for 06SZMO [16]. Given that in the fluorite cell both cations and anions are located in special positions their coordinates were kept fixed. Neither the occupancy of the cation sites was refined in order to avoid strong correlations between free parameters. As can be seen in Fig. 2 quite good agreement between the experimental data and the structural model that includes cation disordering over 4a positions in the fluorite unit cell has been obtained. The refined structural parameters are listed in Table 2. Stability of the molybdates under reducing conditions was examined by XRD after annealing in Ar–5% H2 at 900 °C for 10 h. Although no changes in the sample's phase composition were detected, their unit cell parameters decreased substantially on reduction (Fig. 3). 3.2. Electron–proton conductivity of Ln5.4Zr0.6MoO12+ δ (Ln = Nd, Sm, Dy) and La5.8Zr0.2MoO12.1 The equilibrium electrical conductivity of the cubic and rhombohedral molybdates was studied by impedance spectroscopy under oxidizing and reducing conditions. Fig. 4 shows several representative impedance spectra acquired at different temperatures in dry and wet air. It is interesting to note that the spectra of 06DZMO and 02LZMO showed rather dissimilar features related to the grain boundary behavior in different atmospheres. At intermediate (Fig. 4a) and low (Fig. 4c) temperatures under the atmosphere of wet air the spectra of 06DZMO were dominated by two adjacent semicircles, which were unambiguously attributed to bulk (Cb ≈ 7 × 10− 12 F/cm) and grain boundary (Cgb ≈ 3 × 10−10 F/cm) processes. On the contrary, only one semicircle corresponding to the bulk conductivity was present in the impedance spectra of 06DZMO recorded in dry air. Recently, we observed quite similar behavior of the grain boundary conductivity of 06SZMO [16].
Table 2 Final refined structural parameters for Dy5.4Zr0.6MoO12.3a.
Fig. 2. Rietveld refinement profile for the as-prepared Dy5.4Zr0.6MoO12.3.
Atom
Site
x
y
z
Fractional occupancy
100 × Uiso, Å−2
Dy1/Zr1/Mo1 O1
4a 8c
0 1/4
0 1/4
0 1/4
0.7714/0.0857/0.1429 0.8786
1.34(1) 6.5(2)
a
Cubic, Fm3m, a = 5.3036(2) Å, Rp = 0.0187, Rwp = 0.0270, χ2 = 1.799.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Fig. 3. Dependence of the unit cell volume of the fluorite-like molybdates on the rare-earth ionic radius.
We believe that such unusual development of the grain boundary resistance under wet conditions may be related to impurity/dopant segregation or result from the loss of dimensional stability under reducing
conditions. In fact, as has been discussed in the previous section, reduction of the molybdates in Ar–5% H2 is accompanied by a considerable unit cell contraction, which might have affected somehow the integrity of the intergranular contacts. As can be seen in Fig. 4b,d, no grain boundary contribution to the impedance spectra of 02LZMO is visible either in dry or wet air (Cb ≈ 6 × 10−12 F/cm) at all temperatures. The same is true for the spectra obtained under dry/wet Ar atmosphere. At this point it is interesting to mention that the influence of blocking grain boundaries on the total conductivity of some LAMOX electrolytes has been thoroughly studied and was reported to depend on the synthesis method and processing conditions. For instance, D. Marrero-López et al. [19] found out that the grain boundary contribution to the conductivity of La2Mo2O9 progressively decreased as relative density of the ceramics was approaching 100%. Among the samples obtained from freeze-dried precursors, those attaining the highest (nearly theoretical) density showed the least grain boundary impedance. We believe that the absence of resistive grain boundary regions hindering either protonic or ionic transport in 02LZMO might be an intrinsic feature of lanthanum molybdates. On the other hand according to recent studies by J. Liu et al. [20,21] there is no solid evidence that fast grain boundary diffusion paths actually exist in La2Mo2O9 electrolytes as previously postulated by S. Georges et al. [22]. Regardless of the mechanistic details of the grain boundary conductivity in lanthanum molybdates, it is worth noting that reduction of losses related to the grain boundary impedance of alternative solid electrolyte materials can drastically improve performance of electrochemical cells based on them and eventually outweigh issues resulting from somewhat lower bulk conductivity [2].
Fig. 4. Impedance spectra of (a, c) 06DZMO and (b, d) 02LZMO at intermediate and low temperatures in dry and wet air. Solid lines are results of the non-linear least square fit, while broken lines represent extrapolation of the fit results to infinite frequency.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Fig. 5. Arrhenius plots of the bulk conductivity of (a) 02LZMO, (b) 06NZMO, (c) 06SZMO, and (d) 06DZMO in dry and wet air and Ar.
Fig. 5 presents the bulk conductivity of 06NZMO, 06SZMO, 06DZMO and 02LZMO in dry and wet atmospheres. A systematic increase of total conductivity in wet air/Ar as compared to the conductivity in dry gases is indicative of hydration of the samples resulting in proton conductivity. All curves given in Fig. 5 demonstrate an inflection point, whose location on the graph depends on the composition of the sample. By analogy with rare-earth tungstates the change in the activation energy (Table 3) with temperature observed in Ln6MoO12 − δ molybdates can be attributed to a shift from predominant oxygen-ion/proton conduction at low temperature to predominant electron/hole conduction in the high temperature range [5]. Under wet conditions such shift is usually related to dehydration of ceramics and concomitant depletion of the protons within the bulk of the samples. Interestingly, the onset of electron conduction in 06SZMO and 06DZMO takes place already at 500 °C in air, while 02LZMO is to be heated at least up to 750 °C for the electronic defects to dominate their total conductivity.
The total conductivity of 06NZMO, 06SZMO, 06DZMO and 02LZMO measured in dry Ar–5% H2 atmosphere is presented in Arrhenius form in Fig. 6. Under these conditions the reduction reaction seems to be the dominant defect equilibrium that renders the three molybdate Table 3 Activation energy for bulk conduction in Ln6 − xZrxMoO12+ δ in dry Ar. Sample no
Composition
Activation energy, eV
Temperature range, °C
1
02LZMO
2
06NZMO
3
06SZMO
4
06DZMO
1.21 0.78 1.48 0.91 1.48 0.75 1.38 0.69
Above 700 Below 700 Above 500 Below 500 Above 425 Below 425 Above 425 Below 425
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Fig. 6. Arrhenius plots of the bulk conductivity of 02LZMO, 06NZMO, 06SZMO and 06DZMO in dry Ar–5% H2.
electronic conductors. However, the 02LZMO is strikingly different from the rest of the samples. While 06SZMO and 06DZMO demonstrate a pronounced tendency to be reduced in Ar–5% H2 their conductivity reaching 0.2–0.25 S/cm at 800 °C, 02LZMO exhibits much lower conductivity (2.4 × 10− 2 S/cm at 800 °C) and higher activation energy (≈ 0.4 eV) below 500 °C. All these facts taken together suggest that 02LZMO might be less prone to reduction at low oxygen partial pressures. These data also seem to indicate that 02LZMO has higher proton transport numbers than the rest of the rare-earth molybdates do under strongly reducing conditions. In order to gain further insight into the nature of predominant defect species the equilibrium conductivity of 02LZMO, 06NZMO and 06SZMO was studied as a function of oxygen partial pressure (Fig. 7). The results clearly show two distinct types of behavior. At high temperatures (800 °C) the dependence of the total conductivity of 02LZMO on oxygen partial pressure resembles that of a typical mixed electronic– ionic conductor. There is an extensive region of n-type conductivity under reducing conditions, while under oxidizing conditions a significant region of p-type conductivity can be observed. At intermediate
oxygen partial pressures, the total conductivity of 02LZMO is essentially independent of p(O2), which is indicative of predominant ionic conductivity. The log-log slopes of the conductivity curve appear to be +1/4 and − 1/4 under the oxidizing and reducing conditions respectively, with a gradual shift towards −1/6 at the lowest oxygen partial pressures. In the case of 06NZMO and 06SZMO quite different behaviors were observed. The total conductivity of both samples is dominated by electrons at 800 °C except for a narrow region of roughly p(O2)-independent conductivity demonstrated by 06NZMO under the most oxidizing conditions, i.e. at p(O2) N 0.21 atm. As the temperature is decreased the p-type conductivity of 06NZMO becomes much more pronounced and the ionic regime widens. The tendency of ionic charge carriers to dominate defect equilibria at lower temperatures has been previously reported for the Ln6WO12 proton conductors as well [5]. It should be noted that the abovementioned dependence of the total conductivity of 02LZMO, 06NZMO and 06SZMO on oxygen partial pressure agrees well with the Arrhenius plots given in Fig. 5. Another noteworthy feature of the Ln6MoO12 − δ family of oxides is a significant correlation between their hydration behavior and the nature of the lanthanide species. As can be seen in Fig. 5, in wet air the relative contribution of protons to the total conductivity of the molybdates decreases as the rare-earth ionic radius shrinks (i.e. in the series La ≈ Nd N Sm N Dy). A similar tendency has been previously reported for the Ln6WO12 tungstates [5]. However, in the atmosphere of wet Ar (Fig. 5b), i.e. under mild reducing conditions where the p(O2) independent ionic/protonic region occurs, a tremendous increase in the conductivity of 06NZMO relative to dry Ar takes place. Hydration of the oxygen vacancies abundantly present in 06NZMO under these condition is probably responsible for this behavior. Although the relationships between hydration thermodynamics, concentration/mobility of the protonic defects and materials' physical parameters still have to be established for many ceramic proton conductors, high molar volume and high (preferentially cubic) symmetry as well as basicity of the lanthanide cations are among the factors favoring proton conductivity in oxides [2]. Particularly relevant in this regard is the fact that extrinsic oxygen vacancies introduced into the structure of LAMOX electrolytes as charge compensating species generally do not result in any improvement of their ionic conductivity [23]. The same holds true for Ca-doped “La6WO12” [24]. Therefore, the lattice contraction may be at least partially responsible for the drop in proton conductivity of Ln6(Mo,W)O12 − δ observed across the rare-earth series, albeit it is still under debate whether the smaller acceptor dopants actually alter the immediate environment of the inherent oxygen vacancies or induce trapping of ionic/protonic charge carriers, which reduces their mobility. On the other hand, one cannot definitively rule out the possibility that the transport properties of La5.8Zr0.2MoO12.1 reported here were affected by the Zr4+ dopant to a lesser degree than those of Ln6MoO12 (Ln = Nd, Sm, Dy). The effect of Zr4 + on the conductivity of the rare-earth molybdates was previously discussed in [16], where we demonstrated that it acts as a typical donor species. 4. Conclusions
Fig. 7. The total conductivity of 02LZMO, 06NZMO, and 06SZMO as a function of oxygen partial pressure. Broken lines are guides to the eye.
The fluorite-like Ln5.4Zr0.6MoO12.3 − δ (Ln = Nd, Sm, Dy) oxides have similar total conductivity of the order of 5 × 10−4 S/cm at 800 °C in air. Their conductivity seems to be dominated by ionic defects at low temperatures, while at high temperatures or under the atmosphere of Ar– 5% H2 electronic conduction prevails. Under oxidizing conditions electron holes were found to be dominant charge carriers in 02LZMO and 06NZMO at high and intermediate temperatures respectively. We argue that proton conduction may exist in 06NZMO, 06SZMO and 06DZMO in the temperature range of 250–530 °C in wet atmospheres. The relative contribution of protons to the total conductivity of Ln6 − xZrxMoO12+ x/2 − δ in air gradually decreases on going from La to Dy. 06NZMO demonstrated much better hydration characteristics than the rest of the molybdates did under mild reducing conditions.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Among the samples studied, La5.8Zr0.2MoO12.1 showed the highest total conductivity in wet air (about 2.5 × 10−5 S/cm at 500 °C), which was found to be entirely due to the bulk transport. The total conductivity of La5.8Zr0.2MoO12.1 was found to be dominated by ionic charge carriers at intermediate oxygen partial pressures and high temperatures. Acknowledgments This work was supported by the Russian Foundation for Basic Research (grant no. 13-03-00680) and the Chemistry and Materials Science Division of the Russian Academy of Sciences (basic research program no. 2: Advanced Metallic, Ceramic, Glassy, Polymeric, and Composite Materials). We also acknowledge the financial support provided by the Spanish Government through the National research program (grants MAT2010-16007, MAT2013-42407 and ENE201347826-C4-1-R, co-financed by FEDER funds). References [1] T. Norby, M. Wideroe, R. Glockner, Y. Larring, Dalton Trans. (2004) 3012. [2] K. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. [3] V. Besikiotis, S. Ricote, M. Jensen, T. Norby, R. Haugsrud, Solid State Ionics 229 (2012) 26. [4] T. Shimura, S. Fujimoto, H. Iwahara, Solid State Ionics 143 (2001) 117. [5] R. Haugsrud, Solid State Ionics 178 (2007) 555.
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[6] A. Magrasó, C. Frontera, D. Marrero-López, P. Núñez, Dalton Trans. (2009) 10273. [7] A. Magrasó, J.M. Polfus, C. Frontera, J. Canales-Vázquez, L.-E. Kalland, C.H. Hervoches, S. Erdal, R. Hancke, M.S. Islam, T. Norby, R. Haugsrud, J. Mater. Chem. 22 (2012) 1762. [8] М.М. Ivanova, G.М. Balagina, Е.Ya. Rode, Izv. Akad. Nauk SSSR Inorg. Mater. 6 (1970) 914. [9] M. Yoshimura, A. Ruanet, Mater. Res. Bull. 11 (1976) 151. [10] V.К. Yanovskii, V.I. Voronkova, Izv. Akad. Nauk SSSR, Inorg. Mater. 19 (1983) 416. [11] M.-H. Chambrier, A. Le Bail, F. Giovanelli, A. Redjaimia, P. Florian, D. Massiot, E. Suard, F. Goutenoire, Inorg. Chem. 53 (2014) 147. [12] Rode Е.Ya., D. Karpov, Izv. Akad. Nauk SSSR Inorg. Mater. 2 (1966) 683. [13] L.L.Y. Chang, B. Phillips, Inorg. Chem. 3 (1964) 1792. [14] L.L.Y. Chang, M.G. Scroger, B. Phillips, J. Inorg. Nucl. Chem. 28 (1966) 1179. [15] E.A. Tkachenko, P.P. Fedorov, Inorg. Mater. 39 (2003) S25. [16] S.N. Savvin, A.V. Shlyakhtina, I.V. Kolbanev, A.V. Knotko, D.A. Belov, L.G. Shcherbakova, P. Núñez, Solid State Ionics 262 (2014) 713. [17] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR, 2004, pp. 86–748. [18] M. Amsif, A. Magrasó, D. Marrero-López, J. Ruiz-Morales, J. Canales-Vázquez, P. Núñez, Chem. Mater. 24 (2012) 3868. [19] D. Marrero-López, J. Canales-Vázquez, J. Ruiz-Morales, A. Rodríguez, J. Irvine, P. Núñez, Solid State Ionics 176 (2005) 1807. [20] J. Liu, R. Chater, B. Hagenhoff, R. Morris, S. Skinner, Solid State Ionics 181 (2010) 812. [21] J. Liu, R. Chater, R. Morris, S. Skinner, Solid State Ionics 189 (2011) 39. [22] S. Georges, S. Skinner, P. Lacorre, M. Steil, Dalton Trans. (2004) 3101–3105. [23] D. Marrero-López, D. Pérez-Coll, J. Ruiz-Morales, J. Canales-Vázquez, M. MartínSedeño, P. Núñez, Electrochim. Acta 52 (2007) 5219. [24] R. Haugsrud, C. Kjølseth, J. Phys. Chem. Solids 69 (2008) 1758.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates☆ S.N. Savvin a, A.V. Shlyakhtina b,⁎, A.B. Borunova b, L.G. Shcherbakova b, J.C. Ruiz-Morales a, P. Núñez a,⁎⁎ a b
Department of Inorganic Chemistry and Institute of Materials and Nanotechnology, University of La Laguna, 38200 La Laguna, Tenerife, Spain Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991 Russia
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 10 November 2014 Accepted 2 December 2014 Available online xxxx Keywords: Proton-conducting membranes Proton conductivity Electron conductivity
a b s t r a c t Zr-doped rare-earth molybdates (Nd5.4Zr0.6MoO12.3, Sm5.4Zr0.6MoO12.3, Dy5.4Zr0.6MoO12.3 and La5.8Zr0.2MoO12.1) demonstrate appreciable mixed electron–proton conductivity in the low and intermediate temperature range under wet oxidizing and mild reducing conditions. Proton contribution to their total conductivity decreases as the lanthanide cation radius decreases. Among the samples studied, La5.8Zr0.2MoO12.1 showed the highest total conductivity of about 2.5 × 10−5 S/cm at 500 °C in wet air. Its impedance spectra did not provide any evidence of the grain boundary contribution, which seems to be an inherent feature of lanthanum molybdates. Exposure of Nd5.4Zr0.6MoO12.3 to wet argon was found to increase its total conductivity by almost one order of magnitude relative to the values obtained in dry argon. Although all the rare-earth molybdates studied in this work became essentially electronic conductors in Ar–5% H2 atmosphere, La5.8Zr0.2MoO12.1 demonstrated much lower propensity to reduction than the rest of the samples. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Proton-conducting oxides attracted great attention over the last decades primarily due to the fact that quite a few of them at least potentially can be used as electrolytes in high-temperature fuel cells and steam electrolyzers or mixed electron–proton conductors in gas separation membranes [1]. However, apart from being good proton conductors state-of-the-art materials such as acceptor-doped BaCeO3 and BaZrO3 suffer from several major drawbacks, which impede their immediate implementation into practice [2]. The tendency for the alkaline-earth cerrates/zirconates to react with CO2 and H2O, large grain boundary resistance and complexity of their fabrication are among such drawbacks. On the other hand, lanthanide based oxides, such as fluorite-like rare-earth molybdates and tungstates or Ce/Zr pyrochlores generally possess much better chemical stability than the perovskites and as will be discussed later some of them seem to lack any grain boundary contribution to the total conductivity [3].
☆ This work was presented during the 11th International Symposium on Systems with Fast Ionic Transport, Gdansk, Poland 25–29.06.2014 as poster presentation. ⁎ Correspondence to: A.V. Shlyakhtina, Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991 Russia. Tel.:+7 495 9397950; fax: +7 499 2420253. ⁎⁎ Correspondence to: P. Núñez, Department of Inorganic Chemistry and Institute of Materials and Nanotechnology, University of La Laguna, 38200 La Laguna, Tenerife, Spain. Tel.:+34 922318501; fax: +34 922 318461. E-mail addresses: [email protected], [email protected] (A.V. Shlyakhtina), [email protected] (P. Núñez).
Both oxygen-ion and proton conductivity of the Ln6WO12 tungstates are known to decrease across the rare-earth series in the order La N Nd N Gd N Er [4–7]. This decrease is accompanied by drastic changes in their crystal structure as the rare-earth ionic radius gets smaller. For instance, while La6WO12 adopts a highly disordered fluorite-like cubic structure, medium-sized rare-earth elements (Nd–Gd) bring about tetragonal distortions of the Ln6WO12 crystal structure and, finally, the smallest rare-earth cations (Tb–Lu) give rise to rhombohedral tungstates [5]. It should be mentioned that unlike the tungstates, the crystal structure of the medium-sized rare-earth molybdates (Sm, Eu, Gd, Dy) remains cubic although La6MoO12, Nd6MoO12, and Ho6MoO12 are known to exist both as rhombohedral (R3) and fluorite-like ( Fm3m) cubic polymorphs. Available data on phase relations in the Ln2O3–WO3 [8–14] and Ln2O3–MoO3 [15] systems is scarce and somewhat contradictory. Among the tungstates the La2O3–WO3 quasi-binary system seems to have received most attention over the past years. In fact, hightemperature phase equilibria occurring in this system are quite complex [9] and structural similarities between phases found in the La2O3-rich portion of the phase diagram have recently resulted in some debate on whether the “La6WO12” proton conductor is actually stable at room temperature [11]. The phase diagram of the Sm2O3–WO3 system studied by L.L.Y. Chang et al. [13,14] in the 1960s at first glance does not seem to be as complicated as that of La2O3–WO3. The compound Sm6WO12 was described as a defect pyrochlore with the cubic cell parameter of
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Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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is available on phase relations in the Ln2O3-rich field of the Ln2O3– MoO3 systems. Recently we have demonstrated that Zr-doped samarium molybdates (Sm5ZrMoO12.5 and Sm5.4Zr0.6MoO12.3) possess mixed electron– proton conductivity at intermediate temperatures [16]. In this work we report on the synthesis, crystal structure and conductivity of the fluorite-like Ln5.4Zr0.6MoO12.3 (Ln = Nd, Sm, Dy) and rhombohedral (La5.8Zr0.2MoO12.1) molybdates — potential intermediatetemperature mixed electron–proton conductors.
2. Experimental
Fig. 1. (a) XRD patterns of (1) La5.8Zr0.2MoO12.1 (1600 °C, 3 h), (2) Sm5.4Zr0.6MoO12.3 (1600 °C, 3 h) and (3) Dy5.4Zr0.6MoO12.3 (1600 °C, 3 h), indices are given for the fluorite cubic cell, and refer to the patterns 2 and 3 only; (b) results of the Le-Bail refinement of the XRD data obtained for La5.8Zr0.2MoO12.1; the inset shows that the rhombohedral unit cell proposed does not account for some weak superstructure reflections; (c) results of the Le-Bail refinement of the XRD profile for the sample of nominal composition Nd5.4Zr0.6MoO12.3, the inset highlights the presence of a strong peak splitting.
10.8 Å. Authors [14] speculate that above 1800 °C there might exist complete miscibility between Sm6WO12 and Sm14W4O33, for their crystal structures are apparently quite similar at high temperatures. Both compounds have homogeneity ranges which become wider as the temperature increases reaching ~ 5 and ~ 3.3 mol% Sm 2O 3 at 1700 °C for Sm6WO12 and Sm14W4O33 respectively. Much less data
Nd5.4Zr0.6MoO12 + δ (06NZMO), Sm5.4Zr0.6MoO12 + δ (06SZMO), Dy5.4Zr0.6MoO12 + δ (06DZMO) and La5.8Zr0.2MoO12.1 (02LZMO) were prepared by the mechanical activation method. After preheating the starting Ln2O3 oxides at 1000 °C for 2 h, they were mixed with the rest of the components (ZrO2, MoO3) and co-milled in the SPEX8000 ball mill for 1 h. MoO3 was previously activated in the high energy Aronov ball mill for 4 min. The mechanically activated mixtures of the oxides were unaxially pressed at 914 MPa and sintered at 1400 °C and 1600 °C for 3 h. The geometric density of the asprepared ceramics ranged from 80.5 to 92.5% of the theoretical one. All samples were characterized both structurally and electrically. Powder X-ray diffraction patterns were obtained on a DRON-3M automatic diffractometer (filtered CuKα radiation, step scan mode) in the angular range 2θ = 10–65° and on a PANalytical X'Pert Pro diffractometer equipped with a Ge(111) incident beam monochromator (CuKα1 radiation) and X'Celerator silicon strip detector. In the latter case the patterns were collected in the 2θ range of 5–120°, with a step size of 0.017° and counting time of 300 s per step. Structure refinement was performed using the GSAS suite of Rietveld refinement software [17]. Electrical conductivity of the samples was studied by impedance spectroscopy. Prior to making measurements both faces of the ceramic pellets prepared as described above were covered by Pt paste (Metalor, # 6695) and fired at 950 °C for 1 h. Afterwards, each pellet was settled in a gas-tight sample stage outfitted with an outer ceramic housing, gas inlet and outlet tubes and a thermocouple located in the close vicinity of the sample. The temperature dependence of the total (electronic and ionic) grain interior conductivity in different dry and wet atmospheres (air, Ar, Ar–5% H2) was extracted from impedance spectra obtained using a Solartron 1260 frequency response analyzer. The spectra were recorded in the frequency range of 0.1 Hz to 1 MHz on cooling from 850 °C to 250 °C; the root mean square ac voltage amplitude was set to 150 mV. Depending on the atmosphere and temperature it took from 2 to 5 h for the samples to reach the equilibrium conductivity values. Relative humidity of the gases fed into the sample stage was controlled by passing them over a freshly dehydrated silica gel (designated “dry”) or through a water saturator held at 20 °C (designated “wet”), which ensured constant water content of about 2%. Under reducing conditions total ac conductivity was measured as a function of the oxygen partial pressure p(O2) in the atmosphere of CO/CO2, while under oxidizing conditions dry oxygen and Ar were mixed in different ratios to yield atmospheres with p(O2) ranging from ≈ 1 × 10− 5 atm to ≈ 1 atm at 800 °C. Most of the impedance spectra obtained under mild reducing (Ar) and oxidizing (air) conditions were fitted to an equivalent circuit consisting of two parallel (RQ) elements placed in series, which represented bulk and grain boundary contributions to conductivity. In some instances, e.g. at high temperatures or in the case of 06DZMO and 02LZMO compounds, only one semicircle appeared in the spectra. In these cases the equivalent circuit was modified accordingly in order to take into account the absence of the bulk or grain boundary semicircle.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Table 1 Characteristics of the as prepared samples. Sample no
Composition
Heat treatment
Color
Relative density, %
Structure type
Cell parameters, Å
1 2 3 4
La5.8Zr0.2MoO12.1 Nd5.4Zr0.6MoO12.3 Sm5.4Zr0.6MoO12.3 Dy5.4Zr0.6MoO12.3
Air, 1600 °С, 3 h Air, 1400 °С, 3 h Air, 1600 °С, 3 h Air, 1600 °С, 3 h
Yellow Vinous Bright yellow Orange-yellow
92.5 92 80.5 91.2
Fluorite-like; rhombohedral Fluorite; 2 phases Fluorite Fluorite
a = 3.9785(1); c = 9.8633(2) a1 = 5.4572(1); a2 = 5.4269(1); 5.3991(1) 5.3036(2)
3. Results and discussion 3.1. Crystal structure of Ln5.4Zr0.6MoO12 + La5.8Zr0.2MoO12.1
δ
(Ln = Nd, Sm, Dy) and
According to the JCPDS PDF database and in contrast to La6WO12 (# 16-0391), whose crystal structure, to a first approximation, can be considered cubic defect fluorite, La6MoO12 (record # 34-1220) crystallizes in the rhombohedral structure of space group R3. Nd6MoO12 reportedly can adopt either cubic (# 24-1111) or rhombohedral (# 35-0246) crystal structure, while the other two molybdates, namely Sm6MoO12 (# 24-1121) and Dy6MoO12 (# 25-0331), were found to have only one stable polymorph, with a defect-fluorite cubic structure. The XRD patterns of the as-prepared Ln5.4Zr0.6MoO12.3 (Ln = Nd, Sm, Dy) and La5.8Zr0.2MoO12.1 as well as their cell parameters, color and relative density are given in Fig. 1 and Table 1, respectively. All reflections observed in the patterns of 06SZMO and 06DZMO were indexed on a face-centered cubic lattice (space group Fm3m), which resulted in quite similar cell parameters for both phases. However, closer inspection revealed that the XRD data obtained for the larger rare-earth molybdates, i.e. 02LZMO and 06NZMO, were more complex. Apart from a number of low-intensity and presumably superstructure reflections, the XRD pattern of 02LZMO contained intense peaks which were split in a way consistent with the trigonal distortion of an fcc lattice. It should be noted that our attempts to prepare single-phase La5.4Zr0.6MoO12.3 material did not succeed. Since its XRD patterns, regardless of the way of synthesis and conditions of thermal treatment, persistently showed traces of the La2Zr2O7 pyrochlore impurity we opted to reduce the amount of Zr and prepared less heavily doped La5.8Zr0.2MoO12.1 instead. Interestingly, M. Amsif et al. [18] recently observed a diffraction pattern quite similar to that given in Fig. 1b. They suggested that additional weak reflections present in the XRD pattern of La6Mo0.8W0.2O12 − δ could have resulted from a 7 × 7 superstructure formed in the ab plane of the rhombohedral subcell. Ordering of oxygen vacancies, typically observed in fluorite-based systems, was considered the prime reason for the superstructure to appear.
Indexing of the main reflections in the XRD pattern of 02LZMO allowed us to determine parameters of the subcell (Table 1). The values obtained agree quite well with those reported in [18] if allowance for the smaller ionic radius of Zr4+ is made. The XRD profile of 06NZMO showed clearly that the sample was not phase-pure and contained small amount of impurity whose cell parameter was close to that of the principal fluorite phase. Indeed, the experimental profile exhibited a significant splitting of all but the lowest angle diffraction peaks, which could be satisfactorily accounted for by including a second cubic phase in the Le-Bail refinement (Fig. 1c). Cell parameters of both co-existing fluorite phases are given in Table 1. The XRD pattern of 06DZMO was analyzed by the Rietveld method (Fig. 2). As mentioned above quite close values of the cubic cell parameter were obtained after indexing the diffraction patterns of 06SZMO and 06DZMO, which implied that both phases have similar crystal structures. Therefore, in the course of Rietveld refinement atomic positions in the unit cell of 06DZMO as well as initial values for isotropic displacement parameters were set equal to those reported for 06SZMO [16]. Given that in the fluorite cell both cations and anions are located in special positions their coordinates were kept fixed. Neither the occupancy of the cation sites was refined in order to avoid strong correlations between free parameters. As can be seen in Fig. 2 quite good agreement between the experimental data and the structural model that includes cation disordering over 4a positions in the fluorite unit cell has been obtained. The refined structural parameters are listed in Table 2. Stability of the molybdates under reducing conditions was examined by XRD after annealing in Ar–5% H2 at 900 °C for 10 h. Although no changes in the sample's phase composition were detected, their unit cell parameters decreased substantially on reduction (Fig. 3). 3.2. Electron–proton conductivity of Ln5.4Zr0.6MoO12+ δ (Ln = Nd, Sm, Dy) and La5.8Zr0.2MoO12.1 The equilibrium electrical conductivity of the cubic and rhombohedral molybdates was studied by impedance spectroscopy under oxidizing and reducing conditions. Fig. 4 shows several representative impedance spectra acquired at different temperatures in dry and wet air. It is interesting to note that the spectra of 06DZMO and 02LZMO showed rather dissimilar features related to the grain boundary behavior in different atmospheres. At intermediate (Fig. 4a) and low (Fig. 4c) temperatures under the atmosphere of wet air the spectra of 06DZMO were dominated by two adjacent semicircles, which were unambiguously attributed to bulk (Cb ≈ 7 × 10− 12 F/cm) and grain boundary (Cgb ≈ 3 × 10−10 F/cm) processes. On the contrary, only one semicircle corresponding to the bulk conductivity was present in the impedance spectra of 06DZMO recorded in dry air. Recently, we observed quite similar behavior of the grain boundary conductivity of 06SZMO [16].
Table 2 Final refined structural parameters for Dy5.4Zr0.6MoO12.3a.
Fig. 2. Rietveld refinement profile for the as-prepared Dy5.4Zr0.6MoO12.3.
Atom
Site
x
y
z
Fractional occupancy
100 × Uiso, Å−2
Dy1/Zr1/Mo1 O1
4a 8c
0 1/4
0 1/4
0 1/4
0.7714/0.0857/0.1429 0.8786
1.34(1) 6.5(2)
a
Cubic, Fm3m, a = 5.3036(2) Å, Rp = 0.0187, Rwp = 0.0270, χ2 = 1.799.
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Fig. 3. Dependence of the unit cell volume of the fluorite-like molybdates on the rare-earth ionic radius.
We believe that such unusual development of the grain boundary resistance under wet conditions may be related to impurity/dopant segregation or result from the loss of dimensional stability under reducing
conditions. In fact, as has been discussed in the previous section, reduction of the molybdates in Ar–5% H2 is accompanied by a considerable unit cell contraction, which might have affected somehow the integrity of the intergranular contacts. As can be seen in Fig. 4b,d, no grain boundary contribution to the impedance spectra of 02LZMO is visible either in dry or wet air (Cb ≈ 6 × 10−12 F/cm) at all temperatures. The same is true for the spectra obtained under dry/wet Ar atmosphere. At this point it is interesting to mention that the influence of blocking grain boundaries on the total conductivity of some LAMOX electrolytes has been thoroughly studied and was reported to depend on the synthesis method and processing conditions. For instance, D. Marrero-López et al. [19] found out that the grain boundary contribution to the conductivity of La2Mo2O9 progressively decreased as relative density of the ceramics was approaching 100%. Among the samples obtained from freeze-dried precursors, those attaining the highest (nearly theoretical) density showed the least grain boundary impedance. We believe that the absence of resistive grain boundary regions hindering either protonic or ionic transport in 02LZMO might be an intrinsic feature of lanthanum molybdates. On the other hand according to recent studies by J. Liu et al. [20,21] there is no solid evidence that fast grain boundary diffusion paths actually exist in La2Mo2O9 electrolytes as previously postulated by S. Georges et al. [22]. Regardless of the mechanistic details of the grain boundary conductivity in lanthanum molybdates, it is worth noting that reduction of losses related to the grain boundary impedance of alternative solid electrolyte materials can drastically improve performance of electrochemical cells based on them and eventually outweigh issues resulting from somewhat lower bulk conductivity [2].
Fig. 4. Impedance spectra of (a, c) 06DZMO and (b, d) 02LZMO at intermediate and low temperatures in dry and wet air. Solid lines are results of the non-linear least square fit, while broken lines represent extrapolation of the fit results to infinite frequency.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Fig. 5. Arrhenius plots of the bulk conductivity of (a) 02LZMO, (b) 06NZMO, (c) 06SZMO, and (d) 06DZMO in dry and wet air and Ar.
Fig. 5 presents the bulk conductivity of 06NZMO, 06SZMO, 06DZMO and 02LZMO in dry and wet atmospheres. A systematic increase of total conductivity in wet air/Ar as compared to the conductivity in dry gases is indicative of hydration of the samples resulting in proton conductivity. All curves given in Fig. 5 demonstrate an inflection point, whose location on the graph depends on the composition of the sample. By analogy with rare-earth tungstates the change in the activation energy (Table 3) with temperature observed in Ln6MoO12 − δ molybdates can be attributed to a shift from predominant oxygen-ion/proton conduction at low temperature to predominant electron/hole conduction in the high temperature range [5]. Under wet conditions such shift is usually related to dehydration of ceramics and concomitant depletion of the protons within the bulk of the samples. Interestingly, the onset of electron conduction in 06SZMO and 06DZMO takes place already at 500 °C in air, while 02LZMO is to be heated at least up to 750 °C for the electronic defects to dominate their total conductivity.
The total conductivity of 06NZMO, 06SZMO, 06DZMO and 02LZMO measured in dry Ar–5% H2 atmosphere is presented in Arrhenius form in Fig. 6. Under these conditions the reduction reaction seems to be the dominant defect equilibrium that renders the three molybdate Table 3 Activation energy for bulk conduction in Ln6 − xZrxMoO12+ δ in dry Ar. Sample no
Composition
Activation energy, eV
Temperature range, °C
1
02LZMO
2
06NZMO
3
06SZMO
4
06DZMO
1.21 0.78 1.48 0.91 1.48 0.75 1.38 0.69
Above 700 Below 700 Above 500 Below 500 Above 425 Below 425 Above 425 Below 425
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
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Fig. 6. Arrhenius plots of the bulk conductivity of 02LZMO, 06NZMO, 06SZMO and 06DZMO in dry Ar–5% H2.
electronic conductors. However, the 02LZMO is strikingly different from the rest of the samples. While 06SZMO and 06DZMO demonstrate a pronounced tendency to be reduced in Ar–5% H2 their conductivity reaching 0.2–0.25 S/cm at 800 °C, 02LZMO exhibits much lower conductivity (2.4 × 10− 2 S/cm at 800 °C) and higher activation energy (≈ 0.4 eV) below 500 °C. All these facts taken together suggest that 02LZMO might be less prone to reduction at low oxygen partial pressures. These data also seem to indicate that 02LZMO has higher proton transport numbers than the rest of the rare-earth molybdates do under strongly reducing conditions. In order to gain further insight into the nature of predominant defect species the equilibrium conductivity of 02LZMO, 06NZMO and 06SZMO was studied as a function of oxygen partial pressure (Fig. 7). The results clearly show two distinct types of behavior. At high temperatures (800 °C) the dependence of the total conductivity of 02LZMO on oxygen partial pressure resembles that of a typical mixed electronic– ionic conductor. There is an extensive region of n-type conductivity under reducing conditions, while under oxidizing conditions a significant region of p-type conductivity can be observed. At intermediate
oxygen partial pressures, the total conductivity of 02LZMO is essentially independent of p(O2), which is indicative of predominant ionic conductivity. The log-log slopes of the conductivity curve appear to be +1/4 and − 1/4 under the oxidizing and reducing conditions respectively, with a gradual shift towards −1/6 at the lowest oxygen partial pressures. In the case of 06NZMO and 06SZMO quite different behaviors were observed. The total conductivity of both samples is dominated by electrons at 800 °C except for a narrow region of roughly p(O2)-independent conductivity demonstrated by 06NZMO under the most oxidizing conditions, i.e. at p(O2) N 0.21 atm. As the temperature is decreased the p-type conductivity of 06NZMO becomes much more pronounced and the ionic regime widens. The tendency of ionic charge carriers to dominate defect equilibria at lower temperatures has been previously reported for the Ln6WO12 proton conductors as well [5]. It should be noted that the abovementioned dependence of the total conductivity of 02LZMO, 06NZMO and 06SZMO on oxygen partial pressure agrees well with the Arrhenius plots given in Fig. 5. Another noteworthy feature of the Ln6MoO12 − δ family of oxides is a significant correlation between their hydration behavior and the nature of the lanthanide species. As can be seen in Fig. 5, in wet air the relative contribution of protons to the total conductivity of the molybdates decreases as the rare-earth ionic radius shrinks (i.e. in the series La ≈ Nd N Sm N Dy). A similar tendency has been previously reported for the Ln6WO12 tungstates [5]. However, in the atmosphere of wet Ar (Fig. 5b), i.e. under mild reducing conditions where the p(O2) independent ionic/protonic region occurs, a tremendous increase in the conductivity of 06NZMO relative to dry Ar takes place. Hydration of the oxygen vacancies abundantly present in 06NZMO under these condition is probably responsible for this behavior. Although the relationships between hydration thermodynamics, concentration/mobility of the protonic defects and materials' physical parameters still have to be established for many ceramic proton conductors, high molar volume and high (preferentially cubic) symmetry as well as basicity of the lanthanide cations are among the factors favoring proton conductivity in oxides [2]. Particularly relevant in this regard is the fact that extrinsic oxygen vacancies introduced into the structure of LAMOX electrolytes as charge compensating species generally do not result in any improvement of their ionic conductivity [23]. The same holds true for Ca-doped “La6WO12” [24]. Therefore, the lattice contraction may be at least partially responsible for the drop in proton conductivity of Ln6(Mo,W)O12 − δ observed across the rare-earth series, albeit it is still under debate whether the smaller acceptor dopants actually alter the immediate environment of the inherent oxygen vacancies or induce trapping of ionic/protonic charge carriers, which reduces their mobility. On the other hand, one cannot definitively rule out the possibility that the transport properties of La5.8Zr0.2MoO12.1 reported here were affected by the Zr4+ dopant to a lesser degree than those of Ln6MoO12 (Ln = Nd, Sm, Dy). The effect of Zr4 + on the conductivity of the rare-earth molybdates was previously discussed in [16], where we demonstrated that it acts as a typical donor species. 4. Conclusions
Fig. 7. The total conductivity of 02LZMO, 06NZMO, and 06SZMO as a function of oxygen partial pressure. Broken lines are guides to the eye.
The fluorite-like Ln5.4Zr0.6MoO12.3 − δ (Ln = Nd, Sm, Dy) oxides have similar total conductivity of the order of 5 × 10−4 S/cm at 800 °C in air. Their conductivity seems to be dominated by ionic defects at low temperatures, while at high temperatures or under the atmosphere of Ar– 5% H2 electronic conduction prevails. Under oxidizing conditions electron holes were found to be dominant charge carriers in 02LZMO and 06NZMO at high and intermediate temperatures respectively. We argue that proton conduction may exist in 06NZMO, 06SZMO and 06DZMO in the temperature range of 250–530 °C in wet atmospheres. The relative contribution of protons to the total conductivity of Ln6 − xZrxMoO12+ x/2 − δ in air gradually decreases on going from La to Dy. 06NZMO demonstrated much better hydration characteristics than the rest of the molybdates did under mild reducing conditions.
Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003
S.N. Savvin et al. / Solid State Ionics xxx (2014) xxx–xxx
Among the samples studied, La5.8Zr0.2MoO12.1 showed the highest total conductivity in wet air (about 2.5 × 10−5 S/cm at 500 °C), which was found to be entirely due to the bulk transport. The total conductivity of La5.8Zr0.2MoO12.1 was found to be dominated by ionic charge carriers at intermediate oxygen partial pressures and high temperatures. Acknowledgments This work was supported by the Russian Foundation for Basic Research (grant no. 13-03-00680) and the Chemistry and Materials Science Division of the Russian Academy of Sciences (basic research program no. 2: Advanced Metallic, Ceramic, Glassy, Polymeric, and Composite Materials). We also acknowledge the financial support provided by the Spanish Government through the National research program (grants MAT2010-16007, MAT2013-42407 and ENE201347826-C4-1-R, co-financed by FEDER funds). References [1] T. Norby, M. Wideroe, R. Glockner, Y. Larring, Dalton Trans. (2004) 3012. [2] K. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. [3] V. Besikiotis, S. Ricote, M. Jensen, T. Norby, R. Haugsrud, Solid State Ionics 229 (2012) 26. [4] T. Shimura, S. Fujimoto, H. Iwahara, Solid State Ionics 143 (2001) 117. [5] R. Haugsrud, Solid State Ionics 178 (2007) 555.
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Please cite this article as: S.N. Savvin, et al., Crystal structure and proton conductivity of some Zr-doped rare-earth molybdates, Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.12.003